Chapter 6: Enzymes: The Catalysts of Life

Enzyme Catalysis

Nearly all cellular reactions involve catalysts called enzymes

Activation Energy (EA)

The minimum amount of energy required to give rise to products

Transition State

An intermediate chemical state where the reactants have a higher energy than the initial reactants

Activation Energy Barrier

The rate of reaction is always proportional to the fraction of molecules with an energy greater than or equal to the activation energy

Catalyst

Enhances the rate of reaction by lowering the activation energy required.

Enzymes

organic catalysts

Active Site

A groove or pocket of amino acids that accomadates the intended substrate with high affinity

All catalysts share three basic properties

1.They increase reaction rates by lowering the
E_A required

2.They form transient, reversible complexes with substrate molecules

3.They change the rate at which equilibrium is achieved, not the position of the equilibrium

Amino Acids that are involved in active sites

Cysteine, histidine, aspartate, lysine, glutamate, and serine

Why do Cysteine, histidine, aspartate, lysine, glutamate, and serine, make up active sites?

They can participate in binding the substrate and several serve as donors or acceptors of protons

Cofactors

Needed for catalytic activity, often becaue they function as electron acceptors. Normally metal ions or coenzymes.

Denaturation

The point in which temperature is too high and the enzyme loses activity.

Sensitivity to pH

Most enzymes are active within a pH range of about 3–4 units

Substrate Binding

Substrate binding is readily reversible

What type of bonds form between enzymes and their substrates?

hydrogen bonds, ionic bonds, or both, Never Covalent

Substrate Activation

When the active site recognizes and binds the appropriate substrate and also provides the right environment for catalysis

Bond distortion(Substrate Activation)

makes the bond more susceptible to catalytic attack

Proton transfer(Substrate Activation)

Which increases reactivity of substrate by removing or adding a proton.

Electron transfer,(Substrate Activation)

Which increases reactivity of substrate by removing or adding a electron

Ribozymes

Catalytic RNA molecules.

Enzymes are influenced (mostly inhibited) by

Products

Alternative substrates

Substrate analogues

Drugs

Toxins

Allosteric effectors

Irreversible inhibitors

Bind the enzyme covalently, causing permanent loss of catalytic activity, and are generally toxic to cells

Reversible inhibitors

Bind enzymes noncovalently and can dissociate from the enzyme.

Competitive inhibitors

bind the active site of an enzyme and compete with substrate for the active site

Noncompetitive inhibitors

bind the enzyme molecule outside the active site

substrate-level regulation

A regulation that depends on the interactions of substrates and products with an enzyme

Increases in substrate levels result in increased reaction rates, whereas increased product levels lead to lower rates

feedback (or end-product) inhibition

the final product of an enzyme pathway negatively regulates an earlier step in the pathway

Allosteric enzymes

have two conformations, one in which it has affinity for the substrate(s) and one in which it does not

allosteric effector

regulates enzyme activity by binding and stabilizing one of the conformations

covalent modification

Enzyme

Activity is regulated by addition or removal of groups, such as phosphate, methyl, and acetyl groups

Protein kinases

catalyze the phosphorylation of other proteins

protein phosphatases

catalyze the removal of phosphate groups from proteins