Phosphoribosylamine—glycine ligase
class of enzymes
Phosphoribosylamine—glycine ligase, also known as glycinamide ribonucleotide synthetase (GARS), (EC 6.3.4.13) is an enzyme that catalyzes the chemical reaction
ATP + 5-phospho-D-ribosylamine + glycine
⇌
{\displaystyle \rightleftharpoons }
ADP + phosphate + N1-(5-phospho-D-ribosyl)glycinamide
which is the second step in purine biosynthesis.
The 3 substrates of this enzyme are ATP, 5-phospho-D-ribosylamine, and glycine, whereas its 3 products are ADP, phosphate, and N1-(5-phospho-D-ribosyl)glycinamide.
This enzyme belongs to the family of ligases, specifically those forming generic carbon-nitrogen bonds.
In bacteria, GARS is a monofunctional enzyme (encoded by the purD gene). The purD genes often contain PurD RNA motif in their 5' UTR. In yeast, GARS is part of a bifunctional enzyme (encoded by the ADE5/7 gene) in conjunction with phosphoribosylformylglycinamidine cyclo-ligase (AIRS). In higher eukaryotes, including humans, GARS is part of a trifunctional enzyme in conjunction with AIRS and with phosphoribosylglycinamide formyltransferase (GART), forming GARS-AIRS-GART.
Nomenclature
The systematic name of this enzyme class is 5-phospho-D-ribosylamine:glycine ligase (ADP-forming). Other names in common use include:
phosphoribosylglycinamide synthetase
glycinamide ribonucleotide synthetase
phosphoribosylglycineamide synthetase
glycineamide ribonucleotide synthetase
2-amino-N-ribosylacetamide 5'-phosphate kinosynthase
5'-phosphoribosylglycinamide synthetase
GAR synthetase
Mechanism
GARS operates via an ordered, sequential mechanism. 5-phospho-D-ribosylamine (PRA) binds first, then ATP, and finally glycine. Phosphate is released first, followed by ADP and GAR. The oxygen in the ribose ring of PRA is important in substrate binding, likely due to favorable energetics from hydrogen bonding and the ring conformation it confers. In addition, the phosphate group of GAR has been implicated in GARS substrate recognition.
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