Pathway of anaerobic poly-beta-hydroxybutyrate degradation by Ilyobacter delafieldii

Abstract

Ilyobacter delafieldii produced an extracellular poly-[3-hydroxybutyrate (PHB) depolymerase when grown on PHB; activity was not detected in cultures grown on 3-hydroxybutyrate, crotonate, pyruvate or lactate. PHB depolymerase activity was largely associated with the PHB granules (supplied as growth substrate), and only 16 % was detected free in the culture supernatant. Monomeric 3-hydroxybutyrate was detectable as a product of depolymerase activity. The monomer was fermented to acetate, butyrate and H 2. After activation by coen- zyme A transfer from acetyl-CoA or butyryl-CoA, the resultant 3-hydroxybutyryl-CoA was oxidized to ace-toacetyl-CoA (producing NADH), followed by thiolytic cleavage to yield acetyl-CoA which was further me-tabolized to acetyl-phosphate, then to acetate with concomitant ATP production. The reducing equivalents (NADH) could be disposed of by the evolution of H 2, or by a reductive pathway in which 3-hydroxybutyryl-CoA was dehydrated to crotonyl-CoA and reduced to butyryl-CoA. In cocultures of I. delafieldii with Des-ulfovibrio vulgaris on PHB, the H 2 partial pressure was much lower than in the pure cultures, and sulfide was produced. Thus interspecies hydrogen transfer caused a shift to increased acetate and H 2 production at the expense of butyrate.