Enzyme activity complexes, assembly

Many other oligomeric enzymes and other complex assemblies of more than one kind of protein subunit are known. For example, the 2-oxoacid dehydrogenases are huge 2000- to 4000-kDa complexes containing three different proteins with different enzymatic activities in a cubic array (Fig. 15-14). The filaments of striated muscle (Chapter 19), antibodies and complement of blood (Chapter 31), and the tailed bacteriophages (Box 7-C ) all have complex molecular architectures. 

The rate-limiting step in the kinetic pathway of nucleotide incorporation is the conversion of the E p/t dNTP complex to the activated complex, E p/t dNTP (Step 3 in Fig. 1). This step is crucial in many respects. First, it is essential for the phosphoryl transfer reaction to occur. During the E p/t dNTP to E p/t dNTP transition, all the components of the active site are assembled and organized in a topological and geometrical arrangement that allows the enzyme to proceed with the chemical step (Step 4). Second, Step 3 plays a major role in the mechanism of discrimination between correct versus incorrect nucleotides. Interpretation of the kinetic measurements has led to the hypothesis that the E p/t dNTP... 

The chemical modification of redox enzymes with electron relay groups permits the mediated electron transfer and the electrical wiring of the proteins [83-85] (Figure 5A). The covalent attachment of electron-relay units at the protein periphery, as well as inner sites, yields short inter-relay electron-transfer distances. Electron hopping or tunneling between the periphery and the active site allows electrical communication between the redox enzyme and its environment. The simplest systems of this kind involve electron relay-functionalized enzymes diffusionally communicating with electrodes [83], but more complex assemblies including immobilized enzymes have also been reported. 

The multienzyme complexes are self-assembling emd will reassemble to an active complex after resolution of the individual enzymes. The core enzyme of the complex is the dihydrolipoyl acyltransferase (E2) the oxo-acid dehydrogenase (El) and dihydrolipoyl dehydrogenase (E3) subunits form noncovalent bonds to this central catalytic unit. 

The proteolytic reactions of the hemostatic system are neither catalytically efficient nor localized when proteinase and proteinase precursor only are present. The rapid, localized proenzyme activation required for normal hemostatic response occurs only in a complex of proteinase, proteinase precursor, and cofactor protein assembled on the surface of a damaged cell membrane, or in vitro, on the surface of phospholipid bilayers. The catalytic efficiency of an enzyme-catalyzed reaction is expressed by the ratio of the kinetic constants /ic and kJKm). In the activation complexes, kJKm values can be greater than lO M s . With proteinase and proenzyme alone, the kJKm values are only approximately 100 M s and thus the reactions are 10 times less efficient. Expressed in terms of the same amount of product formed in the two situations, a 10 increase represents the difference between requiring 1 minute and about 6 months to form the product ... 

Most processes in cells do not take place independently of one another or at a constant rate. Instead, the catalytic activity of enzymes or the assembly of a macromolecular complex is so regulated that the amount of reaction product or the appearance of the complex is just sufficient to meet the needs of the cell. As a result, the steady-state concentrations... 

Taanman, J.W. and Capaldi, R.A., Subunit Via of yeast cytochrome c oxidase is not necessary for assembly of the enzyme complex but modulates the enzyme activity. Isolation and characterization of the nuclear-coded gene, J Biol Chem 268 (1993) 18754—18761. 

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