Bioorganic Chemistry: A Chemical Approach to Enzyme Action by Hermann Dugas

By Hermann Dugas

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In mammals, the methionine substrate is an essential amino acid in the diet. ATP 1 enzyme. Mg2+ SAM Homocysteine is the amino acid that results after S-adenosyl methionine donates its methyl group, and then the product S-adenosyl homocysteine is hydrolyzed by a molecule of water. ) in biological systems: tetrahydrofolic acid. In some bacteria, homocysteine can be converted back to methionine by methylation with methylcobalamin (the methyl derivative of vitamin B 12), in the presence of other required compounds or cofactors.

As such, only small proteins (polypeptides) are synthesized by this means; the antibiotic gramicidin S providing an important example (5). Gramicidin S is an interesting antibiotic for a number of reasons. First, it contains phenylalanine in the D-configuration. The occurrence of D-amino acids in nature is very rare and only L-amino acids are found in proteins. Second, gramicidin S contains the amino acid ornithine, which is not a normal constituent of proteins. Gramicidin S is a cyclic decapeptide isolated from Bacillius brevis whose antibiotic activity is derived from its ability to complex alkali metal ions and transport them across membranes.

This, very likely, results from a conformational change on the ribosome which again requires energy at the expense of a molecule of GTP. ----_.... ~ 5' tRNNMet ? OH ? 3'mRNA R-tRNAR §O o O-C:::7 I AH 0 CH-R H N" I I CH 3 " /,,-/CH-~H S new peptide bond Fig. 1. Peptide bond formation in the biological system. The entire process takes place on the ribosome and involves two binding sites, the P (peptidyl) and A (aminoacyl) sites. The reaction is catalyzed by the enzyme peptidyl transferase. It is the message carried by mRNA (which in tum is dictated by the genetic material DNA) which determines by specific interactions as to which aminoacyl-tRNA will bind at the P and A sites.

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