title: eIF-2B and the exchange of guanine nucleotides bound to eIF-2.
abstract: Available data for the formation of the ternary complex eIF-2 X GTP X methionyl-tRNAi involved in eukaryotic initiation and of the inhibition of ternary complex formation by GDP have been examined with a view to determining the mechanism by which eIF-2B facilitates nucleotide exchange. Two mechanisms have been considered--first a displacement reaction in which eIF-2B displaces GDP and GTP in a manner analogous to a "ping-pong" enzyme mechanism, and secondly the possibility that binding of eIF-2B to eIF-2 nucleotide complexes enhances the rate of nucleotide exchange without itself inducing nucleotide displacement. Comparison has been made between the properties of eIF-2 and eIF-2B and of the bacterial elongation factors Tu and Ts. It seems most probable that, as previously suggested by others for Ts, eIF-2B effectively catalyses an exchange reaction through a "ping-pong" type mechanism. Possible explanations of data suggesting otherwise are put forward. Both eIF-2 and bacterial Tu are complex allosteric proteins subject to a variety of influences which in the case of eIF-2 include phosphorylation of the alpha subunit. This phosphorylation appears to change the equilibria in the reaction mechanism such that the transferred entity (eIF-2) becomes firmly bound to the catalyst (eIF-2B). Minimum rate constants for the formation of eIF-2 X eIF-2B from eIF-2 X GDP and eIF-2 X GTP and reverse reactions are derived. These values suggest that the initiation factors are likely to have to operate in a restricted environment if rates of protein synthesis seen in vivo are to be sustained.
mesh_terms: D015852:Eukaryotic Initiation Factor-2; D006150:Guanine Nucleotides / Q000378:metabolism*; D006153:Guanosine Diphosphate / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism*; D007700:Kinetics; D046911:Macromolecular Substances; D010448:Peptide Initiation Factors / Q000378:metabolism*; D010766:Phosphorylation; D011506:Proteins / Q000378:metabolism*; D012346:RNA, Transfer, Amino Acyl / Q000378:metabolism
chemical_list: D015852:Eukaryotic Initiation Factor-2; D006150:Guanine Nucleotides; D046911:Macromolecular Substances; D010448:Peptide Initiation Factors; D011506:Proteins; D012346:RNA, Transfer, Amino Acyl; C026464:tRNA(m)(Met), methionine-; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate