Functional Roles of Subunit Assembly and Disassembly

Monomeric intermediates during the assembly of oligomeric proteins are usually inactive and the formation of native quaternary structures are often a prerequisite for catalytic activity. One clear reason for this is that the active sites of some enzymes are located at the interface between subunits and are formed by amino acid residues from different subunits. Such examples are aspartate transcarbamoylase from K coli5) and ribulose bisphosphate carboxylase/oxygenase from Rhodospirillum rubrum.6) 

H+-ATPase in membranes of mitochondria, chloroplasts and bacteria catalyzes ATP synthesis coupled with electron transport.7) The catalytic portion of the enzyme Fi is composed of five different subunits, denoted a through e in order of decreasing molecular weight. The ATPase activity can be reconstituted from isolated a, fi and y subunits, each of which is devoid of catalytic activity in the separated state.7) Interestingly, the y subunit is indispensable for the functional reconstitution, although the catalytic site is suggested to be located on the fi subunit or at the interface of the a and fi subunits.7  

Yeast phosphoglycerate mutase is composed of four identical subunits. Renaturation experiments of the denatured enzyme revealed that monomeric intermediates formed during the reconstitution of tetrameric structure are partially active, but that the assembly of subunits enhances the catalytic activity about three fold.8  

There are multienzyme complexes that efficiently catalyze sequential reactions in some metabolic pathways. The overall rate of a sequential reaction is greatly enhanced by the assembly of subunits possessing sequential metabolic activities, because the metabolites are transferred directly from one active site to another without diffusing in the solution.9 Typical examples are the mammalian pyruvate dehydrogenase complex (three catalytic subunits)10 and the bacterial trytophan synthase (two catalytic subunits).11  

As demonstrated by the examples above, recent studies aimed at understanding the relationship between the quaternary structure and the physiological activity of oligomeric proteins have produced a considerable amount of information which explains many aspects of this phenomenon. In the process, a number of experimental methods have been used. The following sections provide an overview of new approaches which are generally applicable for elucidating the structure-function relationship of oligomeric proteins. 

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Fig. 3.1 Classification of functional roles of subunit assembly and disassembly.4 a) The assembly of identical subunits is essential for the 9catalytic activity, b) The assembly of nonidentical subunits is essential for the catalytic activity, c) The assembly of active subunits enhances the catalytic activity, d) Sequential metabolic reactions are efficiently catalyzed by the assembly of subunits, e) The assembly of subunits is required for the expression of regulatory properties, f) The assembly of nonidentical subunits diminishes the catalytic activity. See text for details. (Reproduced with permission from S. Tokushige, Kagaku Zokan, 103, 41 (1984), in Japanese)).

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