Amino acids transformation

The association between vitamin B6 deficiency and transamination emerged from 1945 when Schlenk and Fisher noted that pyridoxine-deficient rats had a diminished capacity for transamination. In the same year Gunsalus and his colleagues found transamination in Streptococcus faecalis depended on pydridoxal phosphate. The properties of the heat-stable component in purified glutamic-oxaloacetate transaminase were similar to those of pydridoxal phosphate. Later pyri-doxal phosphate was established as an essential coenzyme in many amino acid transformations. 

Pyridoxine (B ) Pyridoxal phosphate Pyridoxamine phosphate Amino acid transformations... 

The amino acid and nucleotide biosynthetic pathways make repeated use of the biological cofactors pyridoxal phosphate, tetrahydrofolate, and A-adenosylmethionine. Pyridoxal phosphate is required for transamination reactions involving glutamate and for other amino acid transformations. One-carbon transfers require S-adenosyhnethionine and tetrahydrofolate. Glutamine amidotransferases catalyze reactions that incorporate nitrogen derived from glutamine. 

Transamination is just one of a wide range of amino acid transformations that are catalyzed by PLP enzymes. The other reactions catalyzed by PLP enzymes at the a-carbon atom of amino acids are decarboxylations, deam-inations, racemizations, and aldol cleavages (Figure 23.12). In addition, PLP enzymes catalyze elimination and replacement reactions at the P-carbon atom (e.g., tryptophan synthetase Section 24.2.11) and the y-carbon atom (e.g., cytathionine P-synthase, Section 24.2.9) of amino acid substrates. Three common features of PLP catalysis underlie these diverse reactions. 

TMany of the PLP enzymes that catalyze amino acid transformations, such as serine hydroxymethyltransferase, have a similar structure and are clearly related by divergent evolution. Others, such as tryptophan synthetase, have quite different overall structures. Nonetheless, the active sites of these enzymes are remarkably similar to that of aspartate aminotraas-ferase, revealing the effects of convergent evolution. 

Several different kinds of amino acid transformations are catalyzed by PLP-requiring enzymes. The most common are decarboxylation, transamination, racemization (the interconversion of L- and D-amino acids), C —bond cleavage, and a,j8-elimination. 

Explain why the ability of PLP to catalyze an amino acid transformation is greatly reduced if a PLP-requrring enzymatic reaction is carried out at a pH at which the pyridine nitrogen is not protonated. 

The vitamin Be group of coenzymes consists of pyridoxine, pyridoxal, and pyridoxamine and their metabolically active phosphorylated forms. They are striking for the variety of enzymic reactions in which they are important, and many amino acid transformations, including various transaminations and decarboxylations, are vitamin B dependent. Compounds with vitamin B activity are apparently not stored in the body in large amounts, and biochemical evidence of B deficiency can develop quickly if intake is inadequate (S4). 

Vitamin Bg The Vitamin Needed for Amino Acid Transformations 1151... 

VITAMIN Bj THE VITAMIN NEEDED FOR AMINO ACID TRANSFORMATIONS... 

Pyridoxal 5 -phosphate (PLP, Figure 14.32) is a cofactor of enzymes that are responsible for amino acids transformations such as racemization, transamination, and decarboxylation. Its enzymology has been reviewed recently [86, 87]. When PLP is embedded in an enzyme it usually forms the so-called internal aldimine with the e-group of a lysine residue in the active site, or an external aldimine with amino acid substrates or inhibitors. The similarity with the Schiff... 

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