Comparison of polymerization holes “b.” Structures of (A) rfD-BβD432A+GH and (B) normal with both GPRP and GHRP (rfD+BOTH). For both structures, the difference electron density |F_o − F_c| is contoured at 3.0σ and is shown in green. The |2F_o − F_c| electron density maps are contoured at 1.0σ shown in gray. Clear positive difference electron density for GHRP is evident in hole “b” of normal rfD+BOTH and is absent in the rfD-BβD432A+GH structure. This suggests that BβD432A has an impaired hole “b” and does not bind the peptide. Note the positions of BβGlu397 and BβAsp398. In normal rfD+BOTH, both residues flip toward hole “b” to interact with GHRP. In rfD-BβD432A+GH, these residues are nowhere near hole “b.”