Inactivation and cleavage of liver 6-P-gluconate dehydrogenase during irradiation in the presence of vanadate

6-Phosphogluconate dehydrogenase and its crystal structures

6-Phosphogluconate dehydrogenase (6PGDH; EC 1.1.1.44) catalyses the oxidative decarboxylation of 6-phosphogluconate to ribulose 5-phosphate in the context of the oxidative part of the pentose phosphate pathway. Depending on the species, it can be a homodimer or a homotetramer. Oligomerization plays a functional role not only because the active site is at the interface between subunits but also due to the interlocking tail-modulating activity, similar to that of isocitrate dehydrogenase and malic enzyme, which catalyse a similar type of reaction. Since the pioneering crystal structure of sheep liver 6PGDH, which allowed motifs common to the β-hydroxyacid dehydrogenase superfamily to be recognized, several other 6PGDH crystal structures have been solved, including those of ternary complexes. These showed that more than one conformation exists, as had been suggested for many years from enzyme studies in solution. It is inferred that an asymmetrical conformation with a rearrangement of one of the two subunits underlies the homotropic cooperativity. There has been particular interest in the presence or absence of sulfate during crystallization. This might be related to the fact that this ion, which is a competitive inhibitor that binds in the active site, can induce the same 6PGDH configuration as in the complexes with physiological ligands. Mutagenesis, inhibitors, kinetic and binding studies, post-translational modifications and research on the enzyme in cancer cells have been complementary to the crystallographic studies. Computational modelling and new structural studies will probably help to refine the understanding of the functioning of this enzyme, which represents a promising therapeutic target in immunity, cancer and infective diseases. 6PGDH also has applied-science potential as a biosensor or a biobattery. To this end, the enzyme has been efficiently immobilized on specific polymers and nanoparticles. This review spans the 6PGDH literature and all of the 6PGDH crystal structure data files held by the Protein Data Bank.

Chemistry and mechanism of vanadate-promoted photooxidative cleavage of myosin

Irradiation of the stable myosin subfragment 1(S1).MgADP.orthovanadate (Vi) complex results in oxidation of an active site serine (Ser-180) to a serine aldehyde [Cremo, C. R., Grammer, J. C., & Yount, R. G. (1989) J. Biol. Chem. 264, 6608-6611]. This photomodified S1 will reform a new MgADP.Vi complex and upon a second irradiation, the S1 heavy chain is cleaved into 21 kDa NH2-terminal and 74 kDa COOH-terminal fragments. When S1, in which the side chain of Ser-180 was tritiated, was photocleaved tritium was released from the protein suggesting that cleavage was occurring at Ser-180. The 21 kDa NH2-terminal fragment was resistant to carboxypeptidase digestion, and the 74 kDa COOH-terminal fragment yielded no sequence by Edman degradation, indicating that parts of Ser-180 went to each fragment. To identify these parts, the two cleavage fragments were isolated and chemically (21 kDa) or enzymatically (74 kDa) cleaved, and the resulting peptides were separated by reversed phase HPLC. The peptides immediately down- and up-stream from Ser-180 were isolated and the blocking groups were identified by mass spectrometry. The 21 kDa fragment peptide was blocked with a carboxamide on Glu-179 (confirmed by HPLC and capillary electrophoresis in comparison with peptide standards), while the NH2 group of Gly-181 of the 74 kDa fragment was blocked with an oxalyl group (verified by enzymatic analysis for oxalate). The side chain of Ser-180 was released as formate. O2 is required for photocleavage. Cleavage experiments in the presence of 18O2 showed one atom of 18O labeled the oxalyl group. A mechanism in which O2 adds to a free radical on the alpha-carbon of Ser-180 with a subsequent Criegee type rearrangement is proposed to explain both the kinetics and products of the photocleavage.