chapter 7 chem 341 suroviec fall 2013. i. introduction the structure and mechanism can reveal quite...

Chapter 7

Chem 341

Suroviec Fall 2013

I. Introduction

• The structure and mechanism can reveal quite a bit about an enzyme’s function

A. Reaction Kinetics

S P

Progress of this reaction can be expressed as a velocity

A. Reaction Kinetics

• When enzyme concentration is help constant the reaction velocity will vary with [A]

II. Michaelis – Menton Eqn.

A. Rate equations describe chemical processes

• Unimolecular reactions

• Bimolecular reactions

B. Michaelis Menton Equation is rate equation

• Simplest cases enzyme binds to substrate before converting to product

• The reaction of E + P converting back to ES is a step that we assume does NOT happen.

B. Michaelis Menton Equation

• We can measure by choosing the experimental conditions

• We then further choose experimental conditions to simplify the calculations: STEADY STATE

C. Michaelis Constant (Km) and Initial Velocity () and Maximal Velocity (Vmax)

• Occurs a high substrate concentration when enzyme is saturated

• At substrate concentration at which [S] = KM

• So if enzyme has small Km achieves max catalytic efficiency at low [S]

• KM is unique for each enzyme- substrate pair

C. Michaelis Constant (Km) and Initial Velocity () and Maximal Velocity (Vmax)

• Michaelis Constant is a combination of 3 rate constants that is experimentally determined.

C. Michaelis Constant (Km) and Initial Velocity () and Maximal Velocity (Vmax)

D. kcat catalytic constant = turnover #

• How fast an enzyme operates after it has selected and bound its substrate.

• Number of catalytic cycles that each active site undergoes per unit time

E. kcat/KM = measure of efficiency

• Enzyme effectiveness depends on ability to bind substrates and rapidly convert

F. Analysis: Find Vmax , KM

• High values of [S] lead to vo asymptotically reaching Vmax

• Use linear plot – Lineweaver-Burk

G. Exceptions to M-M Model

1. Multi-substrate reaction

G. Exceptions to M-M Model

2. Multi-step reactions

3. Non-hyperbolic reaction

II. Inhibition

• Substance that reduces an enzyme’s activity by influencing– Binding of substrate– Turnover number

• Variety of mechanisms– Irreversible enzyme inhibitors

• Inactivators– Reversible

• Diminish enzyme’s activity by interacting reversibly• Structurally resemble substrates• Affect catalytic activity without interfering with substrate binding

A. Competitive Inhibition•Compete directly with normal substrate for binding site•Resemble substrate•Specifically binds to active site

• Product inhibition

A. Competitive Inhibition

• Transition state analogs

• Depends on inhibitor binding selectively with RAPID equilibrium

A. Competitive Inhibition

• M-M equation for competitive inhibition reaction

1. KI can be measured

B. Transition State Inhibitors

• Inhibitor binds to ES complex• Not to free enzyme

C. Mixed Inhibition

• Interact in a way with the enzyme that affects substrate binding

III. Allosteric Regulation

• Organism must be able to regulate catalytic activities – Metabolic processes– Respond to changes in environment

1. Control of enzyme availability• Amount of given enzyme in a cell depends on its rate of synthesis and its

rate of degradation2. Control of enzyme activity

• Catalytic activity controlled through structural alteration• Can cause large changes in enzymatic activity