GMP synthetase enzyme, molecular model. This enzyme, guanine monophosphate synthetase, catalyses the reaction that converts xanthosine monophosphate to guanosine monophosphate. This is a glutamine-hydrolysing reaction. This enzyme plays a key role in the metabolism of purines, widely involved in nucleic acid synthesis and hence in genetic materials. It is thought that genetic defects in such enzymes can lead to an increased risk of genetic defects and conditions such as cancer. The structure here consists of two protein chains (purple and red).