SECTION 6.4
Inhibition
93
FIGURE 6-6
Effects of three types of reversible inhibitors on the velocity-substrate plots
and on the Lineweaver-Burk double-reciprocal plots.
Ki,
but Vmax is unchanged. Thus, the profiles of plots of
v against [S] show the same Vmax in the presence and the
absence of inhibitor but with an apparent change in
Km,
to a larger value (ATmapp), because S and I compete for
binding at the same site and a higher concentration of S is
required to achieve half-maximal velocity. Therefore, the
value of
Km
is increased in the presence of inhibitor and
depends on the concentration of I. The Lineweaver-Burk
plot for competitive inhibition shows that the lines for [S]
and ([S] + [I]) intersect at the same point on the ordinate
(because Vmax is unaltered) but have different slopes and
intercepts on the abscissa (because of differences in Am)-
Figure
6 - 6
illustrates these plots, and Table 6-2 shows the
values for apparent
Km.
Competitive inhibition occurs in four different circum-
stances.
(1) Structural analogues compete with the substrate for
binding at the active site of the enzyme. Several
specific examples are described below.
Inhibition of Succinate Dehydrogenase
A classic
example
of competitive
inhibition
occurs
with
the
succinate dehydrogenase reaction:
COCT
I
CH2
I
+ FAD
CH2
I
COCT
Succinate
COO-
Succinate
dehydrogenase
Q
_
II
+ fadh2
COQ-
Fumarate
In this reaction, flavin adenine dinucleotide (FAD), a coen-
zyme, serves as a hydrogen acceptor. This enzyme is com-
petitively inhibited by malonate, oxalate, or oxaloacetate,
all structural analogues of succinate.
co c r
COCT
1
COCT
1
c = o
CH2
1
1
COCT
1
CH2
COCT
1
co c r
Malonate
Oxalate
Oxaloacetate
Inhibition of Folate Synthesis
Competitive inhibi-
tion of a biosynthetic step in folate synthesis accounts for
the antimicrobial action of sulfonamides, which are struc-
tural analogues of p-aminobenzoic acid (PABA):
p-Aminobenzoic acid
Sulfanilamide
PABA is used by bacteria in the synthesis of folic acid
(pteroylglutamic acid), which functions as a coenzyme
in one-carbon transfer reactions that are important in
amino acid metabolism, in the synthesis of RNA and
DNA, and thus in cell growth and division. Sulfonamides
inhibit the bacterial enzyme responsible for incorpora-
tion of PABA into 7,8-dihydropteroic acid (Figure 6-7)
and lead to the inhibition of growth (bacteriostasis) of a
wide range of gram-positive and gram-negative microor-
ganisms. Microorganisms susceptible to sulfonamides are
those which synthesize their own folic acid or which can-
not absorb folic acid derived from the host. Sulfonamides,
however, have no effect on host cells (or other mammalian
cells) that require preformed folic acid.
Inhibition
of
Dihydrofolate
Reductase
Folate-
dependent reactions in the body are inhibited by folate ana-
logues (or antagonists, e.g., methotrexate). Before it can
function as a coenzyme in one-carbon transfer reac-
tions,
folate
(F) must be reduced by dihydrofolate
reductase
to
tetrahydrofolate
(F H 4 ).
Dihydrofolate
