Symmetry and Asymmetry of the Pyruvate Dehydrogenase Complexes from Azotobacter vinelandii and Escherichia coli as Reflected by Fluorescence and Spin‐Label Studies

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Abstract
Fluorescence‐lifetime measurements of FAD bound to lipoamide dehydrogenase from Azotobacter vinelandii and Escherichia coli were performed. It is shown from these results that the two FAD groups in the isolated dimeric enzyme, as well as in the enzyme in the intact complex of E. coli, are in non‐equivalent surroundings. This contrasts with the near equivalence of the FAD groups of both the enzyme and complex isolated from A. vinelandii. Reduction of the complex with Mg2+, thiamine pyrophosphate and pyruvate or with NADH enables the attachment of a maleimide analogue specifically to the lipoyl moieties of the transacetylase(s). Spin label [N‐(1‐oxyl‐2,2,5,5‐tetramethyl‐3‐pyrrolidinyl)maleimide] introduced in such a way proves the existence of at least two different micro‐environments around the lipoyl moieties in complex isolated from A. vinelandii. Electron paramagnetic resonance spectra of the specifically spin‐labelled complexes from E. coli and A. vinelandii, when dissolved in tricine [N‐tris(hydroxymethyl)‐methylglycine] buffer, show interactions of at least two electron spins with each other, which indicate that the lipoyl moieties are rather close together.Fluorescent label [N‐(1‐anilinonaphthyl‐4)maleimide] is specifically attached to the lipoyl moiety of the high‐Mr transacetylase of the freshly isolated complex from A. vinelandii. From the large differences in the apparent lifetimes τp and τm, as detected by phase fluorimetry, it is shown that this fluorescent label is distributed in different micro‐environments. The differences observed in energy transfer between fluorescent label, attached to the lipoyl moiety of the high‐Mr transacetylase, indicate different conformations of the complex from A. vinelandii. Upon introduction of the label after reduction with NADH a much larger energy transfer, thus a shorter distance, is observed between the label and FAD than when reduction is performed with Mg2+, thiamine pyrophosphate and pyruvate. A similar conformation dependence upon reduction is found for the pyruvate dehydrogenase complex from E. coli. It is thus proposed that the transacetylase of E. coli and the high‐Mr transacetylase of A. vinelandii are both non‐symmetrically distributed within the complex.