Folylpolyglutamate synthase: Difference between revisions

[[Folylpolyglutamate synthase]] or '''synthetase''' (FPGS) attaches gamma-glutamate units to various forms of folate (vitamin B9). In eukaryotic organisms, polyglutamylation keeps folate in the compartment, while folate lacking it can travel to other compartments. To be effective, antifolates (competitive inhibitors whose structure is similar to tetrahydrofolate) have to undergo polyglutamylation as well. Thus, defects in FPGS have consequences for the [[one-carbon metabolism]] relying on availability of folate as well as treatment of cancer relying on antifolates.

FPGS is active in intracellular folate homeostasis.  Polyglutamated folates are substrates for generation of the primary metyhl group donor S-adenosylmethionine (SAM)<ref>PMID:26895662</ref>, among others. The FPGS  enzyme catalyzes formation of an amide bond between the amino group of glutamate and the gamma carboxylate group of tetrahydrofolate (which contains a single glutamate unit) or of the free gamma carboxylate of the glumatamyl tail of polyglutamyl tetrahydrofolate. This reaction requires ATP; before the amide bond forms, the carboxylate reacts with ATP to form a phosphate ester intermediate and ADP. In the second step, phosphate is the leaving group as the amide bond forms. The deconjugation reaction (i.e. removing glutamate units) is catalyzed by a hydrolase (folate gamma-glutamyl hydrolase, or GGH) and proceeds independent of ATP. Apart from reacting with various folates, FPGS also acts on anti-folates (competitive inhibitores of enzymes that act of folates). The polyglutaminylation of anti-folates makes them effective inhibitors.

Folylpolyglutamate synthase is a single subunit enzyme with two domains (reload <scene name='74/748269/Cv/7'>initial scene</scene>). The three substrates (folate, ATP, glutamate) bind on the same face in a <scene name='74/748269/Conserved/1'>highly conserved patch</scene> near the domain boundary. Once <scene name='74/748269/Folate/2'>folate is bound</scene>, the enzyme undergoes a <scene name='74/748269/Folate_binding/5'>conformational change</scene> into the active form.  

The <scene name='74/748269/Glu/3'>Yersinia pestis structure</scene> 3qcz, shows glutamate bound near ATP while the folate binding site is empty. There is no structure yet with all three substrates bound simultaneously.

The nucleotide-binding pocket of FPGS occupies a <scene name='74/748269/Cv/4'>narrow channel between the N- and C-terminal domains</scene> of the protein and <scene name='74/748269/Cv/6'>contains the nucleotide and a divalent ion</scene><ref>PMID:18566510</ref>.