Amino acid selection in protein biosynthesis

The enzymology of short-chain fatty acyl-coenzyme A synthetase from seeds of Pinus radiata. Kinetic studies and a proposed reaction mechanism

1. Short-chain fatty acyl-CoA synthetase from seeds of Pinus radiata was examined by acetate- and propionate-dependent PP(i)-ATP exchange. Reaction mixtures came to equilibrium almost instantly as judged by rates of exchange and analysis of an incubation mixture. 2. The activity of the enzyme was correlated with the concentration of MgP(2)O(7) (2-) but not with the concentration of Mg(2+), as judged by PP(i)-ATP exchange and fatty acyl AMP-dependent synthesis of ATP in the presence of PP(i). In PP(i)-ATP exchange assays, no clear relationship between activity and any single species of ATP was apparent. 3. High concentrations of fatty acid inhibited PP(i)-ATP exchange. PP(i)-dATP exchange was less than PP(i)-ATP exchange at low concentrations of fatty acid, but at higher concentrations PP(i)-dATP exchange exceeded PP(i)-ATP exchange. The rate of synthesis of fatty acyl-CoA in the presence of dATP was less than with ATP. 4. ATP and propionate inhibited the synthesis of ATP from propionyl-AMP and PP(i). The inhibition by ATP was competitive with respect to propionyl-AMP and non-competitive with respect to PP(i). The inhibition by propionate was non-competitive with respect to propionyl-AMP and PP(i). 5. AMP was a competitive inhibitor of propionyl-AMP-dependent synthesis of ATP and competitively inhibited propionate-dependent PP(i)-ATP exchange when ATP was the variable substrate. 6. It was concluded that the first partial reaction catalysed by the enzyme is ordered; ATP is the first substrate to react with the enzyme and PP(i) is probably the only product released.

Amino acid activation in mammalian brain. Purification and characterization of tryptophan-activating enzyme from buffalo brain

l-Tryptophan-activating enzyme [l-tryptophan-tRNA ligase (AMP), EC 6.1.1.2] of water-buffalo brain was purified to near homogeneity by heat and pH treatments, ammonium sulphate fractionation, column chromatography on DEAE-cellulose, hydroxyapatite and Amberlite CG-50, and gel filtration on Sephadex G-200. The purified enzyme catalyses tryptophanyl-tRNA formation with yeast tRNA, but not with Escherichia coli tRNA. The enzyme exhibits multiple peaks of activity in Sephadex gel filtration with molecular weights corresponding to 155000, 105000 and 50000. However, only one peak of activity with molecular weight of 155000 can be detected when the enzyme is subjected to gel filtration at high concentration. Disc gel electrophoresis in the presence of sodium dodecyl sulphate reveals a single band with molecular weight of 55000. The activity of the enzyme is concentration dependent. Different K(m) and V(max.) values are obtained at different enzyme concentrations. These data suggest that this enzyme may exist in different quaternary structures, each with its own kinetic constants. The enzyme activity is inhibited by p-chloromercuribenzoate, and is not protected by the presence of the substrates, l-tryptophan, Mg(2+), ATP, in any combination.

Isolation of an organ-specific leucyl-tRNA synthetase from soybean seedling

The leucyl-tRNA synthetase activity from cotyledons of 4-day-old soybean seedlings has been fractionated into three components. One of these exclusively acylates two of the six tRNA(Leu) species present in this tissue. The remaining two enzyme fractions charge the other four tRNA(Leu) species equally well. Soybean hypocotyls appear to contain only the last two enzyme fractions.

Properties and substrate specificity of the leucyl-, the threonyl- and the valyl-transfer-ribonucleic acid synthetases from Aesculus species

1. Leucyl- and threonyl-tRNA synthetases were partially purified up to 100-fold and 30-fold respectively from cotyledons of Aesculus hippocastanum and were largely separated from the other aminoacyl-tRNA synthetases. Valyl-tRNA synthetase was purified 25-fold from cotyledons of Aesculus californica. 2. Some properties are reported for the three enzymes when assayed by the [(32)P]pyrophosphate-ATP exchange technique. 3. beta-(Methylenecyclopropyl)alanine, isoleucine, azaleucine, norleucine and gamma-hydroxynorvaline acted as alternative substrates for the leucyl-tRNA synthetase; the enzyme's affinity for beta-(methylenecyclopropyl)-alanine and for isoleucine was about 80-fold less than that exhibited for leucine. 4. alpha-Cyclopropylglycine and alpha-cyclobutylglycine acted as alternative substrates for the valyl-tRNA synthetase.

Properties and substrate specificities of the phenylalanyl-transfer-ribonucleic acid synthetases of Aesculus species

1. Phenylalanyl-tRNA synthetases have been partially purified from cotyledons of seeds of Aesculus californica, which contains 2-amino-4-methylhex-4-enoic acid, and from four other species of Aesculus that do not contain this amino acid. The A. californica preparation was free from other aminoacyl-tRNA synthetases, and the contaminating synthetase activity in preparations from A. hippocastanum was decreased to acceptable limits by conducting assays of pyrophosphate exchange activity in 0.5m-potassium chloride. 2. The phenylalanyl-tRNA synthetase from each species activated 2-amino-4-methylhex-4-enoic acid with K(m) 30-40 times that for phenylalanine. The maximum velocity for 2-amino-4-methylhex-4-enoic acid was only 30% of that for phenylalanine with the A. californica enzyme, but the maximum velocities for the two substrates were identical for the other four species. 3. 2-Amino-4-methylhex-4-enoic acid was not found in the protein of A. californica, so discrimination against this amino acid probably occurs in the step of transfer to tRNA, though subcellular localization, or subsequent steps of protein synthesis could be involved. 4. Crotylglycine, methallylglycine, ethallylglycine, 2-aminohex-4,5-dienoic acid, 2-amino-5-methylhex-4-enoic acid, 2-amino-4-methylhex-4-enoic acid, beta-(thien-2-yl)alanine, beta-(pyrazol-1-yl)alanine, phenylserine and m-fluorophenylalanine were substrates for pyrophosphate exchange catalysed by the phenylalanyl-tRNA synthetases of A. californica or A. hippocastanum. Allylglycine, phenylglycine and 2-amino-4-phenylbutyric acid were inactive.

Partial purification and properties of an enzyme from rat liver that catalyses the sulphation of L-tyrosyl derivatives

1. An enzyme that catalyses the transfer of sulphate from adenosine 3'-phosphate 5'[(35)S]-sulphatophosphate to l-tyrosine methyl ester and tyramine was purified approx. 70-fold from female rat livers. 2. The partially purified preparation is still contaminated with adenosine 3'-phosphate 5'-sulphatophosphate-phenol sulphotransferase (EC 2.8.2.1), but a partial separation of the two enzymes can be achieved by chromatography on columns of Sephadex G-200 and DEAE-Sephadex. 3. The enzyme responsible for the sulphation of l-tyrosine methyl ester and tyramine is activated by dithiothreitol, 2-mercaptoethanol and GSH, the degree of activation being more marked with preparations previously stored at 0 or -10 degrees C. In contrast, the enzymic sulphation of p-nitrophenol is inhibited by all three thiols. Again, there is a quantitative difference in the degree of inhibition of the two enzymes by o-iodosobenzoate, p-chloromercuribenzoate, N-ethylmaleimide and iodoacetate. 4. Mixed-substrate experiments support the hypothesis that the enzyme responsible for the sulphation of l-tyrosine methyl ester and tyramine is separate from that responsible for the sulphation of p-nitrophenol. However, p-nitrophenol is a potent inhibitor of the sulphation of both tyrosyl derivatives whereas these latter compounds have no effect on the sulphation of p-nitrophenol.