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GMP reductase EC 1.7.1.7 is an enzyme that catalyzes the irreversible and NADPH-dependent reductive deamination of GMP into IMP.
NADPH + guanosine 5-phosphate = NADP + inosine 5-phosphate + NH3
By converting guanosine nucleotides to inosine nucleotides, which serve as precursors to both adenosine and G nucleotides, it helps maintains intracellular balance of A and G nucleotides. GMP can be broken down by other enzymes, but GMPR catalyzes the only recognized route for converting GMP to AMP. Whereas the conversion of GMP to IMP involves a single enzyme, GMPR, the conversion of IMP to GMP involves two enzymes. First, inosine monophosphate dehydrogenase catalyzes the conversion of IMP to XMP; then GMP synthetase catalyzes the conversion of XMP to GMP. These two pathways are inversely regulated, with conditions favoring IMPDH expression decreasing GMPR expression. In melanocytic cells, GMP reductase gene expression may be regulated by MITF. It is activated by GTP and inhibited by xanthosine 5'-monophosphate.
The amino acid sequence that makes up the GMP reductase is similar across organisms. In humans, there are hGMPR1 and hGMPR2, 2 GMP reductases that are different in their amino acid sequence but has the same function overall. Although hGMPR1 and hGMPR2 do not have an identical amino acid sequence, they have similar kinetic properties and they both use NADPH as a coenzyme for their catalyzed reaction. Aside from human erythrocytes, GMPR has been isolated from E.coli as well as rodents.