lock and key hypothesis of enzyme action supports that

Lock-and-key model

strong>Lock-and-key model n., [lɑk ænd ki ˈmɑdl̩] Definition: a model for enzyme-substrate interaction

Table of Contents

Lock-and-key model Definition

Lock-and-key model is a model for enzyme-substrate interaction suggesting that the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another. In this model, enzymes are depicted as highly specific. They must bind to specific substrates before they catalyze chemical reactions . The term is a pivotal concept in enzymology to elucidate the intricate interaction between enzymes and substrates at the molecular level. In the lock-and-key model, the enzyme-substrate interaction suggests that the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another. Like a key  into a  lock , only the correct size and shape of the substrate ( the key ) would fit into the  active site  ( the keyhole ) of the enzyme ( the lock ).

Compare: Induced fit model   See also: enzyme , active site , substrate

Lock-and-key vs. Induced Fit Model

At present, two models attempt to explain enzyme-substrate specificity; one of which is the lock-and-key model , and the other is the Induced fit model . The lock and key model theory was first postulated by  Emil Fischer   in 1894. The lock-and-key enzyme action proposes the high specificity of enzymes. However, it does not explain the stabilization of the transition state that the enzymes achieve. The induced fit model (proposed by Daniel Koshland in 1958) suggests that the active site continues to change until the substrate is completely bound to the active site of the enzyme, at which point the final shape and charge are determined. Unlike the lock-and-key model, the induced fit model shows that enzymes are rather flexible structures. Nevertheless, Fischer’s Lock and Key theory laid an important foundation for subsequent research, such as during the refinement of the enzyme-substrate complex mechanism, as ascribed in the induced fit model. The lock-and-key hypothesis has opened ideas where enzyme action is not merely catalytic but incorporates a rather complex process in how they interact with the correct substrates with precision.

Key Components

Components of the lock and key model:

• Enzyme : the enzyme structure is a three-dimensional protein configuration, with an active site from where the substrate binds.
• Substrate : often an organic molecule, a substrate possesses a structural feature that complements the geometry of the enzyme’s active site.

In the lock and key model, both the enzymes and the substrates facilitate the formation of a complex that lowers the activation energy needed for a chemical transformation to occur. Such reduction in the activation energy allows the chemical reaction to proceed at a relatively faster rate, making enzymes crucial in various biological and molecular processes.

Lock-and-key Model Examples

Some of the common examples that are often discussed in the context of the Lock and Key Model are as follows:

• Enzyme lactate dehydrogenase with a specific active site for its substrates, pyruvate and lactate. The complex facilitates the interconversion of pyruvate and lactate during anaerobic respiration
• Enzyme carbonic anhydrase with a specific active site for the substrates carbon dioxide and water. The complex facilitates the hydration of carbon dioxide, forming bicarbonate
• Enzyme lysozyme binding with a bacterial cell wall peptidoglycan, which is a vital immune function

Choose the best answer. 

Send Your Results (Optional)

• Aryal, S. and Karki, P. (2023).  “Lock and Key Model- Mode of Action of Enzymes”. Microbenotes.com. https://microbenotes.com/lock-and-key-model-mode-of-action-of-enzymes/
• Farhana, A., & Lappin, S. L. (2023, May).  Biochemistry, Lactate Dehydrogenase . Nih.gov; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK557536/

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Last updated on January 11th, 2024

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4.7: Enzyme Action

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Learning Objectives

• To describe the interaction between an enzyme and its substrate.

Enzyme-catalyzed reactions occur in at least two steps. In the first step, an enzyme molecule (E) and the substrate molecule or molecules (S) collide and react to form an intermediate compound called the enzyme-substrate (E–S) complex . (This step is reversible because the complex can break apart into the original substrate or substrates and the free enzyme.) Once the E–S complex forms, the enzyme is able to catalyze the formation of product (P), which is then released from the enzyme surface:

\[S + E \rightarrow E–S \tag{\(\PageIndex{1}\)}\]

\[E–S \rightarrow P + E \tag{\(\PageIndex{2}\)}\]

Hydrogen bonding and other electrostatic interactions hold the enzyme and substrate together in the complex. The structural features or functional groups on the enzyme that participate in these interactions are located in a cleft or pocket on the enzyme surface. This pocket, where the enzyme combines with the substrate and transforms the substrate to product is called the active site of the enzyme (Figure \(\PageIndex{1}\)).

The active site of an enzyme possesses a unique conformation (including correctly positioned bonding groups) that is complementary to the structure of the substrate, so that the enzyme and substrate molecules fit together in much the same manner as a key fits into a tumbler lock. In fact, an early model describing the formation of the enzyme-substrate complex was called the lock-and-key model (Figure \(\PageIndex{2}\)). This model portrayed the enzyme as conformationally rigid and able to bond only to substrates that exactly fit the active site.

Working out the precise three-dimensional structures of numerous enzymes has enabled chemists to refine the original lock-and-key model of enzyme actions. They discovered that the binding of a substrate often leads to a large conformational change in the enzyme, as well as to changes in the structure of the substrate or substrates. The current theory, known as the induced-fit model , says that enzymes can undergo a change in conformation when they bind substrate molecules, and the active site has a shape complementary to that of the substrate only after the substrate is bound, as shown for hexokinase in Figure \(\PageIndex{3}\). After catalysis, the enzyme resumes its original structure.

The structural changes that occur when an enzyme and a substrate join together bring specific parts of a substrate into alignment with specific parts of the enzyme’s active site. Amino acid side chains in or near the binding site can then act as acid or base catalysts, provide binding sites for the transfer of functional groups from one substrate to another or aid in the rearrangement of a substrate. The participating amino acids, which are usually widely separated in the primary sequence of the protein, are brought close together in the active site as a result of the folding and bending of the polypeptide chain or chains when the protein acquires its tertiary and quaternary structure. Binding to enzymes brings reactants close to each other and aligns them properly, which has the same effect as increasing the concentration of the reacting compounds.

Example \(\PageIndex{1}\)

• What type of interaction would occur between an OH group present on a substrate molecule and a functional group in the active site of an enzyme?
• Suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you just identified.
• An OH group would most likely engage in hydrogen bonding with an appropriate functional group present in the active site of an enzyme.
• Several amino acid side chains would be able to engage in hydrogen bonding with an OH group. One example would be asparagine, which has an amide functional group.

Exercise \(\PageIndex{1}\)

• What type of interaction would occur between an COO − group present on a substrate molecule and a functional group in the active site of an enzyme?

One characteristic that distinguishes an enzyme from all other types of catalysts is its substrate specificity . An inorganic acid such as sulfuric acid can be used to increase the reaction rates of many different reactions, such as the hydrolysis of disaccharides, polysaccharides, lipids, and proteins, with complete impartiality. In contrast, enzymes are much more specific. Some enzymes act on a single substrate, while other enzymes act on any of a group of related molecules containing a similar functional group or chemical bond. Some enzymes even distinguish between D- and L-stereoisomers, binding one stereoisomer but not the other. Urease, for example, is an enzyme that catalyzes the hydrolysis of a single substrate—urea—but not the closely related compounds methyl urea, thiourea, or biuret. The enzyme carboxypeptidase, on the other hand, is far less specific. It catalyzes the removal of nearly any amino acid from the carboxyl end of any peptide or protein.

Enzyme specificity results from the uniqueness of the active site in each different enzyme because of the identity, charge, and spatial orientation of the functional groups located there. It regulates cell chemistry so that the proper reactions occur in the proper place at the proper time. Clearly, it is crucial to the proper functioning of the living cell.

A substrate binds to a specific region on an enzyme known as the active site, where the substrate can be converted to product. The substrate binds to the enzyme primarily through hydrogen bonding and other electrostatic interactions. The induced-fit model says that an enzyme can undergo a conformational change when binding a substrate. Enzymes exhibit varying degrees of substrate specificity.

Concept Review Exercises

• Distinguish between the lock-and-key model and induced-fit model of enzyme action.
• Which enzyme has greater specificity—urease or carboxypeptidase? Explain.
• The lock-and-key model portrays an enzyme as conformationally rigid and able to bond only to substrates that exactly fit the active site. The induced fit model portrays the enzyme structure as more flexible and is complementary to the substrate only after the substrate is bound.
• Urease has the greater specificity because it can bind only to a single substrate. Carboxypeptidase, on the other hand, can catalyze the removal of nearly any amino acid from the carboxyl end of a peptide or protein.

What type of interaction would occur between each group present on a substrate molecule and a functional group of the active site in an enzyme?

• CH(CH 3 ) 2
• COO −

For each functional group in Exercise 1, suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you identified.

For each functional group in Exercise 2, suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you identified.

• hydrogen bonding
• ionic bonding
• dispersion forces
• The amino acid has a polar side chain capable of engaging in hydrogen bonding; serine (answers will vary).
• The amino acid has a negatively charged side chain; aspartic acid (answers will vary).
• The amino acid has a polar side chain capable of engaging in hydrogen bonding; asparagine (answers will vary).
• The amino acid has a nonpolar side chain; isoleucine (answers will vary).

Structural Biochemistry/Protein function/Lock and Key

In the Lock and Key Model, first presented by Emil Fisher, the lock represents an enzyme and the key represents a substrate. It is assumed that both the enzyme and substrate have fixed conformations that lead to an easy fit. Because the enzyme and the substrate are at a close distance with weak attraction, the substrate must need a matching shape and fit to join together. At the active sites, the enzyme has a specific geometric shape and orientation that a complementary substrate fits into perfectly. The theory behind the Lock and Key model involves the complementarity between the shapes of the enzyme and the substrate. Their complementary shapes make them fit perfectly into each other like a lock and a key. According to this theory, the enzyme and substrate shape do not influence each other because they are already in a predetermined perfectly complementary shape. As a result, the substrate will be stabilized. This theory was replaced by the induced fit model which takes into account the flexibility of enzymes and the influence the substrate has on the shape of the enzyme in order to form a good fit.

The active site is the binding site for catalytic and inhibition reaction of the enzyme and the substrate; structure of active site and its chemical characteristic are of specificity for binding of substrate and enzyme. Three models of enzyme-substrate binding are the lock-and-key model, the induced fit model, and the transition-state model. The lock-and-key model assumes that active site of enzyme is good fit for substrate that does not require change of structure of enzyme after enzyme binds substrate.

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Molecular Recognition: Lock-and-Key, Induced Fit, and Conformational Selection

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Fluctuation fit ; Population selection ; Population shift ; Preexisting equilibrium ; Selected fit

In the most general sense, molecular recognition corresponds to the mechanism by which two or more molecules come together to form a specific complex. These types of specific molecular interactions span biology and include processes as diverse as enzyme catalysis, antibody–antigen recognition, protein synthesis, and transcriptional regulation to name but a few. As a consequence of the universal importance of molecular recognition in biological function, understanding how molecules specifically recognize and interact with one another is of fundamental importance.

Introduction

Early studies on the kinetics and specificity of enzyme-catalyzed reactions led to the conclusion that substrates combine with the enzyme at a specific active site location on the enzyme surface. This understanding generated new questions as to how enzymes could carry out chemical transformations...

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Jencks WP. Binding energy, specificity, and enzymic catalysis: the circe effect. Adv Enzymol Relat Areas Mol Biol. 1975;43:219–410.

Koshland DE. Application of a theory of enzyme specificity to protein synthesis. Proc Natl Acad Sci USA. 1958;44(2):98–104.

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Sullivan SM, Holyoak T. Enzymes with lid-gated active sites must operate by an induced fit mechanism instead of conformational selection. Proc Natl Acad Sci USA. 2008;105(37):13829–34.

Tsai CJ, Kumar S, Ma B, Nussinov R. Folding funnels, binding funnels, and protein function. Protein Sci. 1999a;8(6):1181–90.

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Voet D, Voet JG. Biochemistry. Hoboken: Wiley; 2004.

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Holyoak, T. (2013). Molecular Recognition: Lock-and-Key, Induced Fit, and Conformational Selection. In: Roberts, G.C.K. (eds) Encyclopedia of Biophysics. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-16712-6_468

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A theory to explain the mechanism of enzymatic reactions, in which it is proposed that the enzyme and substrate(s) bind temporarily to form an enzyme–substrate complex. The binding site on the enzyme is known as the ‘active site’ and is structurally complementary to the substrate(s). Thus the enzyme and substrate(s) are said to fit together as do a lock and a key.

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Chapter 18: Amino Acids, Proteins, and Enzymes

18.6 enzyme action, learning objective.

• Describe the interaction between an enzyme and its substrate.

Enzyme-catalyzed reactions occur in at least two steps. In the first step, an enzyme molecule (E) and the substrate molecule or molecules (S) collide and react to form an intermediate compound called the enzyme-substrate (E–S) complex . (This step is reversible because the complex can break apart into the original substrate or substrates and the free enzyme.) Once the E–S complex forms, the enzyme is able to catalyze the formation of product (P), which is then released from the enzyme surface:

S + E → E–S E–S → P + E

Hydrogen bonding and other electrostatic interactions hold the enzyme and substrate together in the complex. The structural features or functional groups on the enzyme that participate in these interactions are located in a cleft or pocket on the enzyme surface. This pocket, where the enzyme combines with the substrate and transforms the substrate to product is called the active site  of the enzyme ( Figure 18.10 “Substrate Binding to the Active Site of an Enzyme” ). It possesses a unique conformation (including correctly positioned bonding groups) that is complementary to the structure of the substrate, so that the enzyme and substrate molecules fit together in much the same manner as a key fits into a tumbler lock. In fact, an early model describing the formation of the enzyme-substrate complex was called the lock-and-key model  ( Figure 18.11 “The Lock-and-Key Model of Enzyme Action” ). This model portrayed the enzyme as conformationally rigid and able to bond only to substrates that exactly fit the active site.

Figure 18.10 Substrate Binding to the Active Site of an Enzyme.  The enzyme dihydrofolate reductase is shown with one of its substrates: NADP+ (a) unbound and (b) bound. The NADP+ (shown in red) binds to a pocket that is complementary to it in shape and ionic properties.

Figure 18.11 The Lock-and-Key Model of Enzyme Action.  (a) Because the substrate and the active site of the enzyme have complementary structures and bonding groups, they fit together as a key fits a lock. (b) The catalytic reaction occurs while the two are bonded together in the enzyme-substrate complex.

Working out the precise three-dimensional structures of numerous enzymes has enabled chemists to refine the original lock-and-key model of enzyme actions. They discovered that the binding of a substrate often leads to a large conformational change in the enzyme, as well as to changes in the structure of the substrate or substrates. The current theory, known as the induced-fit model , says that enzymes can undergo a change in conformation when they bind substrate molecules, and the active site has a shape complementary to that of the substrate only after the substrate is bound, as shown for hexokinase in Figure 18.12 “The Induced-Fit Model of Enzyme Action” . After catalysis, the enzyme resumes its original structure.

Figure 18.12 The Induced-Fit Model of Enzyme Action.  (a) The enzyme hexokinase without its substrate (glucose, shown in red) is bound to the active site. (b) The enzyme conformation changes dramatically when the substrate binds to it, resulting in additional interactions between hexokinase and glucose.

The structural changes that occur when an enzyme and a substrate join together bring specific parts of a substrate into alignment with specific parts of the enzyme’s active site. Amino acid side chains in or near the binding site can then act as acid or base catalysts, provide binding sites for the transfer of functional groups from one substrate to another or aid in the rearrangement of a substrate. The participating amino acids, which are usually widely separated in the primary sequence of the protein, are brought close together in the active site as a result of the folding and bending of the polypeptide chain or chains when the protein acquires its tertiary and quaternary structure. Binding to enzymes brings reactants close to each other and aligns them properly, which has the same effect as increasing the concentration of the reacting compounds.

• What type of interaction would occur between an OH group present on a substrate molecule and a functional group in the active site of an enzyme?

Suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you just identified.

• An OH group would most likely engage in hydrogen bonding with an appropriate functional group present in the active site of an enzyme.
• Several amino acid side chains would be able to engage in hydrogen bonding with an OH group. One example would be asparagine, which has an amide functional group.

Skill-Building Exercise

What type of interaction would occur between an COO − group present on a substrate molecule and a functional group in the active site of an enzyme?

One characteristic that distinguishes an enzyme from all other types of catalysts is its substrate specificity . An inorganic acid such as sulfuric acid can be used to increase the reaction rates of many different reactions, such as the hydrolysis of disaccharides, polysaccharides, lipids, and proteins, with complete impartiality. In contrast, enzymes are much more specific. Some enzymes act on a single substrate, while other enzymes act on any of a group of related molecules containing a similar functional group or chemical bond. Some enzymes even distinguish between D- and L-stereoisomers, binding one stereoisomer but not the other. Urease, for example, is an enzyme that catalyzes the hydrolysis of a single substrate—urea—but not the closely related compounds methyl urea, thiourea, or biuret. The enzyme carboxypeptidase, on the other hand, is far less specific. It catalyzes the removal of nearly any amino acid from the carboxyl end of any peptide or protein.

Enzyme specificity results from the uniqueness of the active site in each different enzyme because of the identity, charge, and spatial orientation of the functional groups located there. It regulates cell chemistry so that the proper reactions occur in the proper place at the proper time. Clearly, it is crucial to the proper functioning of the living cell.

Concept Review Exercises

Distinguish between the lock-and-key model and induced-fit model of enzyme action.

Which enzyme has greater specificity—urease or carboxypeptidase? Explain.

• The lock-and-key model portrays an enzyme as conformationally rigid and able to bond only to substrates that exactly fit the active site. The induced fit model portrays the enzyme structure as more flexible and is complementary to the substrate only after the substrate is bound.
• Urease has the greater specificity because it can bind only to a single substrate. Carboxypeptidase, on the other hand, can catalyze the removal of nearly any amino acid from the carboxyl end of a peptide or protein.

Key Takeaways

• A substrate binds to a specific region on an enzyme known as the active site, where the substrate can be converted to product.
• The substrate binds to the enzyme primarily through hydrogen bonding and other electrostatic interactions.
• The induced-fit model says that an enzyme can undergo a conformational change when binding a substrate.
• Enzymes exhibit varying degrees of substrate specificity.

What type of interaction would occur between each group present on a substrate molecule and a functional group of the active site in an enzyme?

• CH(CH 3 ) 2

For each functional group in Exercise 1, suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you identified.

For each functional group in Exercise 2, suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you identified.

1. a. hydrogen bonding

    b. ionic bonding

    c. hydrogen bonding

    d. dispersion forces  

3. a. The amino acid has a polar side chain capable of engaging in hydrogen bonding; serine (answers will vary).

    b. The amino acid has a negatively charged side chain; aspartic acid (answers will vary).

    c. The amino acid has a polar side chain capable of engaging in hydrogen bonding; asparagine (answers will vary).

• The Basics of General, Organic, and Biological Chemistry v. 1.0. Provided by : Saylor Academy. Located at : https://saylordotorg.github.io/text_the-basics-of-general-organic-and-biological-chemistry/ . License : CC BY-NC: Attribution-NonCommercial . License Terms : This text was adapted by Saylor Academy under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License without attribution as requested by the work's original creator or licensor.

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Chapter 5: Enzyme Action

• Kanwal Hafeez
• March 28, 2024

Table of Contents

Chapter 5 delves into the dynamic realm of enzyme action, pivotal for accelerating metabolic reactions within organisms. Enzymes, specialized proteins acting as biological catalysts, exhibit specificity through their unique active sites that precisely bind substrates. This lock-and-key mechanism ensures efficiency in catalyzing specific reactions, crucial for cellular processes’ regulation. Understanding the interplay between enzymes and their substrates unveils insights into metabolic pathways’ intricacies, essential for comprehending biochemical regulations within cells.

Catalyst Definition:

• Catalyst: A substance that increases the rate of a chemical reaction without undergoing any permanent change in its own chemical composition.

Enzyme Definition:

• Enzyme: Proteins functioning as biological catalysts, crucial in accelerating metabolic reactions within living organisms.

Enzyme Action:

• Substrate: The specific molecule upon which an enzyme acts.
• Active Site: Region on the enzyme where the substrate binds, forming the enzyme-substrate complex.
• Enzyme-Substrate Complex: Temporary structure formed when the enzyme and substrate interact.
• Product: The resulting molecule(s) after the enzymatic reaction.

Specificity of Enzymes:

• Complementary Shape: The active site of an enzyme has a specific three-dimensional shape that matches the substrate.
• Fit: Described by the ‘lock and key’ hypothesis, where the active site (lock) precisely fits the substrate (key).
• Importance: This specificity ensures that each enzyme catalyzes a particular reaction, contributing to the efficiency and regulation of cellular processes.

Understanding enzyme action and specificity is fundamental to comprehending the intricacies of metabolic pathways and how enzymes contribute to the regulation of biochemical reactions within cells.

GCSE Biology – What are Enzymes?

Effects of Temperature and pH on Enzyme Activity:

1. progress measurement of enzyme-catalyzed reactions:.

• Measurement: Followed by assessing the concentrations of reactants and products.
• Techniques: Spectrophotometry, colorimetry, or other methods depending on the nature of the reaction.

2. Effects of Temperature and pH on Enzyme Activity:

• Optimal Range: Enzymes have an optimum temperature for activity.
• Effect: Below optimal range – Slow reaction due to low kinetic energy. Above optimal range – Denaturation, loss of enzyme structure.
• Investigation: Experiments involve varying temperature and measuring reaction rates.
• Optimal pH: Enzymes work best within a specific pH range.
• Effect: Deviation from optimal pH can alter enzyme shape and affect active site interactions.
• Investigation: Experiments involve adjusting pH and monitoring enzyme activity.

3. Explanation of Temperature and pH Effects:

• Temperature: Higher temperature increases kinetic energy, enhancing collision frequency.
• pH: pH affects ionisation states, influencing enzyme-substrate interactions.
• Temperature: Excessive heat can disrupt the enzyme’s three-dimensional structure, affecting the fit with substrates.
• pH: Changes in pH alter charge distributions, impacting the shape of the active site.
• Temperature: Beyond the optimal range, heat induced denaturation, leading to irreversible loss of enzyme structure.
• pH: Extreme pH values can cause denaturation.
• Temperature: Increased temperature raises collision frequency, potentially leading to more effective collisions.
• pH: Optimal pH ensures proper ionisation states for enzyme-substrate interactions.

GCSE Biology – Enzymes – How Temperature and pH Affect Rate of Reaction

Understanding these factors is crucial for optimising experimental conditions and comprehending the intricacies of enzyme function in various biological processes.

Enzymes – IGCSE Biology Revision Notes

Mastering the nuances of enzyme activity, including the effects of temperature and pH, is pivotal for optimizing experimental conditions and comprehending the intricacies of biological processes. By exploring how enzymes catalyze reactions and how external factors influence their function, researchers gain invaluable insights into the regulation of biochemical pathways within living organisms.

This understanding not only enhances our knowledge of fundamental biological principles but also underpins advancements in fields such as medicine, biotechnology, and pharmacology, paving the way for innovative solutions to complex biological challenges.

Related Posts.

Chapter 19: relationships of organisms with one another and with the environment.

Energy Flow: 1. Principal Source of Energy: 2. Dependence on Photosynthesis: 3. Flow of Energy Through Food Chains and Webs:

Chapter 18: Biotechnology and Genetic Modification 

1. Role of Yeast 2. Bacteria in Biotechnology 3. Why Bacteria in Biotechnology 4. Fermenters in Large-Scale Production 5. Enzymes

Chapter 17: Inheritance – Variation 

1. Description of Variation: 2. Continuous Variation: 3. Discontinuous Variation: 4. Causes of Variation: 5. Examples of Continuous and Discontinuous

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2.7.2: Enzyme Active Site and Substrate Specificity

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Enzymes catalyze chemical reactions by lowering activation energy barriers and converting substrate molecules to products.

Learning Objectives

• Describe models of substrate binding to an enzyme’s active site.
• The enzyme ‘s active site binds to the substrate.
• Increasing the temperature generally increases the rate of a reaction, but dramatic changes in temperature and pH can denature an enzyme, thereby abolishing its action as a catalyst.
• The induced fit model states an substrate binds to an active site and both change shape slightly, creating an ideal fit for catalysis.
• When an enzyme binds its substrate it forms an enzyme-substrate complex.
• Enzymes promote chemical reactions by bringing substrates together in an optimal orientation, thus creating an ideal chemical environment for the reaction to occur.
• The enzyme will always return to its original state at the completion of the reaction.
• substrate : A reactant in a chemical reaction is called a substrate when acted upon by an enzyme.
• induced fit : Proposes that the initial interaction between enzyme and substrate is relatively weak, but that these weak interactions rapidly induce conformational changes in the enzyme that strengthen binding.
• active site : The active site is the part of an enzyme to which substrates bind and where a reaction is catalyzed.

Enzyme Active Site and Substrate Specificity

Enzymes bind with chemical reactants called substrates. There may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. In some reactions, a single-reactant substrate is broken down into multiple products. In others, two substrates may come together to create one larger molecule. Two reactants might also enter a reaction, both become modified, and leave the reaction as two products.

The enzyme’s active site binds to the substrate. Since enzymes are proteins, this site is composed of a unique combination of amino acid residues (side chains or R groups). Each amino acid residue can be large or small; weakly acidic or basic; hydrophilic or hydrophobic; and positively-charged, negatively-charged, or neutral. The positions, sequences, structures, and properties of these residues create a very specific chemical environment within the active site. A specific chemical substrate matches this site like a jigsaw puzzle piece and makes the enzyme specific to its substrate.

Active Sites and Environmental Conditions

Environmental conditions can affect an enzyme’s active site and, therefore, the rate at which a chemical reaction can proceed. Increasing the environmental temperature generally increases reaction rates because the molecules are moving more quickly and are more likely to come into contact with each other.

However, increasing or decreasing the temperature outside of an optimal range can affect chemical bonds within the enzyme and change its shape. If the enzyme changes shape, the active site may no longer bind to the appropriate substrate and the rate of reaction will decrease. Dramatic changes to the temperature and pH will eventually cause enzymes to denature.

Induced Fit and Enzyme Function

For many years, scientists thought that enzyme-substrate binding took place in a simple “lock-and-key” fashion. This model asserted that the enzyme and substrate fit together perfectly in one instantaneous step. However, current research supports a more refined view called induced fit. As the enzyme and substrate come together, their interaction causes a mild shift in the enzyme’s structure that confirms an ideal binding arrangement between the enzyme and the substrate. This dynamic binding maximizes the enzyme’s ability to catalyze its reaction.

Enzyme-Substrate Complex

When an enzyme binds its substrate, it forms an enzyme-substrate complex. This complex lowers the activation energy of the reaction and promotes its rapid progression by providing certain ions or chemical groups that actually form covalent bonds with molecules as a necessary step of the reaction process. Enzymes also promote chemical reactions by bringing substrates together in an optimal orientation, lining up the atoms and bonds of one molecule with the atoms and bonds of the other molecule. This can contort the substrate molecules and facilitate bond-breaking. The active site of an enzyme also creates an ideal environment, such as a slightly acidic or non-polar environment, for the reaction to occur. The enzyme will always return to its original state at the completion of the reaction. One of the important properties of enzymes is that they remain ultimately unchanged by the reactions they catalyze. After an enzyme is done catalyzing a reaction, it releases its products (substrates).

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• OpenStax College, Biology. October 16, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44429/latest...ol11448/latest . License : CC BY: Attribution
• Boundless. Provided by : Boundless Learning. Located at : www.boundless.com//biology/de...llosteric-site. License : CC BY-SA: Attribution-ShareAlike
• cofactor. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/cofactor. License : CC BY-SA: Attribution-ShareAlike
• coenzyme. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/coenzyme. License : CC BY-SA: Attribution-ShareAlike
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• OpenStax College, Enzymes. October 16, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44429/latest...e_06_05_05.jpg . License : CC BY: Attribution
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• active site. Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/active%20site. License : CC BY-SA: Attribution-ShareAlike
• substrate. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/substrate. License : CC BY-SA: Attribution-ShareAlike
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