92 Chapter 4
The activity of an enzyme, as measured by the rate at which
its substrates are converted to products, is influenced by such
factors as (1) the temperature and pH of the solution; (2) the
concentration of cofactors and coenzymes, which are needed
by many enzymes as “helpers” for their catalytic activity;
(3) the concentration of enzyme and substrate molecules in the
solution; and (4) the stimulatory and inhibitory effects of some
products of enzyme action on the activity of the enzymes that
helped to form these products.Effects of Temperature and pH
An increase in temperature will increase the rate of non-
enzyme-catalyzed reactions. A similar relationship between
temperature and reaction rate occurs in enzyme-catalyzed reac-
tions. At a temperature of 0 8 C the reaction rate is immeasur-
ably slow. As the temperature is raised above 0 8 C the reaction
rate increases, but only up to a point. At a few degrees above
body temperature (which is 37 8 C) the reaction rate reaches a
plateau; further increases in temperature actually decrease the
rate of the reaction ( fig. 4.3 ). This decrease is due to the altered
tertiary structure of enzymes at higher temperatures.
A similar relationship is observed when the rate of an
enzymatic reaction is measured at different pH values. Each
enzyme characteristically exhibits peak activity in a very nar-
row pH range, which is the pH optimum for the enzyme. If
the pH is changed so that it is no longer within the enzyme’s
optimum range, the reaction rate will decrease ( fig. 4.4 ). This
decreased enzyme activity is due to changes in the conforma-
tion of the enzyme and in the charges of the R groups of the
amino acids lining the active sites.
The pH optimum of an enzyme usually reflects the pH of
the body fluid in which the enzyme is found. The acidic pH
optimum of the protein-digesting enzyme pepsin, for example,Figure 4.3 The effect of temperature on enzyme
activity. This effect is measured by the rate of the enzyme-
catalyzed reaction under standardized conditions as the
temperature of the reaction is varied.10Enzyme activity20
Temperature (°C)30 37 40 100CLINICAL APPLICATION
Different diseased organs can liberate different isoenzy-
matic forms of an enzyme into the blood, aiding the diag-
nosis of disease. Creatine phosphokinase ( CPK or CK ),
for example, is normally located in the cytosol and catalyzes
the production of ATP (section 4.3) from phosphocreatine.
The isoenzymatic forms are coded by different genes on dif-
ferent chromosomes, and clinical tests are available using
antibodies to distinguish between these isoenzymes. The
CK-BB form (or CK-1) is released into the blood mainly from
damaged brain and lungs; the CK-MB form (or CK-2) is
released mainly by damaged cardiac muscle in a myocar-
dial infarction ( heart attack); and the CK-MM form (or CK-3)
is released mainly by damaged skeletal muscle.Clinical Investigation CLUES
Sheryl had an elevated level of the CK-MB isoenzyme of
creatine phosphokinase.- What are the different isoenzymes of CK, and what
is their medical significance? - What is the most likely explanation of Sheryl’s
symptoms and laboratory findings?
| CHECKPOINT
- Use the lock-and-key model to explain how enzymes
function as catalysts. - Explain how enzymes are named, and the nature of
isoenzymes.
4.2 Control of Enzyme Activity
The rate of an enzyme-catalyzed reaction depends on the
concentration of the enzyme and the pH and temperature
of the solution. Genetic control of enzyme concentration, for
example, affects the rate of progress along particular meta-
bolic pathways and thus regulates cellular metabolism.
LEARNING OUTCOMES
After studying this section, you should be able to:- Describe the effects of pH and temperature on
enzyme-catalyzed reactions, and the nature of
cofactors and coenzymes. - Explain the law of mass action in reversible
reactions. - Describe a metabolic pathway and how it is affected by
end-product inhibition and inborn errors of metabolism.