Homocysteine biosynthesis control

Homocysteine biosynthesis requires the confluence of two pathways, one providing the 4-carbon moiety via 0-phosphohomo-serine, the other providing the sulfur moiety via cysteine (Fig. 3), Sach of the converging pathways contains two branch points, and additional branch points occur at threonine, methionine and AdoMet. A priori, two predictions can be made regarding the control patterns of homocysteine biosynthesis in the system. One is that the control patterns will be complex, in order that a fine balance can be maintained between the interlocking and multiply branched pathways. The second is that novel control patterns may occur... 

The biosynthesis of adenosine is theoretically controlled by several processes namely (1) the biosynthesis of adenosine from AMP by 5 -nucleotidase [EC 3.1.3.5], (2) from S-adenosyl homocysteine by S-adenosyl homocystine hydrolase [EC 3.3.1.1], (3) the metabolism of adenosine to AMP by adenosine kinase [EC 2.7.1.20], and (4) to inosine by adenosine deaminase (ADA) [EC 3.5.4.2], Interestingly, both 5 -nucleotidase and ADA activities were found to be highest in the leptomeninges of rat brain in contrast, the adenosine kinase activity was widely distributed throughout the brain parenchyma, which has negligible ADA activity... 

---