A2p2 tetramer

Preparation of mixed-metal hemoglobin hybrids is achieved by separation of hemoglobin into its constituent a and P chains, followed by demetallation of one of the chains, reconstitution with MP, and chain recombination, yielding the tetrameric [a2(MP), P2(Fe P)] or [a2(Fe P), P2(MP)] species [11]. Thus, MP FeP electron transfer might in principle occur between aj/Pj or ai/P2 subunits. However, the distance between aj and Pj hemes is over 10 A greater than the and P2. This extra distance is expected, and indeed is found, to reduce ET rates by several orders of magnitude. Hence, for all practical purposes we may treat the a2P2 tetramer in terms of two independent [otj, P2] electron transfer complexes. 

A (3 replacement reaction catalyzed by the PLP-dependent tryptophan synthase converts indoleglycerol phosphate and serine to tryptophan. Tryptophan synthase from E. coli consists of two subunits associated as an a2P2 tetramer (Fig. 25-3). The a subunit catalyzes the cleavage (essentially a reverse aldol) of indoleglycerol phosphate to glyceraldehyde 3-phosphate and free indole (Fig. 25-2, step s).67 The P subunit contains PLP. It presumably generates, from serine, the Schiff base of aminoacrylate, as indicated in Fig. 25-2 (step f). The enzyme catalyzes the addition of the free indole to the Schiff base to form tryptophan. The indole must diffuse for a distance of 2.5 ran... 

The first X-ray structure of a nitrile hydratase was determined for the enzyme from Rhodococcus sp. R312 25. In this study an a2P2-tetramer conformation has been found in the native enzyme. Characterization of the highly related Rhodococcus sp. N-771 nitrile hydratase, however, revealed a dimeric species in solution for this enzyme1121. 

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