|Description|| At least three pathways (alanine biosynthesis I, alanine biosynthesis II, and alanine biosynthesis III) contribute to the synthesis of alanine.
Taxonomic distribution for the three pathways are as follows:
alanine biosynthesis I: archaea, eubacteria, and eukaryotes
alanine biosynthesis II: archaea and eukaryotes
alanine biosynthesis III: archaea, eubacteria, and eukaryotes.
A - main eubacterial pathway with the key enzyme AtvA ( Valine--pyruvate aminotransferase) – “Alanine biosynthesis I pathway”;
A.120 - Alanine biosynthesis I pathway + alanine biosyntheis III pathway ( cysteine desulfurase) = IlvE + AvtA + IscS
A.123 - Alanine biosynthesis I pathway + alanine biosyntheis III pathway ( cysteine desulfurase) + alanine racemase = IlvE + AvtA + IscS + *AlaR;
A.1234 – All above + AlaB (glutamate-pyruvate aminotransferase pathway (eukaryotic);
B - Alanine biosyntheis III pathway, mediated by cysteine desulfurase (archaea, eubacteria, and eukaryotes) = IscS;
C - Alanine biosyntheis III pathway( cysteine desulfurase) and alanine racemase;
IscS + *AlaR, with IlvE or without
D - Alanine biosynthesis II - glutamate-pyruvate aminotransferase pathway (eukaryotic) = AlaB;
E – just alanine racemase present
|Notes|| Alanine biosynthesis I is established only by existence of the relevant enzymes. Its contribution to alanine synthesis remains speculative because alanine auxotrophs have not yet been isolated. Because alanine but not valine represses AvtA, its primary purpose is probably synthesis of L-alanine.
The alanine biosyntheis III pathway, mediated by cysteine desulfurase activity is required to donate sulfane sulfur for the synthesis of Fe-S clusters, thiamine, thionucleosides in tRNAs, biotin, lipoic acid, and molybdopterin probably contributes only a minor amount of the cell's alanine requirement, as judged by the cell's total requirement for sulfane sulfur.
In many eubacteria, alanine racemase occurs as a homodimer. These subunits constitute a major and minor component. In E coli these genes are denoted as dadX (major) and alr(minor).
Bioinformatic analyses currently recognize the AvtA structure as similar to the class I or class I/II structures of ATs, which represent the most common types of ATs present in C. glutamicum. Nevertheless, AvtA is the only AT with exceptionally high activity toward L-alanine as an amino donor instead of preferably using L-glutamate.
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