[The effect of phosphorylation on catalytic function of muscle pyruvate dehydrogenase complex]

It has been shown that phosphorylation of the pyruvate dehydrogenase complex from pigeon breast muscle by endogenous ATP-dependent protein kinase suppresses the substrate conversion in the pyruvate: acceptor oxidoreductase reactions and nonoxidative reactions monitored by pyruvate decline in the absence of CoA and NAD. To identify the catalytic step blocked by phosphorylation, CD spectroscopy was used which revealed the appearance and decay of the charge transfer complex between component E1 and thiamine pyrophosphate during the enzymatic reaction. Phosphorylation of the pyruvate dehydrogenase complex while lowering the affinity for thiamine pyrophosphate does not preclude the formation of holo-E1 but inhibits its interaction with pyruvate. Phosphorylated pyruvate dehydrogenase, like the dephosphorylated enzyme, reacts with 2-hydroxyethyl thiamine pyrophosphate in half of the active sites. In the presence of deacylating agents (CoA or dithiothreitol) all the sites are reactive. A conclusion is drawn that the alternating functioning of the active centers is preserved in reductive acetylation of the acceptor substrates by phospho-E1.

[Study of the pyruvate dehydrogenase complex by circular dichroism]

A comparative study of the pyruvate dehydrogenase complex and its pyruvate dehydrogenase component was carried out by using the circular dichroism method. It was found that the spectral properties of the pyruvate dehydrogenase complex are determined by those of its first component: i) the spectrum of the thiamine pyrophosphate-free pyruvate dehydrogenase complex displayed the main characteristics of the pyruvate dehydrogenase component; ii) the appearance of the charge transfer complex band during thiamine pyrophosphate saturation was revealed for the both proteins; iii) in both cases the charge transfer complex band disappeared after the interaction of the holoform with pyruvate and reappeared after the addition of dithiothreitol used as a deacetylating reagent. Coenzyme A in the same reaction selectively deacetylated the pyruvate dehydrogenase complex (but not its pyruvate dehydrogenase component). The spectral dynamics of pyruvate dehydrogenase reflects the functional changes in the enzyme active centers during the catalytic act. The similarity of the spectral behaviour of pyruvate dehydrogenase within the complex structure and in the isolated state provides support for the earlier proposed mechanism of the pyruvate dehydrogenase action and ensures a methodological basis for its direct investigation within the complex structure.

Half-of-the-site reactivity of the decarboxylating component of the pyruvate dehydrogenase complex from pigeon breast muscle with respect to 2-hydroxyethyl thiamine pyrophosphate

The holopyruvate dehydrogenase is characterized by the charge transfer complex formation between tryptophan residue and thiamine pyrophosphate in each of two active centres. Interaction of apoenzyme with one mole of 2-hydroxyethyl thiamine pyrophosphate results in appearance of the same spectral band which does not change in intensity with further increase in ligand concentration. 2-hydroxyethyl thiamine pyrophosphate: acceptor oxidoreductase activity abolishes after oxidation of only one tryptophan residue per mole of the protein or blocking of one of the active centres with inactive analogue of the coenzyme. In the latter case the charge transfer complex band induced by interaction of apoenzyme with 2-hydroxyethyl thiamine pyrophosphate was not shown at all. These facts testify to half-of-the-site reactivity of pyruvate dehydrogenase with respect to 2-hydroxyethyl thiamine pyrophosphate.

Localization of tryptophan residues in thiamine pyrophosphate-binding sites of pyruvate dehydrogenase from pigeon breast muscle

Interaction of N-bromosuccinimide with the pyruvate dehydrogenase component of the pigeon breast muscle pyruvate dehydrogenase complex results in a rapid and specific modification of two tryptophan residues per mole of the protein and in complete inactivation of the enzyme. Modification of the enzyme excludes the development of the negative Cotton effect with a maximum at 330 nm, characteristic of the charge transfer complex between the protein tryptophan residue and thiamine pyrophosphate. Modification of one tryptophan residue was shown to result in the absence of the band at 330 nm in one of the active centers of pyruvate dehydrogenase, while oxidation of two tryptophan residues excludes the formation of a charge transfer complex in both centers. The conclusion was drawn about the presence in the pyruvate dehydrogenase active centers of two tryptophan residues involved in the formation of the charge transfer complex with the thiazolium ring of thiamine pyrophosphate.