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.

<p>The specific reactants that enzymes act on.</p> Signup and view all the answers

What does it mean for an enzyme to exhibit stereospecificity?

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

What is the most important part of enzyme specificity?

<p>Reaction specificity.</p> Signup and view all the answers

What is reaction specificity?

<p>The lack of formation of wasteful by-products.</p> Signup and view all the answers

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

<p>Enzyme activity can be regulated.</p> Signup and view all the answers

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

<p>Urease.</p> Signup and view all the answers

All enzymes have been shown to be __________ or __________________.

<p>Proteins, proteins plus cofactors.</p> 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?

<p>They catalyze oxidation-reduction reactions.</p> Signup and view all the answers

What do transferases catalyze?

<p>They catalyze group transfer reactions.</p> Signup and view all the answers

What do hydrolases catalyze?

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

What do lyases catalyze?

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

What do isomerases catalyze?

<p>Isomerases catalyze structural change within a single molecule.</p> Signup and view all the answers

What is the simplest enzymatic reaction?

<p>Isomerases.</p> Signup and view all the answers

What do ligases catalyze?

<p>They catalyze ligation or joining of two substrates.</p> Signup and view all the answers

What are ligases usually referred to as?

<p>Synthetases.</p> Signup and view all the answers

Kinetic experiments examine the relationship between?

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

What does the rate equation reflect?

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

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

<p>Both substrates.</p> Signup and view all the answers

What is the enzyme-substrate complex?

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

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

<p>Both the substrate and the catalyst.</p> Signup and view all the answers

What do enzyme assays measure?

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

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

<p>Because only the nonequivalent bonds are being formed and broken.</p> Signup and view all the answers

When is the substrate chemically altered?

<p>During the rate limiting step.</p> Signup and view all the answers

How does enzyme kinetics differ from simple chemical kinetics?

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

What relationship does the Michaelis-Menten equation describe?

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

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

<p>It is a rectangular hyperbola.</p> 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?

<p>Multisubstrate reactions occurring by several different kinetic schemes.</p> Signup and view all the answers

What do sequential reactions require?

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

In ping-pong reactions, a product is released _______ all the _________ are bound.

<p>before, substrates.</p> Signup and view all the answers

What is an inhibitor?

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

How do reversible inhibitors bind to enzymes?

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

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

<p>Competitive, uncompetitive, noncompetitive, and mixed.</p> Signup and view all the answers

Describe the effect of competitive inhibition on Km and Vmax.

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

Describe the effect of uncompetitive inhibition on Km and Vmax.

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

Describe the effect of noncompetitive inhibition on Km and Vmax.

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

What is an irreversible enzyme inhibitor?

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

How does irreversible inhibition typically occur?

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

What is an important use of irreversible inhibitors?

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

Which organic phosphorus compound irreversibly inhibits serine protease chymotrypsin?

<p>DFP, diisoprophyl fluorophosphate.</p> Signup and view all the answers

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

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

How can enzymes have rapid control of enzyme regulation rates?

<p>Through reversible modulation of the activity of regulated enzymes.</p> Signup and view all the answers

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

<p>It responds to environmental signals.</p> Signup and view all the answers

When do regulated enzymes become more active catalysts?

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

When do regulated enzymes become less active?

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

Inhibition of what conserves both material and energy?

<p>Inhibition of the first enzyme unique to a pathway.</p> Signup and view all the answers

What are allosteric enzymes?

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

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

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

What are affinity labels?

<p>Active-site directed reagents.</p> 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?

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

Regulation by ________________________________ is usually slower than allosteric regulation.

<p>covalent modification.</p> Signup and view all the answers

Covalent modification of regulated enzymes must be _____________

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

What is the most common type of covalent modification?

<p>Phosphorylation of one or more specific serine residues.</p> 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?

<p>The enzyme tryptophan synthase.</p> 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.

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