The incorporation of amino acids

L-a-Aminoadipic acid plays an essential role in bio nthesis, although in the assembled end-product it appears in the D form. Incorporation studies employing a-[6- C]aminoadipic acid show that the incorporation of the L isomer is considerably faster than that of the D isomer [279]. According to studies carried out on mutant strains of Cephalosporium acremonium, if the common lysine—L-a-aminoadipic acid metabolism is inhibited before the latter, then besides lysine the internal addition of a-aminoadipic acid is necessary to make the microorganisms capable of synthesizing -lactam antibiotics [271]. 

Investigations using labelled cysteine are of special interest in attempts to clarify the mechanism of ring-closure of Arnstein tripeptide. Because the label appears stereospecifically at the 6a and/or positions (depending on the labelled compound used, see (293) (294)), during the cyclization process 

As for the valine moiety, in the case of penicillins itsC-1 (i.e. penicillin C-3) has D configuration ((291)). The existence of LLL-ACV and the isotope-labelled precursors suggest that the primary form is in the L configuration the isomerization takes place after formation of the tripeptide. 

The same applies to the Cephalosporium-type biosynthesis. L-Valine is also a cephalosporin precursor, although the valine moiety has a higher rate of oxidation because of its unsaturation and is also oxidized on C-10. Penicillin N and 

Oxidized derivatives of valine are not precursors they presumably do not enter the mycelial math, and thus the functionalization of cephalosporins at 

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Antibiotic A201A. Antibiotic A201A (23), produced by S. capreolus is an /V -dimethyladenine nucleoside stmcturaHy similar to puromycin (19). Compound (23) which contains an aromatic acid and monosaccharide residues (1,4), inhibits the incorporation of amino acids into proteins but has no effect on RNA or DNA synthesis. Compound (23) does not accept polypeptides as does (19), and does appear to block formation of the initiation complex of the SOS subunit. It may block formation of a puromycin-reactive ribosome. 

An area of research of obvious importance and immense scope involves the incorporation of amino acid analogues into proteins. Incorporation is accomplished either by chemical synthesis or by biochemical synthesis using the protein assembly mechanisms of nature. Halogenated, especially fluorinated, amino acid analogues have been particularly useful tools in this research. In this section, selected examples of research using such analogues will be described to illustrate strategies and principles employed. 

Increasing Nutritional Value. Few direct attacks have been reported on increasing the nutritional value of proteins by those chemical modifications that incorporate substances in the protein. A successful exception has been the incorporation of amino acids into proteins. Since a separate presentation on this subject is given in this volume (113), only a brief discussion will be presented here. 

A number of workers have observed amino acids in lipide extracts, including those of microbial origin (11, 12, 17, 29). Recently, Macfar-lane (34) has reported that most of the phospholipide in Clostridium welchii is bound to amino acids and that some of this material occurs as the O-amino acid ester of phosphatidylglycerol. The relatively prominent occurrence of lipides in cell membranes has led to the recurrent suggestion that transport of hydrophilic substances through such membranes would be greatly facilitated by combination with hydrophobic substances. Consequently, most workers who have observed the incorporation of amino acids into lipide fractions quite naturally... 

In eukaryotic cells, the first step in the incorporation of amino acids into polypeptide chains requires... 

In addition to induction of specific proteins, the administration of biotin to deficient rats results in an overall two-fold stimulation of the incorporation of amino acids into proteins. The synthesis of serum albumin in liver is increased two-fold, but at least 10 other proteins show increases in amino incorporation of about five-fold, and some show an eight-fold increase, whereas others show no change (Dakshinamurti and Litvak, 1970 Boeckx and Dakshinamurti, 1974). 

Insulin stimulates protein biosynthesis by a mechanism which is not directly dependent upon its action in stimulating the uptake and accumulation of amino acids by the cells. Insulin increases the incorporation of amino acids into the protein of a number of tissues, an effect which is dependent on the presence of glucose. 

The activity of blasticidin S involves primarily the inhibition of fungal protein synthesis. In the presence of blasticidin S, the incorporation of amino acids into protein is reduced by 80-90% (Gottlieb et al., 19SS Misato et al., 1%1 Huang et al.. 1964a). It was foimd that blasticidin S reduces the formation of aminoacyl-tRNA. Hence the next step, the incorporation of the amino acid, is also block. The site of primary action is not yet known. 

Protein metabolism. Insulin stimulates transport and uptake of amino acids, as well as the incorporation of amino acids into proteins. 

T. Erdos and A. Ullmann, Effect of streptomycin on the incorporation of amino acids labeled with carbon-14 into ribonucleic acid and protein in a cell-free system of a Mycobacterium, Nature, 183 (1959) 618-619. 

The phenicols are transported into bacterial cells by passive or facilitated diffusion. They bind to the 505 subunit of the 705 bacterial ribosome and impair peptidyltransferase activity, thereby interfering with the incorporation of amino acids into newly formed peptides. Chloramphenicol also inhibits mitochondrial protein synthesis in mammalian bone marrow cells but does not significantly affect other intact cells. 

The question arises whether informatin is synthesized synchronously with RNA, but this has not yet been decided. Studies on the incorporation of amino acids into the SOS particles indicate that their proteins are labeled rather slowly (Samarina et al., 1967a). It thus seems probable that they may be repeatedly used in the cycle of RNA transfer, but further investigations are necessary to determine whether that is so. 

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