FIG. 4.

Spo0F:Spo0B association and phosphoryl transfer. (a) Crystal structure of the Spo0B:Spo0F complex viewed perpendicularly to the four-helix bundle. The residues His30 of Spo0B and Asp54 of Spo0F are in close proximity for phosphoryl transfer (reproduced, with permission, from reference 22). (b) A model for the transition state intermediate, produced using the crystal structure of the Spo0F:Spo0B complex by placing a phosphoryl group between the active His30 and Asp54. The P-to-O and P-to-N distances are 2.45 Å each. The model shows that the environment produced by the association of the two molecules stabilizes the transition state interactions through suitable electrostatic and hydrogen-bonding interactions. The phosphorus atom forms partial covalent bonds with O^δ of Asp and N^ɛ of His and is in a pentacoordinated state. The negative charges on the phosphoryl oxygens are compensated through interactions with Mg²⁺ and Lys104 (reproduced, with permission, from reference 29).