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6.5 Multienzyme complexes

In free solution, the rate of an enzyme-catalysed reaction depends on the concentration of the enzyme and the concentration of its substrate. For an enzyme operating at suboptimal concentrations, the reaction is said to be diffusion-limited, since it depends on the random collision of the enzyme and substrate. If we consider a metabolic pathway, the product of one reaction is the substrate for the next enzyme in the pathway. Direct transfer of a metabolite from one enzyme to another would avoid dilution of the metabolite in the bulk aqueous environment and would increase the rate of reaction.

In the cell, enzymes of a particular pathway are frequently organised spatially so that such metabolic channelling can occur. Some enzymes are associated with other enzymes involved in a particular pathway to form multienzyme complexes. For the enzymes in such complexes, the diffusion of the substrate is not rate-limiting. Pyruvate dehydrogenase (Figure 47) is a complex of three different enzymes that collectively catalyse the oxidation of pyruvate as described previously (Section 5.3). In fact, in eukaryotic cells, most enzymes do not diffuse freely in the cytosol but are effectively concentrated in particular parts of the cell along with other enzymes or proteins involved in related processes. Concentration of enzymes in this way can be achieved by specific protein–protein interactions.

Figure 47 Pyruvate dehydrogenase is a multienzyme complex comprising multiples of three different enzymes: eight of lipoamide reductase–transacetylase (a trimer), six of dihydrolipoyl dehydrogenase (a dimer) and 12 of pyruvate decarboxylase (a dimer), giving a total of 60 polypeptide chains per complex.

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