E. coli pyruvate dehydrogenation complex

The decarboxylation reaction, Eq. (7), is visualized as a cleavage of the a-keto acid to yield CO2 and an enzyme-bound aldehyde-thiamine pyrophosphate (RCHO—TPP) compound, i.e., active aldehyde. There is now unequivocal evidence for this reaction since a pyruvic carboxylase (El) has been shown to be an essential component of the E. coli pyruvate dehydrogenation complex (Koike and Reed, 1961 Gounaris and Hager, 1961) and the nature of the aldehyde-TPP compound has been elucidated (Breslow, 1958 Breslow and McNelis, 1959 Krampitz et al., 1961 Holzer and Beaucamp, 1961 Carlson and Brown, 1961). 

The resolution and reconstitution experiments described above indicate that the E. coli pyruvate dehydrogenation complex is a highly organized multienzyme unit. The number of molecules of carboxylase and of flavoprotein per molecule of complex have been calculated. Those numbers are about 12 for the carboxylase and about 6 for the flavoprotein. Each mole-... 

The results of the resolution and reconstitution experiments carried out with the E. coli pyruvate dehydrogenation complex indicate that there are specific binding sites on the lipoic reductase-transacetylase component for the carboxylase and the flavoprotein. In other words, the latter two enzymes appear to be specifically oriented with respect to the lipoic acid bound to the lipoic reductase-transacetylase component. It is evident from the re-... 

Fi(i. 8. A schematic representation of po.ssible interactions between several lipoyllysyl moieties in the E. coli pyruvate dehydrogenation complex. These interactions may involve thiol-disulfide interchange and acetyl transfer. The arrows describe the area covered by each lipoyllysyl moiety. 

Support for the proposal that lipoic acid is bound in the pyruvate dehydrogenation complex in covalent linkage through its carboxyl group was furnished by studies with a hydrolytic enzyme, lipoyl-X hydrolase, obtained from extracts of S. faecalis (Reed et al., 1958b). Incubation of the E. coli pyruvate and a-ketoglutarate dehydrogenation complexes with lipoyl-X hydrolase released approximately 96% of the bound lipoic acid (Koike and Reed, 1960) and resulted in a loss of the DPN-linked a-keto... 

Fkj. 6. Functional form of lipoic acid in Escherichia coli pyruvate and -koto-glutarate dehydrogenation complexes. The earboxyl group of lipoie acid is bound in amide linkage to the e-amino group of a lysine residue, providing a flexible arm of approximately 14 A for the reactive dithiolane ring. 

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