Human Physiology, 14th edition (2016)

90 Chapter 4

Figure 4.1 A comparison of noncatalyzed and catalyzed reactions. The upper figures compare the proportion of reactant
molecules that have sufficient activation energy to participate in the reaction (blue 5 insufficient energy; green 5 sufficient energy).
This proportion is increased in the enzyme-catalyzed reaction because enzymes lower the activation energy required for the reaction
(shown as a barrier on top of an energy “hill” in the lower figures). Reactants that can overcome this barrier are able to participate in the
reaction, as shown by arrows pointing to the bottom of the energy hill.

Number of

reactant molecules

Energy

Activation

energy

Activation energy

Energy

released

by reaction

Energy of reactants

Noncatalyzed reaction

Number of

reactant molecules

Activation

energy is

lowered

Energy of reactants

Energy

Activation energy

Energy

released

by reaction

Catalyzed reaction

Reactants Reactants

Products Products

Figure 4.2 The lock-and-key model of enzyme action. ( a ) Substrates A and B fit into active sites in the enzyme, forming
( b ) an enzyme-substrate complex. This complex then ( c ) dissociates, releasing the products of the reaction and the free enzyme.

Active sites

Substrate B

Substrate A

(a) Enzyme and substrates

Enzyme

(b) Enzyme-substrate complex (c) Reaction products and enzyme (unchanged)

Product C

Product D

A + B

(Reactants) Enzyme

C + D

(Products)