Glycine decarboxylase serine synthesis

Some acetogenic bacteria, which convert CO2 to acetic acid, form pyruvate for synthesis of carbohydrates, etc., by formation of formaldehyde and conversion of the latter to glycine by reversal of the PLP and lipoic acid-dependent glycine decarboxylase, a 4-protein system. The glycine is then converted to serine, pyruvate, oxaloacetate, etc. Propose a detailed pathway for this sequence. 

The synthesis of one glycine and two units of formyltetrahydrofolate via the pathway involving serine described above would lead to the production of excess ycine [i.e., 2 serine 2 glycine -b 2 formyltetrahydrofolate). The excess glycine could be used to resynthesize serine through the action of glycine synthetase (Fig. 4). There is no experimental evidence for the presence of ycine synthetase (or glycine decarboxylase) in nodules. 

The a-oxoamine synthases family is a small group of fold-type I enzymes that catalyze Claisen condensations between amino acids and acyl-CoA thioesters (Figure 16). Members of this family are (1) 8-amino-7-oxononanoate (AON) synthase (AONS), which catalyzes the first committed step in the biosynthesis of biotine, (2) 5-aminolevulinate synthase (ALAS), responsible for the condensation between glycine and succinyl-CoA, which yields aminolevulinate, the universal precursor of tetrapyrrolic compounds, (3) serine palmitoyltransferase (SPT), which catalyzes the first reaction in sphingolipids synthesis, and (4) 2-amino-3-ketobutyrate CoA ligase (KBL), involved in the threonine degradation pathway. With the exception of the reaction catalyzed by KLB, all condensation reactions involve a decarboxylase step. 

As mentioned before, one of the main drawbacks in the application of threonine aldolases is their lack of erithro/threo selectivity (kinetic limitation) and their equilibrium position (thermodynamic limitation). Recently, a tandem use of LD-threonine aldolases with low selectivity and L-amino acid decarboxylases with high selectivity has demonstrated to overcome the kinetic and thermodynamic limitations in the synthesis of phenyl serine (Steinreiber et al. 2007). Starting with benzalde-hyde and glycine, i -phenyl ethanol was obtained in 58% isolated yield and R enantiomeric excess higher than 99% by the action of L-threonine aldolase (L-TA) from Pseudomonas putida, D-threonine aldolase (D-TA) from Alcaligenes xylosoxidans and L-tyrosine decarboxylase (L-TyrDC) from Enterococcus faecalis following the scheme depicted in Fig. 6.5.17. 

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