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15.2.4 Allosterism and cooperativity

The discussion of the mechanism of enzyme action so far has been based on the

assumption that successive substrate molecules bind to the enzyme with the same ease

(affinity). This is not true with some enzymes. With some enzymes successive binding

occurs with either progressive greater ease or with reduced ease and the enzymes are

said to be subject toallosteric control.Historically, all such enzymes were believed to

possess quaternary structure and consist of several protein subunits (oligomers),

which could be identical or different, and to possess multiple catalytic sites. Proteins

subject to allosteric control are not confined to enzymes. Other examples are haemo-

globin and many cell membrane receptors especially those of the G-protein-coupled

receptors (GPCRs) type (Section 17.4.3). Oligomeric enzymes may either display

simple Michaelis–Menten kinetics in which case all substrate molecules bind with

equal ease, as is the case with the tetramer lactate dehydrogenase (Section 15.1.2), or

they display a characteristicsigmoidalrelationship between initial rate and substrate

concentration (Fig. 15.9) indicative of anallosteric enzyme. Examples of such

enzymes are aspartate carbamoylase and phosphofructokinase. Progressive binding

of the substrate molecules to the subunits of an allosteric enzyme may result in either

increased (positive cooperativity) or decreased (negative cooperativity) activity

towards the binding of further substrate molecules. In such cases the substrate

molecules are said to display ahomotropic effect. Changes in catalytic activity

towards the substrate may also be brought about by the binding of molecules other

than the substrate at distinctallosteric binding siteson one or more subunit.

Compounds that induce such changes are referred to asheterotropic effectors. They

are commonly key metabolic intermediates such as ATP, ADP, AMP and Pi. Hetero-

tropic activators increase the catalytic activity of the enzyme, making the curve less

sigmoidal and moving it to the left, whilst heterotropic inhibitors cause a decrease in

activity, making the curve more sigmoidal and moving it to the right (Fig. 15.9). The

diagnosis of cooperativity by use of the Lineweaver–Burk plot is shown in Fig. 15.5c.

The operation of cooperative effects may be confirmed by a Hill plot, which is based

on the equation:

+ Activator

+ Inhibitor

Initial rate

Substrate concentration

No effector

Fig. 15.9Effect of activators and inhibitors on the sigmoidal kinetics of an enzyme subject to allosteric control.

599 15.2 Enzyme steady-state kinetics