Acetoacetate activation in muscle and the nucleotide specificity of succinyl thiokinase

Interactions of GTP with the ATP-grasp domain of GTP-specific succinyl-CoA synthetase

Two isoforms of succinyl-CoA synthetase exist in mammals, one specific for ATP and the other for GTP. The GTP-specific form of pig succinyl-CoA synthetase has been crystallized in the presence of GTP and the structure determined to 2.1 A resolution. GTP is bound in the ATP-grasp domain, where interactions of the guanine base with a glutamine residue (Gln-20beta) and with backbone atoms provide the specificity. The gamma-phosphate interacts with the side chain of an arginine residue (Arg-54beta) and with backbone amide nitrogen atoms, leading to tight interactions between the gamma-phosphate and the protein. This contrasts with the structures of ATP bound to other members of the family of ATP-grasp proteins where the gamma-phosphate is exposed, free to react with the other substrate. To test if GDP would interact with GTP-specific succinyl-CoA synthetase in the same way that ADP interacts with other members of the family of ATP-grasp proteins, the structure of GDP bound to GTP-specific succinyl-CoA synthetase was also determined. A comparison of the conformations of GTP and GDP shows that the bases adopt the same position but that changes in conformation of the ribose moieties and the alpha- and beta-phosphates allow the gamma-phosphate to interact with the arginine residue and amide nitrogen atoms in GTP, while the beta-phosphate interacts with these residues in GDP. The complex of GTP with succinyl-CoA synthetase shows that the enzyme is able to protect GTP from hydrolysis when the active-site histidine residue is not in position to be phosphorylated.

Investigations into the electrooxidation of guanosine-5'-triphosphate at the pyrolytic graphite electrode

The electrochemical oxidation of guanosine-5'-triphosphate has been investigated in phosphate-containing electrolytes in the pH range 1.5-10.9 at a pyrolytic graphite electrode by cyclic sweep voltammetry, spectral studies, bulk electrolysis and related techniques. In this pH range, the oxidation occurred in a single well-defined peak (Ia). The peak potential of oxidation peaks (Ep) was found to be dependent on pH, concentration and sweep rate. The kinetics of the UV-absorbing intermediates was followed spectrophotometrically and the decay of the intermediate occurred in a pseudo-first-order reaction. The first-order rate constants for the disappearance of the UV-absorbing intermediate have also been calculated. The products of the electrode reaction were characterized by HPLC and GC/MS. A tentative mechanism for the formation of the products has also been suggested.

Genetic evidence for the expression of ATP- and GTP-specific succinyl-CoA synthetases in multicellular eucaryotes

Highly ATP- and GTP-specific isoforms of succinyl-CoA synthetase in pigeon incorporate the same alpha-subunit, but different beta-subunits (Johnson, J. D., Muhonen, W. W., and Lambeth, D. O. (1998) J. Biol. Chem. 273, 27573-27579). The sequences of the mature subunits were determined by methods based on reverse transcription-polymerase chain reaction. The 306-residue mature alpha-subunit in pigeon shows >88% identity to its homologues in pig and rat. The sequences of the mature ATP- and GTP-specific beta-subunits (A-beta and G-beta, respectively) in pigeon are 54% identical. These sequences were used to identify expressed sequence tags for human and mouse that were highly homologous to G-beta and A-beta, respectively. The sequences for mature A-beta and G-beta in mouse and human were completed and verified by polymerase chain reaction. The sequence of A-beta in pig was also obtained. The mammalian A-beta sequences show >89% identity to each other; the G-beta sequences are similarly related. However, pairwise comparisons of the A-beta and G-beta sequences revealed <53% identity. Alignment with two sequences of the beta-subunit in Caenorhabditis elegans suggests that the A-beta and G-beta genes arose by duplication early in the evolution of multicellular eucaryotes. The expression of A-beta is strong in numerous mouse and human tissues, which suggests that ATP-specific succinyl-CoA synthetase also plays an important role in species throughout the animal kingdom.

Characterization of the ATP- and GTP-specific succinyl-CoA synthetases in pigeon. The enzymes incorporate the same alpha-subunit

Two succinyl-CoA synthetases, one highly specific for GTP/GDP and the other for ATP/ADP, have been purified to homogeneity from pigeon liver and breast muscle. The two enzymes are differentially distributed in pigeon, with only the GTP-specific enzyme detected in liver and the ATP-specific enzyme in breast muscle. Based on assays in the direction of CoA formation, the ratios of GTP-specific to ATP-specific activities in kidney, brain, and heart are approximately 7, 1, and 0.1, respectively. Both enzymes have the characteristic alpha- and beta-subunits found in other succinyl-CoA synthetases. Studies of the alpha-subunit by electrophoresis, mass spectrometry, reversed-phase high performance liquid chromatography, and peptide mapping showed that it was the same in the two enzymes. Characterization of the beta-subunits by the same methods indicated that they were different, with the tryptic peptide maps providing evidence that the beta-subunits likely differ along their entire sequences. Because the two succinyl-CoA synthetases incorporate the same alpha-subunit, the determinants of nucleotide specificity must reside within the beta-subunit. Determination of the apparent Michaelis constants showed that the affinity of the GTP-specific enzyme for GDP is greater than that of the ATP-specific enzyme for ADP (7 versus 250 microM). Rather large differences in apparent Km values were also observed for succinate and phosphate.