G proteins such as p21ras are designed to hydrolyze the bound GTP very slowly, so as to maintain a special conformation that enables them to bind and activate various cellular receptors and protein kinases. A GTPase activating protein (GAP) can increase this hydrolytic step by up to five orders of magnitude. For this feature these enzymes are called V-type enzymes, even though many G proteins also have significant changes in Kd for the bound substrate or allosteric effector. GAP is an inhibitor, since by activating the GTPase rate it limits the time for the protein to be active. Guanine nucleotide exchange factors (GEFs) lower the affinity for the tightly bound GDP and promote replacement by GTP.
KeywordsGuanine Nucleotide GTPase Activity Active Conformation Guanine Nucleotide Exchange Factor GTPase Activate Protein
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