Kinetic studies of the reactivity of the sulfhydryl groups of glyceraldehyde-3-phosphate dehydrogenase

Thiol-disulphide interchange in tubulin: kinetics and the effect on polymerization

All 20 cysteine residues are accessible to disulphide reagents in the tubulin dimer, whereas only four are accessible in taxol-stabilized microtubules. Reaction rates with disulphide reagents are a function of the reagent, are decreased by G nucleotides, and increased with increase in pH and urea. With transient (stop-flow) kinetics, DTNB [5,5'-dithiobis-(2-nitrobenzoic acid)] and 2,2'-dithiodipyridine progress curves cannot be fitted by the sum of exponential terms based only on classes of cysteines. The mixed disulphide products react further to form both intra- and intermonomer disulphide bonds that can be reversed by reducing agents. With MMTS (methyl methanethiosulphonate) or ODNB (n-octyl-dithio-2-nitrobenzoate), virtually no protein-protein disulphide bonds are formed and the ODNB reaction can be given as the sum of three exponential terms with pseudo-first-order rate constants of 0.206, 0.069 and 0.010 s(-1) at pH 6.5, suggesting three classes of thiol reactivities. Limited cysteine substitution leads to only small changes in tryptophan or CD spectra, whereas complete substitution leads to loss of the helix content. MMTS-induced loss of SH groups leads to progressive increases in the critical concentration and loss of polymerization competence that can be reversed by assembly promoters such as higher protein concentration, taxol or high ionic strength. Under such conditions, the substituted tubulin forms protofilament-based structures such as microtubules, open tubules, sheets and/or bundles.

Glyceraldehyde-3-phosphate dehydrogenase. Investigation on the regions responsible for self-assembly of subunits

1. In glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (EC 1.2.1.12) the four S-loop form the core of the tetramer. 2. Amino acid sequence of the S-loop of the regions of GAPDH from carp muscle was established through the analysis of tryptic digests of the enzyme treated alternatively with bromocyanate and o-iodosobenzoic acid. 3. Enzyme had been oxidized with performic acid. After treatment with trypsin the peptide mixture was fractionated into fragments. 4. CNBr cleavage of this enzyme was performed after S-carboxymethylation. The respective cyanogen bromide fragments have been isolated and characterized. 5. The procedure of protein fragmentation by o-iodosobenzoic acid used to split tryptophanyl peptide bonds. 6. Each peptide obtained after enzymatic or chemical fragmentation was purified to homogeneity by Bio-Gel or Sephadex chromatography, high voltage electrophoresis and descending paper chromatography and characterized by electrochromatography, N- and C-terminal sequence and amino acid composition. 7. The results are compared with those obtained from studies on GAPDH from other sources.

Comparative study of the structure of glyceraldehyde-3-phosphate dehydrogenase from bovine heart muscle

Glyceraldehyde-3-phosphate dehydrogenase with a specific activity of 153 units/mg protein was isolated from bovine heart muscle. Its relative molecular mass was found to be 144,000. The tryptic peptide map and amino acid analysis were obtained. The N-terminal sequence was established as Val-Lys-Val-Gly-Val-Asn-Gly-... and C-terminal as ...-Ala-Ser-Lys-Glu. Fluorescence and optimal rotation dispersion measurements were performed. The data were compared with other glyceraldehyde-3-phosphate dehydrogenases.

Kinetics of inactivation and molecular asymmetry of NAD-specific glyceraldehyde-3-phosphate dehydrogenase of Pisum sativum

Glyceraldehyde-3-phosphate dehydrogenase (D-glyceraldehyde-3-phosphate: NAD+ oxidoreductase (phosphorylating), EC 1.2.1.12) has been purified to homogeneity from pea seeds. The purified enzyme gave a single protein band on polyacrylamide gel electrophoresis (with and without sodium dodecyl sulfate; subunit molecular weight 38 000). It is free from bound nucleotides. The kinetics of heat inactivation of the crude enzyme extract as well as the purified enzyme are biphasic, in that exactly half of the activity is destroyed more rapidly than the residual half. The data are consistent with the rate equation: (formula: (see text): where A0 and A are activities at times zero and t, respectively, and k1 and k2 are first-order rate constants for the fast and slow phases, respectively. Addition of NAD+ slows down thermal inactivation, without altering the overall kinetic pattern. The activity lost due to the fast component (k1) of the reaction is regained on colling ('annealing'), whereas the slow reaction (k2) is not reversed, suggesting the following scheme: formula (see text): This is confirmed by plotting the activity after 'annealing' against initial period of heating. A single first-order rate constant (k2) is observed. The enzyme possesses about one reactive SH group per subunit which can be titrated with 5,5'-dithio-bis-(2-nitrobenzoic acid). Blocking of these groups inactivates the enzyme. Inactivation with 20 micrometer N-ethylmaleimide and 30 micrometer iodoacetate (at pH 8.6 and 33 degrees C) follows simple first-order kinetics (rate constants 0.099 and 0.139 min-1, respectively), suggesting that all SH groups react equally readily with these reagents. Reaction of the enzyme with 0.6 micrometer p-chloromercuri benzoate, however, shows biphasic kinetics similar to thermal inactivation. The reaction of p-chloromercuri benzoate with partially heat-inactivated enzyme (residual activity 37.5%) follows simple first-order kinetics. The molecular asymmetry demonstrated by these results must arise from the unique quaternary structure of the enzyme molecule, which is apparently made up of chemically identical subunits (pseudo-isologous association).

Effect of borate on the catalytic activities of muscle glyceraldehyde-3-phosphate dehydrogenase

The effect of borate on glyceraldehyde-3-phosphate dehydrogenase from human, pig and rabbit muscle was studied. At lower concentration of borate only the dehydrogenase activity is inhibited, reversibly and competitively against NAD. At concentration of borate above 6 mM the plots of 1/v versus borate concentration become nonlinear and the inhibition is extended to the esterase and acetylphosphatase activities. In certain conditions a time-dependent inactivation and reactivation was observed. The direct interaction between borate (if present at concentration of at least 6 mM) and glyceraldehyde-3-phosphate dehydrogenase is postulated, the possible site of the reaction being the histidine residue(s). The esterase activity of the human muscle enzyme and the effect of borate on it are different from the other mammalian enzymes.