Amino acids - continued

Feedback inhibition of amino acid transporters by amino acids synthesized by the cells might be responsible for the well known fact that blocking protein synthesis by cycloheximide in Saccharomyces cerevisiae inhibits the uptake of most amino acids [56]. Indeed, under these conditions, endogenous amino acids continue to accumulate. This situation, which precludes studying amino acid transport in yeast in the presence of inhibitors of protein synthesis, is very different from that observed in bacteria, where amino acid uptake is commonly measured in the presence of chloramphenicol in order to isolate the uptake process from further metabolism of accumulated substances. In yeast, when nitrogen starvation rather than cycloheximide is used to block protein synthesis, this leads to very high uptake activity. This fact supports the feedback inhibition interpretation of the observed cycloheximide effect. 

Amino acids continue to be useful starting materials for the preparation of enantiomerically-pure heterocycles. Henk Hiemstra of the University of Amsterdam and Floris Ruljes of the University of Nijmegen report (J. Am. Chem. Soc. 2004,126,4100) that cyclization of the ally silane 9 followed by ring-closing metathesis leads to the highly-functionalized quinolizidine 11. 

Amino acids, amino esters, and related compounds, a-Amino acids (Continued)... 

For amino acids (continued) Ammonium formate 4.10 138 ml 2 M Formic acid 200 ml 1 M NH4OH (H13)... 

Leucine dH is the enzyme used as the biocatalyst in the process commercialized by Degussa AG (Hanau, Germany) to produce L-tert-leucine (L-Tle).28-60 This UAA has found widespread use in peptidomimetic drugs in development, and the demand for this unique amino acid continues to increase.61-62 This process, which has been the subject of much study, requires a co-factor recycling system (Scheme 19.4, R = Me3C).63-64 Similar to phenylalanine dH, leucine dH has been used to prepare numerous UAAs because of its broad substrate specificity.43-65-66... 

FIGURE 1,19 Synthesis of a polypeptide on the endoplasmic reticulum (ER). The black oval represents a ribosome the line represents a newly made polypeptide chain, (1) Ribosome arrives at EK- (2J Amino terminus of the growing protein is pushed through the membrane of the ER. (3) Amino acids continue to be polymerized at the carboxy terminus of the growing protein. 

Amino acids continue to be polymerized at the carboxy terminus of the growing protein. 

Table 12.1.2. The main pyrolysis products of several amino acids (continued).

Heterocyclic amino acids (continued) CH,—CHCO fim 0 Histidine His H 2.1%... 

MSX occurred within 15 s at 37 C and was 95% effective vdthin the first minute of uptake (compare Figures 6A and B), it is not currently possible to state whether (i) that 95% represents assimilation into amino acids and the remaining 5% uptake as NH4 (ii) the MSX block was only 95% effective and 5% residual incorporation into amino acids continued or (iii) a NH4 transport system is rate-limiting for uptake plus accumulation but inhibited by MSX or regulated rapidly by some intermediate that is itself sensitive to MSX inhibition. 

Although a majority of these less common a-amino acids also belong to the L series, D-amino acids are nevertheless widely distributed in nature, and many have been recognized in both higher and lower organisms. Thus, as new amino acids continue to be discovered, the establishment of their absolute stereochemistry, and especially that at the asymmetric a-carbon, is an important task in the course of their complete structure determination. 

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