TABLE 2.

Comparison of system-level and individual-factor GTP and ATP affinities^a

Reaction	KmGTP (μM)	KmATP (μM)
Protein synthesisb	14	27
IF-2 activation	2
EF-Tu recycling	10
EF-G activation	10
RF-3 activation	2.5
Tyrosyl-tRNA acylation		4
Methionyl-tRNA acylation		430
Serinyl-tRNA acylation		100
Leucyl-tRNA acylation		130-720
Tryptophanyl-tRNA acylation		300

^aThe overall system dependence on [GTP] agrees well with the individually measured affinities of the four GTP binding proteins known to catalyze bacterial protein synthesis (IF-2, EF-Tu, EF-G, and RF-3). However, the overall system dependence on [ATP] is inconsistent with the individually measured affinities of aminoacyl-tRNA synthetases (which catalyze the most energy-demanding steps of protein synthesis). For references see the text. The K_m^ATPs for tRNA acylation reactions are representative (for a more complete list, see references 3 and 17).

^bData for protein synthesis are from the cell-free experiments described in this communication.