BioPHYSICAL chemistry

aqueous solution. The maximum limit of 10^8 –10^9 M−^1 s−^1 has been achieved
by some enzymes.
Enzymes can often catalyze reactions with different substrates, and
molecules that resemble the substrates are sometimes able to occupy
the catalytic site and act as competitive inhibitors. Competitive inhibitors
are able to bind reversibly to the active site and to prevent catalysis of
the substrate. Even if the inhibitor, I, binds only temporarily, the enzyme
efficiency will decrease and effectively change the value of KM, yielding
an apparent KM:

(7.54)

Alternatively, an inhibitor may bind at a site distinct from the substrate
active site and only bind to the enzyme–substrate complex. In this case,
the binding of the inhibitor effectively changes the value of the substrate
concentration:

(7.55)

The type of inhibitor can be identified by use of the double-reciprocal
plots (Figure 7.18). For a competitive inhibitor, the competition between
the substrate and inhibitor for the binding site can be biased to favor the
substrate by adding more substrate. When the substrate concentration
exceeds the inhibitor concentration, the probability of an inhibitor bind-
ing instead of the substrate is minimized. The enzyme shows the same
value of the maximum velocity but the substrate concentration required
to produce the half-maximum velocity has increased, resulting in the appar-
ent increase in KM. On the double-reciprocol plot, this effect is evident as
the lines have the same yintercept, which is proportional to 1/KM, but the

V

k

(^0) K

[]

[]

=

+

cat total =

M uncomp

uncomp

E ][S

α S

α

[]

[]

1

1

+

′

′=

K

K

I

I

ES][I

ESI

V

k

(^0) K

1

[][]

[]

=

+

cat total =+

comp M

comp

ES

α S

α

[]

[]

1

K

K

I

I

E][I

EI

=

CHAPTER 7 KINETICS AND ENZYMES 157

Table 7.3

Experimental parameters for some enzymes.

Enzyme Substrate kcat(s−−^1 ) KM(M) kcat/KM(M−−^1 s−−^1 )

Carbonic anhydrase CO 2 1 × 106 1.2 × 10 −^2 8.3 × 107

Catalase H 2 O 2 4 × 107 1.1 4 × 107

Fumerase Fumarate 8 × 102 5 × 10 −^6 1.6 × 108

Fumerase Malate 9 × 102 2.5 × 10 −^5 3.6 × 107

From Fersht, A. (1999) Structure and Mechanism in Protein Science. W.H. Freeman, New

York, p. 166.