Computational Structural Analysis and Kinetic Studies of a Cytosolic Glutamine Synthetase from Camellia sinensis (L.) O. Kuntze

Structural analysis of a cytosolic glutamine synthetase from Camellia sinensis (CsGS) has been conducted employing computational techniques. This was conducted to compare its structural aspects with other known structures of GS. The disordered residues and their distribution in CsGS are in close comparison to earlier reported GS. The 3-D structure of CsGS also showed high degree of similarity with the only known crystal structure of GS from Zea mays. The K _m values observed with recombinant CsGS for all the three substrates are higher compared to rice, Arabidopsis, maize and human. This suggests lower affinity of CsGS for substrates. Further, kinetic mechanism of CsGS catalysis was investigated using initial velocity analysis and product inhibition studies. Initial velocity data eliminate the possibility of ping-pong mechanism and favor the random mechanism of catalysis. Through product inhibition studies, ADP was found to be a competitive inhibitor with respect to ATP and noncompetitive inhibitor versus both glutamate and ammonium. While, glutamine and inorganic phosphate were found to be non-competitive inhibitors of ATP, glutamate and ammonia. Taken together, these observations are consistent with a random catalysis mechanism for the CsGS where the binding order of certain substrates is kinetically preferred by the enzyme.