Module 8: Enzyme Classification and Catalysis
Enzyme Characteristics & Classification Notes
DLOs (Desired Learning Outcomes)
Define the term catalyst.
A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process.
Describe how catalysts affect reaction rates.
Catalysts provide an alternative reaction pathway with a lower activation energy which accelerates the reaction rate significantly.
Describe how an enzyme increases the rate of a biochemical reaction.
Enzymes are biological catalysts that facilitate biochemical reactions by lowering the activation energy required for the reaction, thus increasing the reaction rate.
Explain the role of the active site of an enzyme and how enzymes differ from other regulatory proteins such as receptors.
The active site of an enzyme is the specific region where substrate molecules bind. It is uniquely shaped to fit its substrate, allowing for a biochemical reaction to occur. Unlike receptors that bind signaling molecules, enzymes catalyze reactions and are not permanently altered by these processes.
Explain how enzymes are named according to the International Union of Biochemistry (IUB) systematic naming scheme.
Enzymes are named based on the reaction they catalyze, typically comprising a root name indicating the substrate or class of reaction followed by a suffix indicating the type of enzyme (e.g., -ase).
Additional DLOs
List the six IUB classes of enzymes and give examples of each class.
Oxidoreductases: Catalyze oxidation-reduction reactions.
Transferases: Transfer functional groups between molecules (e.g., kinases).
Hydrolases: Catalyze hydrolysis reactions (e.g., proteases).
Lyases: Catalyze the addition or removal of groups to form double bonds.
Isomerases: Catalyze isomerization reactions.
Ligases: Catalyze joining of two molecules using ATP energy.
Define the terms cofactor, essential ions, and coenzymes.
Cofactor: A non-protein chemical compound that is required for the biological activity of a protein (e.g., metal ions).
Essential ions: Metal ions necessary for enzyme function, such as Mg²⁺, Zn²⁺.
Coenzymes: Organic non-protein molecules that assist enzymes (e.g., vitamins).
Compare and contrast metal-activated enzymes and metalloenzymes and give examples of each.
Metal-activated enzymes: Require metal ions for activity but can be activated by various concentrations (e.g., enzyme that activates with Mg²⁺).
Metalloenzymes: Contain metal ions as integral parts of their structure; the metal is tightly bound (e.g., carbonic anhydrase).
Define the terms apoenzyme and holoenzyme.
Apoenzyme: The protein portion of an enzyme, inactive until associated with a cofactor.
Holoenzyme: The complete, active form of the enzyme including both the apoenzyme and the cofactor.
Compare and contrast cosubstrates and prosthetic groups and give examples of each.
Cosubstrates: Loosely bound, non-protein molecules that participate in the reaction and are modified (e.g., NAD⁺).
Prosthetic groups: Tightly bound non-polypeptide units essential for the biological activity (e.g., heme in hemoglobin).
Define the term vitamin and explain the importance of these agents in the functioning of enzymes.
Vitamin: Organic compounds required in small quantities for vital metabolism; many vitamins serve as precursors for coenzymes to facilitate enzyme activity.
Mechanisms and Catalysis
Describe the major mechanisms by which reactive groups on amino acids participate in catalysis.
Reactive groups from amino acids participate through: acid-base catalysis, covalent catalysis, and metal ion catalysis, lowering the activation energy.
Define the terms specific acid-base catalysis and general acid-base catalysis, and give examples of each.
Specific acid-base catalysis: Involves only the solvent molecules (water) acting in the proton transfer (e.g., H⁺ transfer in enzyme reactions).
General acid-base catalysis: Involves an acid or base other than water (e.g., histidine acting as a base in serine protease).
Define covalent catalysis.
Covalent catalysis is a mechanism where a transient covalent bond forms between the enzyme and substrate, altering the reaction pathway.
Explain what is meant by a diffusion-controlled reaction.
A diffusion-controlled reaction is one where the rate is limited by the diffusion of substrates to the enzyme, with maximum reaction rates approaching to M⁻¹s⁻¹.
Give examples of several enzymes that approach the theoretical upper limit for second-order reaction rates in solution.
Enzymes like acetylcholinesterase and carbonic anhydrase reach rates near this upper limit due to their efficient binding and catalysis of substrates.
Compare the relative proposed acceleration rates played by mechanistic and binding effects in enzyme catalysis.
Mechanistic effects relate to the actual pathways enzymes take, while binding effects relate to the affinity and orientation of substrates. Both contribute to overall acceleration but vary in their impact depending on the enzyme and reaction context.
Explain and illustrate the proximity effect in enzyme catalysis.
The proximity effect refers to increasing reaction rates by positioning molecules closer together, thus increasing the likelihood of interaction.
Define the term effective molarity.
Effective molarity is a measure of the concentration of reactive molecules in a catalyzed reaction that reflects their enhanced effective concentration due to favorable positioning by the enzyme.
Explain the concept of transition state stabilization in enzyme catalysis.
Transition state stabilization is the process where enzymes bind to the transition state of substrates more tightly than to the substrate itself, lowering the energy barrier and increasing reaction rates.
Define the term transition state analog.
A transition state analog is a molecule that resembles the transition state of a substrate but is not a substrate itself, often used as an inhibitor in studies on enzyme kinetics.
Define the term saturable system. Explain how Vmax illustrates that enzyme systems are saturable.
A saturable system is one where, beyond a certain substrate concentration, the reaction rate no longer increases and reaches a maximum (Vmax), indicating that all active sites of the enzyme molecules are occupied.
Enzymes: Key Concepts
Enzymes only affect kinetics, not thermodynamics of reactions.
Enzymatic catalysis lowers activation energy by stabilizing transition states.
Enzymes are reusable and not consumed in reactions.
Enzymes can catalyze reactions that would take too long without them.
Characteristics of Enzymes
Enzymes are predominantly proteins (with exceptions such as ribozymes).
They act as catalysts, lowering activation energy (rate enhancements can reach ).
Metabolic roles: - Anabolic: Biosynthesis, highly substrate specific.
Catabolic: Broader substrate specificity (e.g., endogenous vs. xenobiotic).
Locations include intracellular (nucleus, cytosol, membranes) and extracellular regions.
Zymogens or proenzymes are inactive precursors of enzymes, which require cleavage to become active (e.g., coagulation cascade).
Natural inhibitors include: - Macroglobulins (e.g., -macroglobulin) - irreversibly inhibit certain proteases.
Serpins (e.g., -antitrypsin) - inhibitors of serine proteases.
Kunitz inhibitors (e.g., pancreatic trypsin inhibitor) - reversible inhibitors.
Kazal inhibitors (e.g., acrosin inhibitor) - reversible inhibitors.
Classification of Enzymes
Simple enzymes: Composed of protein alone.
Complex enzymes (Holoenzymes): Composed of protein and a non-protein cofactor. - Apoenzyme: The protein part when not bound to cofactor.
Cofactors: Non-protein parts that assist, including coenzymes and prosthetic groups.
IUB Classification
Oxidoreductases: Catalyze oxidation-reduction reactions. - Example: glucose oxidase.
Transferases: Move functional groups from one molecule to another. - Example: aspartate aminotransferase.
Hydrolases: Break down compounds with the addition of water. - Example: chymosin.
Lyases: Use water, ammonia, or carbon dioxide to convert substrates. - Example: histidine ammonia-lyase.
Isomerases: Rearrange molecular structures. - Example: xylose isomerase.
Ligases: Join two molecules using ATP energy. - Example: glutathione synthetase.
Cofactors for Enzymes
Essential ions: Such as Mg²⁺, Zn²⁺, Cu²⁺, necessary for function and include in many enzymes like cytochrome oxidase and carbonic anhydrase.
Coenzymes
Coenzymes are modified during reactions and may act as cosubstrates or prosthetic groups. Vitamins serve as precursors for some coenzymes.
Difference between Metalloenzymes and Metal-Activated Enzymes
Metalloenzymes are tightly bound to a metal ion which is an active part of the enzyme; metal-activated enzymes show activity with varying concentrations of metal ions attached but do not necessarily contain them in a defined structure.
Enzymatic Reaction Dynamics
Transition states lower energy states; substrate recognition is selective based on binding site properties.
Consider the enzyme-catalyzed rate difference in catalysis and the mechanisms of serine proteases, cysteine proteases, and aspartyl proteases.
Key Terms
kcat (turnover number): the maximum substrate transformation rate per enzyme per unit of time when saturated; .
Km: the substrate concentration at which the reaction rate is half of Vmax, defining enzyme-substrate affinity.
kcat/Km: specificity constant reflecting enzyme efficiency; optimal ranges are between to M⁻¹s⁻¹ for diffusion-controlled reactions.
Medicinal Applications of Enzymes
Inhibitors: Targeting human or non-human enzymes for therapeutic uses (e.g., cancer treatment, antimicrobial).
Administration of enzymes: therapeutics (e.g., streptokinase) and digestive aids (e.g., lactase).
Diagnostics: Enzyme activity assays to detect health markers (e.g., liver enzymes).
Isoforms and Isozymes: Differences between isoforms (related proteins) and isozymes (variants of an enzyme).
The Concept of Transition States in Enzymatic Reaction Paths
Enzymes stabilize transition states to lower activation energies, evidenced by substantial rate enhancements in enzyme reactions compared to uncatalyzed reactions, highlighting the specificity and efficiency inherent in enzyme catalysis.
Practice Problems
What is a catalyst, and how does it function in a chemical reaction without being consumed?
Answer: A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. It functions by providing an alternative reaction pathway with a lower activation energy.
Explain how catalysts impact the rate of a chemical reaction, specifically mentioning their effect on activation energy.
Answer: Catalysts accelerate the reaction rate significantly by providing an alternative reaction pathway that has a lower activation energy compared to the uncatalyzed reaction.
How do enzymes, as biological catalysts, enhance the speed of biochemical reactions?
Answer: Enzymes enhance the speed of biochemical reactions by lowering the activation energy required for the reaction, thereby increasing the reaction rate.
Describe the significance of an enzyme's active site. How does the function of an enzyme's active site differentiate it from a receptor binding site?
Answer: The active site of an enzyme is the specific region where substrate molecules bind and where the biochemical reaction occurs, due to its unique shape fitted to the substrate. Unlike receptors, which bind signaling molecules (often leading to a cascade of events), enzymes catalyze a chemical transformation and are not permanently altered in the process.
According to the IUB naming scheme, what would be the likely name for an enzyme that breaks down lipids? Justify your answer.
Answer: An enzyme that breaks down lipids would likely be named a "Lipase". This follows the IUB naming scheme where the substrate (lipid) is followed by the suffix "-ase", indicating it's an enzyme, and specifically "hydrolase" in the IUB classification as it breaks down compounds with the addition of water (hydrolysis).
Classify the following enzymes into their respective IUB categories: DNA ligase, alcohol dehydrogenase, hexokinase, pepsin, fumarase, triosephosphate isomerase. For each, briefly explain why it belongs to that class.
Answer:
DNA Ligase: Ligase. It joins two molecules (DNA fragments) using ATP energy.
Alcohol Dehydrogenase: Oxidoreductase. It catalyzes oxidation-reduction reactions (e.g., oxidation of alcohol).
Hexokinase: Transferase. It transfers a phosphate group from ATP to hexose (e.g., glucose).
Pepsin: Hydrolase. It breaks down proteins (peptides) with the addition of water (hydrolysis).
Fumarase: Lyase. It catalyzes the addition or removal of groups to form double bonds (e.g., adds water to fumarate to form malate).
Triosephosphate Isomerase: Isomerase. It catalyzes isomerization reactions (rearranges the molecular structure of triose phosphate).
Differentiate between a cofactor, an essential ion, and a coenzyme, providing an example for each.
Answer:
Cofactor: A non-protein chemical compound required for the biological activity of a protein (e.g., metal ions like Mg²⁺).
Essential ions: Specific metal ions necessary for enzyme function, often included within the broader term cofactor (e.g., Mg²⁺, Zn²⁺).
Coenzymes: Organic non-protein molecules that assist enzymes, often derived from vitamins (e.g., NAD⁺).
Explain the key differences and similarities between metal-activated enzymes and metalloenzymes, providing an example for both.
Answer:
Metalloenzymes: Contain metal ions as integral parts of their structure; the metal is tightly bound and is directly involved in catalysis (e.g., carbonic anhydrase, which contains tightly bound Zn²⁺).
Metal-activated enzymes: Require metal ions for activity but can be activated by various concentrations of less tightly bound metal ions (e.g., an enzyme that shows activity with free Mg²⁺ ions).
Similarity: Both require metal ions for their catalytic activity.
Define apoenzyme and holoenzyme. How do they relate to each other in terms of enzyme activity?
Answer:
Apoenzyme: The protein portion of an enzyme, which is inactive on its own.
Holoenzyme: The complete, active form of the enzyme, consisting of the apoenzyme combined with its cofactor.
The apoenzyme becomes active only when it associates with its specific cofactor to form the holoenzyme.
Identify the main distinctions between cosubstrates and prosthetic groups, giving an example of each.
Answer:
Cosubstrates: Loosely bound, non-protein molecules that participate in the reaction and are chemically modified during the reaction. They dissociate from the enzyme after the reaction (e.g., NAD⁺, which is reduced to NADH).
Prosthetic groups: Tightly bound non-polypeptide units essential for the biological activity of an enzyme. They remain associated with the enzyme throughout the reaction cycle (e.g., heme in hemoglobin).
What is a vitamin, and how do many of these organic compounds contribute to proper enzyme function?
Answer: A vitamin is an organic compound required in small quantities for vital metabolism. Many vitamins serve as precursors for coenzymes (e.g., B vitamins are precursors for NAD⁺, FAD), which are essential for facilitating enzyme activity and enabling various biochemical reactions.
List and briefly describe three major mechanisms by which amino acid reactive groups contribute to enzyme catalysis.
Answer: Reactive groups from amino acids primarily contribute through:
Acid-base catalysis: Amino acid side chains act as proton donors or acceptors, facilitating proton transfer.
Covalent catalysis: A transient covalent bond forms between the enzyme and the substrate, altering the reaction pathway.
Metal ion catalysis: Metal ions assist in substrate binding, activate water molecules, or stabilize charges in the transition state.
Explain the difference between specific acid-base catalysis and general acid-base catalysis, providing an example for each mechanism.
Answer:
Specific acid-base catalysis: Involves only the solvent molecules (usually water or H⁺/OH⁻ from water) acting as the proton donor or acceptor. For example, the transfer of H⁺ in enzyme-catalyzed reactions where water is the sole source of protons.
General acid-base catalysis: Involves an acid or base other than water (i.e., specific functional groups on the enzyme, like amino acid side chains) acting in the proton transfer. For example, a histidine residue acting as a base to abstract a proton in the mechanism of serine proteases.
Describe the principle of covalent catalysis in enzyme mechanisms.
Answer: Covalent catalysis is a mechanism where a reactive group on the enzyme forms a transient covalent bond with the substrate, creating a modified enzyme-substrate intermediate. This alters the reaction pathway, making it energetically more favorable and thus accelerating the reaction.
What characterizes a diffusion-controlled reaction in the context of enzyme kinetics?
Answer: A diffusion-controlled reaction is one where the rate of the reaction is limited by the speed at which the substrate molecules can diffuse through the solvent to encounter and bind with the enzyme's active site. For such reactions, the maximum reaction rates typically approach to M⁻¹s⁻¹.
Name two enzymes known for approaching the theoretical upper limit for second-order reaction rates.
Answer: Acetylcholinesterase and carbonic anhydrase are two enzymes known for reaching rates near the theoretical upper limit for second-order reactions due to their highly efficient binding and catalysis of substrates.
Discuss how mechanistic and binding effects individually and collectively contribute to the acceleration rates observed in enzyme catalysis.
Answer:
Mechanistic effects: Relate to the actual chemical steps taken by the enzyme (e.g., covalent catalysis, acid-base catalysis) that provide an alternative, lower-energy reaction pathway.
Binding effects: Relate to the enzyme's ability to bind substrates with high affinity and in optimal orientation for catalysis, including effects like proximity, orientation, and transition state stabilization.
Collectively: Both effects contribute significantly. Binding positions substrates correctly and stabilizes the transition state, while mechanistic effects directly facilitate bond breaking and forming, leading to substantial overall rate acceleration.
Define the proximity effect and explain how it contributes to increased reaction rates in enzyme catalysis.
Answer: The proximity effect refers to the increase in reaction rates due to the enzyme bringing substrate molecules into close proximity and proper orientation within the active site. By concentrating and correctly aligning the reactants, the enzyme significantly increases the effective concentration and the likelihood of collision and reaction, accelerating the rate.
What is effective molarity, and what does it signify in the context of enzyme-catalyzed reactions?
Answer: Effective molarity is a concept used to describe the enhanced effective concentration of reactive molecules within the enzyme's active site due to favorable positioning. It signifies how much higher the concentration of reactant would have to be in a free solution to achieve the same reaction rate as observed in the enzyme's active site, highlighting the catalytic power of enzymes.
How does an enzyme achieve transition state stabilization, and why is this crucial for enhancing reaction rates?
Answer: An enzyme achieves transition state stabilization by binding to the transition state of the substrate more tightly than to the substrate itself. The active site is optimally shaped and chemically suited to interact with and stabilize the high-energy transition state complex. This stabilization lowers the activation energy of the reaction, which is crucial for increasing reaction rates because it makes the energetically unfavorable transition state more accessible.
Define a transition state analog and describe its typical application in enzymology.
Answer: A transition state analog is a molecule that structurally resembles the transition state intermediate of a substrate but is not a substrate itself. It typically binds much more tightly to the enzyme than the substrate, acting as a potent inhibitor. They are used in enzymology to study enzyme mechanisms, understand transition state stabilization, and as lead compounds for drug discovery.
What does it mean for an enzyme system to be saturable? How does the concept of demonstrate this characteristic?
Answer: An enzyme system is saturable means that there is a finite number of active sites on the enzyme molecules available for substrate binding. Beyond a certain substrate concentration, all active sites become occupied, and adding more substrate will not increase the reaction rate. (maximum velocity) illustrates this by representing the highest reaction rate achievable when all enzyme active sites are saturated with substrate, indicating the system's capacity limit.
If an enzyme increases the rate of a reaction, what can be said about its effect on the (Gibbs free energy change) of the reaction? Explain.
Answer: Enzymes only affect the kinetics (rate) of a reaction, not its thermodynamics. Therefore, an enzyme has no effect on the overall (Gibbs free energy change) of a reaction. The depends only on the free energy of the initial reactants and final products, not on the pathway or the activation energy, which is what enzymes alter.
Explain the relationship between enzymatic catalysis, activation energy, and transition states.
Answer: Enzymatic catalysis accelerates reactions by lowering the activation energy. This is primarily achieved by enzymes providing a unique reaction pathway where they bind to and stabilize the transition state more tightly than the substrate itself. By making the high-energy transition state more stable and thus more accessible, the energy barrier for the reaction is reduced, leading to a faster reaction rate.
Why are enzymes considered catalysts rather than reactants in a biochemical reaction?
Answer: Enzymes are considered catalysts because they increase the rate of a biochemical reaction without being consumed or permanently altered in the process. They participate in the reaction by binding substrates, forming intermediates, and releasing products, but at the end of the reaction, the enzyme is regenerated in its original form and can catalyze further reactions.
Without enzymes, many vital biochemical reactions would proceed too slowly for life to exist. Elaborate on why this is the case.
Answer: Without enzymes, many vital biochemical reactions would have very high activation energies, meaning they would proceed at extremely slow rates, often negligible on a biological timescale. Life processes, such as metabolism, DNA replication, and protein synthesis, require rapid and efficient chemical transformations that would simply not occur fast enough to sustain cellular functions and organism survival in the absence of enzymatic catalysis.
What is the primary chemical nature of most enzymes, and what is a notable exception?
Answer: The primary chemical nature of most enzymes is protein. A notable exception is ribozymes, which are RNA molecules with catalytic activity.
Quantify the typical rate enhancement provided by enzymes.
Answer: Enzymes can provide enormous rate enhancements, typically accelerating reaction rates by factors ranging from to times compared to uncatalyzed reactions. The notes state that rate enhancements can reach .
Compare the substrate specificity of anabolic and catabolic enzymes.
Answer: Anabolic enzymes, involved in biosynthesis, tend to have high substrate specificity, precisely controlling the formation of specific biomolecules. Catabolic enzymes, involved in breaking down molecules, generally have broader substrate specificity, capable of acting on a wider range of related substrates (e.g., breaking down various endogenous or xenobiotic compounds).
Where can enzymes be found within an organism?
Answer: Enzymes can be found in various locations within an organism, including intracellularly (e.g., in the nucleus, cytosol, membranes of organelles like mitochondria and ER) and extracellularly (e.g., digestive enzymes in the gut, circulatory enzymes in the blood).
What are zymogens, and why are they important in regulating enzyme activity?
Answer: Zymogens (or proenzymes) are inactive precursors of enzymes that require specific cleavage or modification to become active. They are important in regulating enzyme activity by controlling when and where an enzyme becomes active, preventing premature or uncontrolled reactions. A classic example is the coagulation cascade, where zymogens are activated sequentially to form a clot only when needed.
List and briefly describe two classes of natural enzyme inhibitors mentioned in the notes.
Answer: Two classes of natural enzyme inhibitors are:
Macroglobulins (e.g., -macroglobulin): Large plasma proteins that irreversibly inhibit certain proteases by trapping them.
Serpins (e.g., -antitrypsin): A family of proteins that inhibit serine proteases, often involved in regulating proteolytic cascades.
How does selective substrate recognition by enzymes relate to their catalytic activity?
Answer: Selective substrate recognition is fundamental to an enzyme's catalytic activity. The unique shape and chemical environment of the active site allow an enzyme to bind only specific substrates, ensuring that the correct reactions occur. This specificity ensures that the enzyme acts on its intended target, preventing unwanted side reactions and improving catalytic efficiency.
Briefly describe the general catalytic strategies employed by serine proteases.
Answer: Serine proteases employ a catalytic triad (typically Asp, His, Ser) for their action. Histidine acts as a general base to activate serine for nucleophilic attack on the peptide bond. The resulting tetrahedral intermediate is stabilized by the oxyanion hole, and covalent catalysis is involved as an acyl-enzyme intermediate is formed, which then undergoes deacylation with water.
Define and explain its significance in enzyme kinetics.
Answer: (turnover number) is the maximum number of substrate molecules transformed into product per enzyme molecule per unit of time when the enzyme is saturated with substrate. It reflects the inherent catalytic efficiency of the enzyme at saturating substrate concentrations, or how fast the enzyme can work once it has bound its substrate. It is calculated as .
What is the significance of in enzyme kinetics?
Answer: (Michaelis constant) is the substrate concentration at which the reaction rate is half of . It serves as an inverse measure of the enzyme's affinity for its substrate; a lower generally indicates a higher affinity (tighter binding) of the enzyme for its substrate, meaning less substrate is needed to reach half .
How does the specificity constant reflect an enzyme's efficiency? What are the optimal ranges for diffusion-controlled reactions?
Answer: The specificity constant is a measure of an enzyme's overall catalytic efficiency, reflecting both the efficiency of catalysis () and the efficiency of substrate binding and release (). A higher value indicates a more efficient enzyme. For diffusion-controlled reactions, where the enzyme's efficiency is limited only by how fast it can encounter its substrate, optimal ranges for the specificity constant are between to M⁻¹s⁻¹.
Provide an example of how enzyme inhibitors are utilized in medicinal applications.
Answer: Enzyme inhibitors are widely used in medicinal applications. For example, inhibitors can target human enzymes for treating diseases like cancer (e.g., kinase inhibitors) or non-human enzymes for antimicrobial therapy (e.g., penicillin inhibits bacterial transpeptidase, an enzyme essential for cell wall synthesis).
How are enzymes themselves administered for therapeutic or nutritional purposes?
Answer: Enzymes can be administered directly for therapeutic or nutritional purposes. Examples include streptokinase used as a therapeutic to dissolve blood clots, or lactase administered as a digestive aid to individuals with lactose intolerance, helping them break down lactose.
Explain the role of enzymes in diagnostic applications.
Answer: Enzymes play a crucial role in diagnostic applications by serving as biomarkers whose activity levels can indicate specific health conditions. For example, elevated levels of certain liver enzymes (e.g., ALT, AST) in the blood can indicate liver damage, while the presence of specific enzymes can also be used in assays to detect other health markers.
Differentiate between enzyme isoforms and isozymes.
Answer:
Isoforms: Generally refers to related proteins that arise from a single gene through alternative splicing or post-translational modifications, leading to slightly different forms of the protein with potentially different functions or locations.
Isozymes: Are variants of an enzyme that catalyze the same reaction but have different amino acid sequences (often encoded by different genes), resulting in different kinetic properties, regulatory mechanisms, or tissue-specific expressions. For example, hexokinase has several isozymes with different values.