Sandbox Reserved 342: Difference between revisions
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== '''XANTHINE-GUANINE PHOSPHORIBOSYLTRANSFERASE''' == | == '''XANTHINE-GUANINE PHOSPHORIBOSYLTRANSFERASE''' == | ||
__TOC__ | __TOC__ | ||
=Introduction= | =Introduction= | ||
Xanthine-guanine Phosphoribosyltansferase (XGPRT) is one of three purine phosphoribosyltransferases (PRTases) that are part of the purine salvage pathway in Escherichia coli<ref name="Vos"> PMID:9743633 </ref>; the other two PRTases in the pathway are HPRT and APRT<ref name="Vos"/> | Xanthine-guanine Phosphoribosyltansferase (XGPRT) is one of three purine phosphoribosyltransferases (PRTases) that are part of the purine salvage pathway in Escherichia coli<ref name="Vos"> PMID:9743633 </ref>; the other two PRTases in the pathway are HPRT and APRT<ref name="Vos"/> | ||
=Structure= | =Structure= | ||
XGPRT is a tetramer<ref name="Vos"/>. | |||
PRTase structures fall into two groups, type I and Type II<ref name="Vos"/>. | PRTase structures fall into two groups, type I and Type II<ref name="Vos"/>. | ||
XGTPase has a conserved sequence,<scene name='Sandbox_Reserved_342/Prib-pp_binding_site/1'>85-IVIDDLVDTG-94</scene>, which is called the PRib-PP (5-phospho-a-D-ribosyl-1-pyrophosphate) binding site<ref name="Vos"/>. This binding site features two adjacent acidic residues, which are surrounded by hydrophobic residues<ref name="Vos"/>. There is a five-stranded b-sheet surrounded by three or four a-helices that creates a conserved structural core containing the PRib-PP binding site<ref name="Vos"/>. Another region of the sequence in XGTPase forms a lobe, which is involved in substrate recognition<ref name="Vos"/>. | XGTPase has a conserved sequence,<scene name='Sandbox_Reserved_342/Prib-pp_binding_site/1'>85-IVIDDLVDTG-94</scene>, which is called the PRib-PP (5-phospho-a-D-ribosyl-1-pyrophosphate) binding site<ref name="Vos"/>. This binding site features two adjacent acidic residues, which are surrounded by hydrophobic residues<ref name="Vos"/>. There is a five-stranded b-sheet surrounded by three or four a-helices that creates a conserved structural core containing the PRib-PP binding site<ref name="Vos"/>. Another region of the sequence in XGTPase forms a lobe, which is involved in substrate recognition<ref name="Vos"/>. | ||
=Function= | =Function= | ||
XGPRT is an enzyme that catalyzes the conversion of guanine, xanthine, and sometimes hypoxanthine, to GMP, XMP, and IMP <ref name="Vos"/>. | |||
<Structure load='1a96' size='300' frame='true' align='right' caption='Insert caption here' scene='Insert optional scene name here' /> | <Structure load='1a96' size='300' frame='true' align='right' caption='Insert caption here' scene='Insert optional scene name here' /> | ||
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=MECHANISM= | =MECHANISM= | ||
In the large mobile loop of the | In the large mobile loop of the XGPRT, there are several residues that are critical for substrate binding and catalysis<ref name="Vos"/>. | ||
==Recognition== | ==Recognition== | ||
==Catalysis== | ==Catalysis== | ||
Magnesium and other divalent cations are necessary for catalysis<ref name="Vos"/>. | Magnesium and other divalent cations are necessary for catalysis because magnesium and PRib-PP binding play a critical role for the PRTase reaction<ref name="Vos"/>. The Mg:PRib-PP complex binds to the active site of PRTases<ref name="Vos"/>. In type I PRTases, XGPRT, catalysis proceeds via SN1 mechanism and it forms a oxocarbonium ion in the transition state<ref name="Vos"/>. It has been suggested, that the magnesium ion departs with the displaced pyrophosphate because there is no magnesium ion at the active site<ref name="Vos"/>. | ||
=References= | =References= | ||
<references/> | <references/> | ||