PROTEINS 45
protein ferritin stores iron in the human body. Immunoglobins are proteins
that react with and neutralize foreign compounds (antigens) in the body. The
protein hormone insulin regulates glucose levels in the blood.
Many enzymes are named in classes by attaching “ ase ” to the type of reaction
they catalyze. ATPase enzymes catalyze the hydrolysis of adenosine triphos-
phate (ATP) to adenosine diphosphate (ADP). Oxidoreductases catalyze oxi-
dation – reduction reactions: Alcohol dehydrogenase catalyzes the oxidation of
an alcohol to an aldehyde. Transferases catalyze the transfer of groups from one
molecule to another. Hydrolases catalyze the breaking of covalent bonds using
water; for example, peptidase hydrolyzes a peptide bond. Lyases either remove
a group by splitting a bond and forming a double bond or add a group to a
double bond to form a single bond; for example, decarboxylase removes a car-
boxyl group to form carbon dioxide. Isomerases catalyze internal atom rear-
rangements in a molecule. Ligases catalyze the formation of covalent bonds.
The general theory of enzyme kinetics is based on work of L. Michaelis and
M. L. Menten, later extended by G. E. Briggs and J. B. S. Haldane. 1a The basic
reactions (E = enzyme, S = substrate, P = product) are shown in equation 2.1 :
ES ES
ES E P+⇔
⇔+
(2.1)
Assuming that the reactions are reversible and that a one - substrate enzyme -
catalyzed reaction is being studied, one can derive the Michaelis – Menten
rate:
V
V
Km=
+
max[]
[]S
S
(2.2)
where V is the initial rate for fi rst - order breakdown of the enzyme – substrate
([ES]) complex into enzyme (E) and product (P); Vmax is the maximum reac-
tion rate for a given concentration of enzyme in the presence of saturating
levels of substrate; [S] is the substrate concentration; and Km is the Michaelis
constant, the concentration of substrate required to achieve one - half the
enzyme ’ s maximal velocity. Equation 2.2 applies to single - substrate reactions
at a constant enzyme concentration. Only nonallosteric enzymes — those not
dependent on binding of a second molecule in addition to the substrate — are
treated by the Michaelis – Menten rate equation. When the substrate concen-
tration is low, the rate of reaction ( V ) increases in direct proportion to the
substrate concentration (fi rst - order kinetics). When the substrate concentra-
tion becomes high enough to saturate all available enzyme active sites, the
reaction rate becomes constant (zero - order kinetics). The graphical represen-
tation of this behavior in shown in Figure 2.12.
The Lineweaver – Burk plot uses the reciprocal of the Michaelis – Menten
equation in the form of the equation of a straight line, y = mx + b , having the
form shown in equation 2.3 :