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Non-competitive reversible inhibition

Anon-competitive reversible inhibitorcombines at a site distinct from that for the

substrate. Whilst the substrate can still bind to its catalytic site, resulting in the

formation of a ternary complex ESI, the complex is unable to convert the substrate

to product and is referred to as adead-end complex. Since this inhibition involves a

site distinct from the catalytic site, the inhibition cannot be overcome by increasing

the substrate concentration. The consequence is thatVmax, but notKm, is reduced

because the inhibitor does not affect the binding of substrate but it does reduce the

amount of free ES that can proceed to the formation of product. With this type of

inhibitionKEIandKESIare identical andKiis numerically equal to both of them. In this

case the Lineweaver–Burk equation (15.5) becomes:

1

v 0 ¼

Km

Vmax

1

½SŠþ

1

Vmax^1 þ

½IŠ

Ki



ð 15 : 12 Þ

Once non-competitive inhibition has been diagnosed (Fig. 15.5a), theKivalue is best

obtained from a secondary plot of either the slope of the primary plot or of 1/V^0 max

(which is equal to the intercept on the 1/v 0 axis) against inhibitor concentration.

Both secondary plots will have an intercept ofKion the inhibitor concentration

axis (Fig. 15.6).

Uncompetitive reversible inhibition

Anuncompetitive reversible inhibitorcan bind only to the ES complex and not to the

free enzyme, so that inhibitor binding must be either at a site created by a conforma-

tional change induced by the binding of the substrate to the catalytic site or directly to

the substrate molecule. The resulting ternary complex, ESI, is also a dead-end complex.

EþS ! ES !EþP

þI#"I

ESI !no reaction

As with non-competitive inhibition, the effect cannot be overcome by increasing the

substrate concentration, but in this case bothKmandVmaxare reduced by a factor of

(1þ[I]/Ki). An inhibitor concentration equal toKiwill therefore halve the values of

bothKmandVmax. With this type of inhibitor,KEIis infinite because the inhibitor

cannot bind to the free enzyme soKiis equal toKESI. The Lineweaver–Burk equation

(15.5) therefore becomes:

1

v 0

¼ Km

Vmax

^1

½SŠ

þ^1

Vmax



1 þ½IŠ

Ki



ð 15 : 13 Þ

The value ofKiis best obtained from a secondary plot of either 1/V^0 maxor I/K^0 m

(which is equal to the intercept on the I/[S] axis) against inhibitor concentration. Both

secondary plots will have an intercept ofKion the inhibitor concentration axis.

Mixed reversible inhibition

For some inhibitors either the ESI complex has some catalytic activity or theKEIand

KESIvalues are neither equal nor infinite. In such case so-calledmixed inhibition

594 Enzymes