Asparagine essentiality

The Src SH2 domain typifies a large number of those characterized to date. The pTyr fits into a pocket on the opposite side of the central sheet to the pY-r3 pocket (Figure 13.27a). All known SH2 domains bind pTyr in essentially the same way, but some have a different pattern of contacts for the residues that follow. For example, in the Grb2 SH2 domain, a tryptophan side chain from the small sheet fills the pY-r3 pocket, and the bound peptide takes a different course, with important interactions to an asparagine at pY-r2. Screens of peptide libraries have detected the importance of this asparagine. The SH2 domain from PFC-yl contacts five mainly hydrophobic residues that follow pTyr. 

Until recently, the catalytic role of Asp ° in trypsin and the other serine proteases had been surmised on the basis of its proximity to His in structures obtained from X-ray diffraction studies, but it had never been demonstrated with certainty in physical or chemical studies. As can be seen in Figure 16.17, Asp ° is buried at the active site and is normally inaccessible to chemical modifying reagents. In 1987, however, Charles Craik, William Rutter, and their colleagues used site-directed mutagenesis (see Chapter 13) to prepare a mutant trypsin with an asparagine in place of Asp °. This mutant trypsin possessed a hydrolytic activity with ester substrates only 1/10,000 that of native trypsin, demonstrating that Asp ° is indeed essential for catalysis and that its ability to immobilize and orient His is crucial to the function of the catalytic triad. 

L-Asparaginase, an enzyme derived from E. coli or Erwinia carotovora, has been employed in cancer chemotherapy where its selectivity depends upon the essential requirement of some tumours for the amino acid L-asparagine. Normal tissues do not require this amino acid and thus the enzyme is administered with the intention of depleting tumour cells of asparagine by converting it to aspartic acid and ammonia. Whilst L-asparaginase showed promise in a variety of experimentally induced tumours, it is only useful in humans for the treatment of acute lymphoblastic leukaemia, although it is sometimes used for myeloid leukaemia. 

Antitumor enzyme, hydrolyzes L-asparagine in bloodstream, depriving tumor cells of the essential amino acid results in inhibition of protein, DNA, and RNA synthesis and cell proliferation derived from Escharichia coli... 

The non-essential amino acids are alanine, arginine, aspartate, asparagine, cysteine, glutamate, glutamine, glycine, proline, serine and tyrosine. A summary of the reactions involved in their synthesis is given in Figure 8.3 and full details of these pathways are provided in Appendix 8.2. 

Most healthy (untransformed) mammalian cells are capable of directly synthesizing asparagine from glutamine (Figure 9.22). Hence, asparagine is generally classified as a non-essential amino acid (i.e. we do not require it as an essential component of our diet). However,... 

It destroys essential amino acid (asparagine) hence leukaemic cells are deprived of amino acid and leads to cell... 

C. acetobutylicum is not too exacting in its growth requirements. Asparagine is needed to effect normal production of solvents in what otherwise would be an acid fermentation. Both biotin and p-aminoben-zoic acid are required in trace quantities, 0.001 and 0.05 /xg/ml, respectively. Iron is essential in small but variable quantities for attainment of maximal fermentation rates (Leviton 1946,1949 Meade et al. 1945). The requirement for iron varies with the composition of the medium potassium is also required (Davies 1942A.B, 1943 Davies and Ste-... 

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