Biochemistry:Linking of pterin, p-aminobenzoic acid and glutamate
Each of the three precursors of folic acid supplements in large map of cellular metabolism, can take different roads and be addressed in metabolic pathways that lead to the biosynthesis of a large number of diverse compounds, as both structure and biological value. Within the biosynthesis of vitamin B9 enzymes are used to "tie" the acid p-amino benzoic acid, neopterin and glutamate, finalizing the synthesis of one molecule of folic acid. The enzymatic chain of synthesis of folate, in the space of the eukaryotic cells, is present within the mitochondria (11). With the exception of the "place of synthesis", since prokaryotes lack mitochondria or other structure comparable to them, assembling of folic acid supplements, from the standpoint of enzyme, proceed in a similar way as in plants than in bacteria. The reactions that are here described are part of the so-called "late" stage of the biosynthesis of folic acid, to mark the distance from "early" stages that delimits the genesis of three biological precursors of vitamin B9.
Looking at Figure 1, you can sequentially subdivide the folic acid synthesis in three steps: formation of dihydropteroic acid, formation of folic acid and reduction of dihydrofolate (DHF) into tetrahydrofolate (THF) that, as previously stated, is the biologically active form of folate. The underlying structure, however, shows the condensation points of three precursors and related enzymes involved.
Figure 7 Schematization of enzymes that lead to the folate biosynthesis
Biosynthesis of diidropteroico acid. The formation of dihydropteroic acid, has been well studied and characterized in Escherichia coli. The first enzyme (EC 220.127.116.11), member of the classe of kinase, ATP dependent, catalyzes the reaction that leads to the formation of an ester of diidroneopteridin pyrophosphate (Figure 8).
Figure 8 dihydroneopterin phosphorylation in dihydroneopterin-diphosphate
The enzyme which "links" the acid p-aminobenzoic acid to the diphosphate-diidroneopterin takes the name of dihydropteroate synthase (EC 18.104.22.168); condensation happens in ATP-dependent reaction. The reaction is turned away a molecule of pyrophosphate and one AMP (12).
Mammals have lost the ability to condense the pterin with p-amino benzoic acid because it does not encode the enzyme diidropteroato synthetase.
Figure 7.8-9 biosynthesis of dihydropteroate
The structure of the enzyme in E. coli, is proposed below. Is possible to see a part of enzyme at the level of catalic site.
Figure 10 "ribbon" of dihydropteroate synthetase in E. coli
It was experimentally deduced that, at the level of the active site of the enzyme dihydropteroate synthetase, there exists a sequence of amino acids that expose, in a specific site, residues with positive charge. In this small area is tied the carboxylic anion of the p-amino benzoic acid(13).
Figure 11 Dihydropteroato synthase, schematization
A Nucleophilic attack of the amine nitrogen to carbon of the pterin determines the movement of electrons to oxygen binding and the dismissal of phosphoric radical, as shown in figure above.
Biosynthesis of dihydrofolate. The second step catalyzes the joining of one molecule of glutamate to a molecule of dihydropteroate, whose summary was analysed in the previous paragraph. The dihydropteroate reacts with glutamate in a reaction catalysed by the enzyme dihydrofolate synthetase, and DHFS, ATP-dependent protein (EC 22.214.171.124).
The reaction of the biosynthesis of dihydrofolate is summarized as follows:
Figure 12 the biosynthesis of dihydrofolate
For some years now, the attention of pharmaceutical research focuses on the operation of dihydrofolate synthase with the aim to refine the methodologies of paramount importance for the inactivation of this enzyme. As an example it is possible to see the "focus" of modern research against malaria, a potentially lethal disease caused by the parasite Plasmodium falciparum that in 2010 has caused over 650,000 deaths in the African territory. The proliferation of lethal Plasmodium is an example of how the inactivation of the enzyme could serve to eradicate, or at least contain, the lethal action of parasite. In this regard it is useful to point out that, in P. falciparum, dihydrofolate synthase is paired, as in other organisms, for another protein: the folipoluglutamate synthase (FPGS) serving for the addition of the polyglutamate tail.
The total absence of the enzyme, at human level, makes plausible a scenario in which a high specificity molecule can inhibit or slow, as a result of direct competition of the substrate, the enzyme dihydrofolate synthase. Developing a chemotherapy with these characteristics, may help to find a molecule with low toxicity to humans, unless further investigation of pharmacokinetics, nature (14)