Structural differences between the active site in the A subunit and the pseudo-active site in the B subunit. The A subunit’s active site has a P-loop (GX 1 X 2 X 3 X 4 GK(T/S)), which is a well-known motif for binding to the phosphate group of ATP, and has a space to bind ATP. On the other hand, the B subunit’s pseudo-active site does not have either the P-loop or space for ATP binding. The B subunit has a loop (that is, the pseudo P-loop) at the position corresponding to the P-loop in the A subunit. The amino acid sequence and backbone geometry of the pseudo P-loop are different from the typical ones of P-loops, and the space around the pseudo P-loop is filled with side chains.
