Biochem Chapter 5 - Properties of Enzymes

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Questions and Answers

What is a catalyst?

A substance that speeds up the attainment of equilibrium.

Does a catalyst change the position of the reaction's equilibrium?

False

What does a catalyst do?

Lowers the amount of energy needed for the reaction to proceed.

Define substrates.

The specific reactants that enzymes act on. Signup and view all the answers

What does it mean for an enzyme to exhibit stereospecificity?

The enzyme acts only on a single stereoisomer of the substrate. Signup and view all the answers

What is the most important part of enzyme specificity?

Reaction specificity. Signup and view all the answers

What is reaction specificity?

The lack of formation of wasteful by-products. Signup and view all the answers

What is an important property that distinguishes biological catalysts from those encountered in a chemistry lab?

Enzyme activity can be regulated. Signup and view all the answers

What was the first enzyme that was crystallized and proved to be a protein?

Urease. Signup and view all the answers

All enzymes have been shown to be or ________.

Proteins, proteins plus cofactors. Signup and view all the answers

What are the four main properties of enzymes?

<ol> <li>They function as catalysts. 2. They catalyze highly specific reactions. 3. They can couple reactions. 4. Their activity can be regulated.</li> </ol> Signup and view all the answers

What are the six classes of enzymes?

<ol> <li>Oxidoreductases 2. Transferases 3. Hydrolases 4. Lysases 5. Isomerases 6. Ligases.</li> </ol> Signup and view all the answers

What do oxidoreductases catalyze?

They catalyze oxidation-reduction reactions. Signup and view all the answers

What do transferases catalyze?

They catalyze group transfer reactions. Signup and view all the answers

What do hydrolases catalyze?

They catalyze hydrolysis where water is the acceptor of the transferred group. Signup and view all the answers

What do lyases catalyze?

They catalyze the lysis of a substrate generating a double bond in non-hydrolytic, non-oxidative, elimination reactions. Signup and view all the answers

What do isomerases catalyze?

Isomerases catalyze structural change within a single molecule. Signup and view all the answers

What is the simplest enzymatic reaction?

Isomerases. Signup and view all the answers

What do ligases catalyze?

They catalyze ligation or joining of two substrates. Signup and view all the answers

What are ligases usually referred to as?

Synthetases. Signup and view all the answers

Kinetic experiments examine the relationship between?

The amount of product formed in a unit of time and the experimental conditions under which the reaction takes place. Signup and view all the answers

What does the rate equation reflect?

The velocity of a reaction depends on the concentration of a substrate. Signup and view all the answers

In a second order reaction, the rate is determined by the concentration of?

Both substrates. Signup and view all the answers

What is the enzyme-substrate complex?

Complex formed when specific substrates fit into the enzyme active site. Signup and view all the answers

What does the overall rate of an enzyme reaction depend on?

Both the substrate and the catalyst. Signup and view all the answers

What do enzyme assays measure?

The amount of product formed in a given time period. Signup and view all the answers

Why is the formation and dissociation of ES complexes very rapid?

Because only the nonequivalent bonds are being formed and broken. Signup and view all the answers

When is the substrate chemically altered?

During the rate limiting step. Signup and view all the answers

How does enzyme kinetics differ from simple chemical kinetics?

Because the rates of enzyme-catalyzed reactions depend on the concentration of the enzyme. Signup and view all the answers

What relationship does the Michaelis-Menten equation describe?

The relationship between the initial velocity of a reaction and the substrate concentration. Signup and view all the answers

What is the shape of a Vo versus [S] curve?

It is a rectangular hyperbola. Signup and view all the answers

Name three statements about the Km (Michaelis Constant).

<ol> <li>Defines the set point for the concentration of a substrate for a biochemical reaction in a cell. 2. Is a measure of the affinity of the substrate for the enzyme, only if kcat is much smaller than either k1 or k-1. 3. Is the substrate concentration at ½ maximal velocity.</li> </ol> Signup and view all the answers

What are kinetic mechanisms?

Multisubstrate reactions occurring by several different kinetic schemes. Signup and view all the answers

What do sequential reactions require?

All substrates to be present before any product is released. Signup and view all the answers

In ping-pong reactions, a product is released _ all the ___ are bound.

before, substrates. Signup and view all the answers

What is an inhibitor?

A small molecule that binds reversibly to the enzyme and interferes with its activity. Signup and view all the answers

How do reversible inhibitors bind to enzymes?

By the same weak, nonequivalent forces that bind substrates and products. Signup and view all the answers

What are the basic types of reversible inhibition, and how are they distinguished?

Competitive, uncompetitive, noncompetitive, and mixed. Signup and view all the answers

Describe the effect of competitive inhibition on Km and Vmax.

Competitive Inhibitors - I binds to E only. Raises Km, Vmax remains unchanged. Signup and view all the answers

Describe the effect of uncompetitive inhibition on Km and Vmax.

Uncompetitive - I binds to ES only. Lowers Vmax and Km, Ratio of Vmax/Km remains unchanged. Signup and view all the answers

Describe the effect of noncompetitive inhibition on Km and Vmax.

Noncompetitive - I binds to E or ES. Km remains unchanged, lowers Vmax. Signup and view all the answers

What is an irreversible enzyme inhibitor?

Forms a stable covalent bond with an enzyme molecule thus removing active molecules from the enzyme population. Signup and view all the answers

How does irreversible inhibition typically occur?

By alkylation and acylation of the side chain of an active-site amino acid residue. Signup and view all the answers

What is an important use of irreversible inhibitors?

To identify the amino acid residues at the active site by specific substitution of their reactive side chains. Signup and view all the answers

Which organic phosphorus compound irreversibly inhibits serine protease chymotrypsin?

DFP, diisoprophyl fluorophosphate. Signup and view all the answers

What is the major biological action of the original organophosphorus nerve gases?

Irreversible inhibition of the serine esterase acetylcholinesterase that catalyzes hydrolysis of the neurotransmitter acetylcholine. Signup and view all the answers

How can enzymes have rapid control of enzyme regulation rates?

Through reversible modulation of the activity of regulated enzymes. Signup and view all the answers

What do regulated enzymes respond to in order to adjust the rates of its metabolic processes?

It responds to environmental signals. Signup and view all the answers

When do regulated enzymes become more active catalysts?

When the concentration of their substrates increase or when the concentrations of the products of their metabolic pathways decrease. Signup and view all the answers

When do regulated enzymes become less active?

When the concentrations of their substrates decrease or when the products of their metabolic pathways accumulate. Signup and view all the answers

Inhibition of what conserves both material and energy?

Inhibition of the first enzyme unique to a pathway. Signup and view all the answers

What are allosteric enzymes?

Enzymes whose properties are affected by changes in structure. Signup and view all the answers

Why do allosteric enzymes not exhibit typical Michaelis-Menten kinetics?

Due to cooperative binding of substrate, as in the case with hemoglobin. Signup and view all the answers

What are affinity labels?

Active-site directed reagents. Signup and view all the answers

What are two methods of regulation?

<ol> <li>Non-covalent allosteric regulation 2. Covalent modification.</li> </ol> Signup and view all the answers

What are the general properties of allosteric enzymes?

<ol> <li>Activities of allosteric regulator enzymes are changed by inhibitors and activators. 2. Allosteric modulators bind non-covalently to the enzymes they regulate. 3. Regulatory enzymes possess quaternary structure. 4. Regulatory enzymes have at least one substrate which has a sigmoidal Vo versus [S] curve. 5. There is a rapid transition between the active (R) and inactive (T) conformations.</li> </ol> Signup and view all the answers

What are the two theories of allosteric regulation?

<ol> <li>Concerted Theory. 2. Sequential Model.</li> </ol> Signup and view all the answers

How can the activity of an enzyme be modified?

Through the covalent attachment and removal of groups on the polypeptide chain. Signup and view all the answers

Regulation by ________________________________ is usually slower than allosteric regulation.

covalent modification. Signup and view all the answers

Covalent modification of regulated enzymes must be _____________

reversible, otherwise it wouldn't be a form of regulation. Signup and view all the answers

What is the most common type of covalent modification?

Phosphorylation of one or more specific serine residues. Signup and view all the answers

What are the metabolic advantages of multi-enzyme complexes and multifunctional enzymes?

<ol> <li>Metabolite channeling. 2. Protects chemically labile intermediates. 3. Enzymes can effectively couple separate reactions.</li> </ol> Signup and view all the answers

What is the best-characterized example of channeling?

The enzyme tryptophan synthase. Signup and view all the answers

Study Notes

Properties of Enzymes

Catalysts speed up the attainment of equilibrium without changing it.
Catalysts lower the activation energy needed for reactions.
Substrates are specific reactants that enzymes act upon.
Stereospecificity refers to enzymes acting only on a specific stereoisomer of the substrate.
Reaction specificity is vital as it prevents the formation of unwanted by-products.

Enzyme Classification and Properties

Enzymes can be proteins or protein-cofactor complexes.
Key enzyme properties include:
- Acting as catalysts
- Catalyzing highly specific reactions
- Coupling reactions for energy transfer
- Being subject to regulatory mechanisms
Six classes of enzymes:
- Oxidoreductases (catalyze oxidation-reduction)
- Transferases (catalyze group transfers with coenzymes)
- Hydrolases (catalyze hydrolysis reactions)
- Lyases (catalyze non-hydrolytic, non-oxidative eliminations)
- Isomerases (catalyze isomerization reactions)
- Ligases (catalyze joining of substrates, often utilizing ATP)

Enzyme Kinetics

Kinetic experiments assess the relationship between product formation and reaction conditions.
The rate equation reflects the dependence of reaction velocity on substrate concentration.
An enzyme-substrate complex forms when substrates fit into the enzyme's active site.

Michaelis-Menten Kinetics

The Michaelis-Menten equation describes the initial velocity of a reaction in relation to substrate concentration, forming a rectangular hyperbola.
The Michaelis constant (Km) reflects substrate affinity and is the substrate concentration at half of the maximum velocity (Vmax).

Enzyme Regulation

Regulation of enzyme activity can occur through reversible modulation via environmental signals.
Allosteric enzymes have regulatory sites distinct from active sites and can exhibit sigmoidal reaction curves due to cooperative binding.
Two primary regulation methods: non-covalent allosteric regulation and covalent modification, with phosphorylation being the most common type of covalent modification.

Inhibition Mechanisms

Inhibitors are small molecules that can reversibly bind to enzymes, affecting their activity.
Types of reversible inhibition include:
- Competitive (increases Km, Vmax unchanged)
- Uncompetitive (lowers Km and Vmax, ratio of Vmax/Km unchanged)
- Noncompetitive (Km unchanged, Vmax lowered)
Irreversible inhibitors form stable covalent bonds, often to the active site, removing active enzymes from the population, as seen with nerve gases inhibiting acetylcholinesterase.

Enzyme Complexes and Channeling

Multi-enzyme complexes enhance the efficiency of metabolic pathways through metabolite channeling.
Channeling protects intermediates from degradation and increases reaction rates, exemplified by the enzyme tryptophan synthase.

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This quiz focuses on the key concepts related to the properties of enzymes as covered in Chapter 5 of Biochemistry. It includes definitions and explanations of terms associated with catalysts and their role in chemical reactions. Test your knowledge with flashcards designed to enhance your understanding of this essential topic.