Biosynthesis of the Diguanosine Nucleotides. II. Mechanism of Action of GTP:GTP Guanylyltransferase on Nucleotide Metabolism in Brine Shrimp Embryos

Abstract

The enzyme GTP:GTP guanylyltransferase (Gp₄G synthetase) from yolk platelets of embryos of the brine shrimp, Artemia salina, catalyzes the synthesis of P¹P⁴-diguanosine 5′-tetraphosphate (Gp₄G) and pyrophosphate (PP_i) from 2 equiv of GTP in a freely reversible reaction. The Michaelis constant (K_m) for GTP in the forward reaction is 2.2 mM, whereas the K_m for Gp₄G and PP_i in the reverse reaction is 1.06 mM and 0.84 mM, respectively. Also, the yolk platelet enzyme catalyzes the synthesis of P¹P³-diguanosine 5′-triphosphate (Gp₃G) and GTP from equivalent amounts of GDP and Gp₄G. Both GDP and Gp₃G inhibit the synthesis of Gp₄G and the kinetic data suggest that either two enzymes are present in the Gp₄G synthetase preparations or that two catalytic sites exist on one protein, one for the synthesis of Gp₃G and one for the synthesis of Gp₄G. Almost 80% of the Gp₄G synthetase in embryos of Artemia salina is localized in yolk platelets, and a small amount of activity is found in the mitochondrial fraction.