3.3 Some Proteins Act as Enzymes to Speed up Biochemical Reactions

catalyst

to break down; A chemical substance that accelerates a reaction without itself being consumed in the overall course of the reaction. Catalysts lower the activation energy of a reaction. Enzymes are biological catalysts.

enzyme

to leaven (as in yeast bread)] A catalytic protein that speeds up a biochemical reaction.

free energy (G)

which is the amount of energy in a system that is available to do work.

HOW ENZYMES WORK

(1) Inducing strain: Once the substrate has bound to the active site, the enzyme causes bonds in the substrate to stretch, putting it in an unstable transition state: (2) Substrate orientation: When free in solution, substrates are moving from place to place randomly while at the same time vibrating, rotating, and tumbling. They only rarely have the proper orientation to react when they collide. The enzyme lowers the activation energy needed to start the reaction, by bringing together specific atoms so that bonds can form. (3) Adding chemical groups: The side chains (R groups) of an enzyme's amino acids may be directly involved in the reaction. For example, in acid-base catalysis, the acidic or basic side chains of the amino acids in the active site transfer H+ ions to or from the substrate, destabilizing a covalent bond in the substrate and permitting the bond to break.

substrate

(1) The molecule or molecules on which an enzyme exerts catalytic action. (2) The base material on which a sessile organism lives.

Induced fit at least partly explains why enzymes are so large. The rest of the macromolecule has at least three roles:

-It provides a framework so the amino acids of the active site are properly positioned in relation to the substrate(s). -It participates in the changes in protein shape and structure that result in induced fit. -It provides binding sites for regulatory molecules

transition state

In an enzyme-catalyzed reaction, the reactive condition of the substrate after there has been sufficient input of energy (activation energy) to initiate the reaction

An enzyme lowers the activation energy for a reaction by enabling the reactants to come together and react more easily; the reactants need lower amounts of kinetic energy to enter their transition state

In this way, an enzyme can change the rate of a reaction substantially.

Enzymes Lower the Energy Barrier

The activation energy (Ea) is lower in an enzyme-catalyzed reaction than in an uncatalyzed reaction, but the free energy released is the same with or without catalysis. A lower activation energy means the reaction will take place at a faster rate.

the activation energy (Ea).

The energy input required for sucrose to reach this state -Once the transition state is reached, the reaction can proceed spontaneously with a release of free energy (ΔG is negative)

Catalyzed Reactions Reach a Maximum Rate

The maximum rate of a catalyzed reaction can be used to measure how efficient the enzyme is—that is, how many molecules of substrate are converted into product by an individual enzyme molecule per unit of time, when there is an excess of substrate present.

RATE OF REACTION

The rate of an uncatalyzed reaction is directly proportional to the concentration of the substrate.

active site

The region on the surface of an enzyme or ribozyme where the substrate binds, and where catalysis occurs.

conformation

The specificity of an enzyme results from the exact three-dimensional shape -Only a narrow range of substrates, with specific shapes, functional groups, and chemical properties, can fit properly and bind to the active site.

cofactor

an inorganic ion that is weakly bound to an enzyme and required for its activities

(3)Prosthetic groups

are organic molecules that are permanently bound to their enzymes. An example is a flavin nucleotide, which binds to succinate dehydrogenase, an important enzyme in energy metabolism.

(2)A coenzyme

is a relatively small, carbon-containing (organic) molecule that is required for the action of one or more enzymes. It binds to the active site of the enzyme, adds or removes a chemical group from the substrate, and then separates from the enzyme to participate in other reactions. A coenzyme differs from a substrate in that it can participate in many different reactions with different enzymes.

(1)Metal ions

such as copper, zinc, and iron bind to certain enzymes and participate in the enzyme-catalyzed reactions. For example, the cofactor zinc binds to the enzyme alcohol dehydrogenase, which catalyzes the breakdown of toxic alcohol.