8885ac19690-7503/1/0411:32 AM Page711mac76mac76:385 19.2 ATP Synthes 711 ADP ADP, and the third was empty. The corresponding B inner circle is made up of the amino-terminal helices of subunit conformations are designated B-ATP, B-ADP, each c subunit; the outer circle, about 55 A in diame- and B-empty(Fig. 19-23c). This difference in nucleo- ter, is made up of the carboxyl-terminal helices. The a tide binding among the three subunits is critical to the and y subunits of FI form a leg-and-foot that projects mechanism of the complex. from the bottom(membrane) side of F1 and stands The Fo complex making up the proton pore is firmly on the ring of c subunits. The schematic drawing composed of three subunits, a, b, and c, in the propor- in Figure 19-23f combines the structural information tion abe C10-12. Subunit c is a small (Mr 8,000), very from studies of bovine Fi and yeast FoFI hydrophobic polypeptide, consisting almost entirely of two transmembrane helices, with a small loop extend- Rotational Catalysis Is Key to the Binding-Change ng from the matrix side of the membrane. The crystal structure of the yeast FoFI, solved in 1999, shows the Mechanism for ATP Synthesis arrangement of the c subunits. The yeast complex has On the basis of detailed kinetic and binding studies of ten c subunits, each with two transmembrane helices the reactions catalyzed by FoFl, Paul Boyer proposed roughly perpendicular to the plane of the membrane and a rotational catalysis mechanism in which the three arranged in two concentric circles(Fig. 19-23d, e). The active sites of FI take turns catalyzing ATP synthesisF1 Fo (d) ADP, and the third was empty. The corresponding
subunit conformations are designated
-ATP,
-ADP, and
-empty (Fig. 19–23c). This difference in nucleo￾tide binding among the three subunits is critical to the mechanism of the complex. The Fo complex making up the proton pore is composed of three subunits, a, b, and c, in the propor￾tion ab2c10–12. Subunit c is a small (Mr 8,000), very hydrophobic polypeptide, consisting almost entirely of two transmembrane helices, with a small loop extend￾ing from the matrix side of the membrane. The crystal structure of the yeast FoF1, solved in 1999, shows the arrangement of the c subunits. The yeast complex has ten c subunits, each with two transmembrane helices roughly perpendicular to the plane of the membrane and arranged in two concentric circles (Fig. 19–23d, e). The inner circle is made up of the amino-terminal helices of each c subunit; the outer circle, about 55 Å in diame￾ter, is made up of the carboxyl-terminal helices. The  and subunits of F1 form a leg-and-foot that projects from the bottom (membrane) side of F1 and stands firmly on the ring of c subunits. The schematic drawing in Figure 19–23f combines the structural information from studies of bovine F1 and yeast FoF1. Rotational Catalysis Is Key to the Binding-Change Mechanism for ATP Synthesis On the basis of detailed kinetic and binding studies of the reactions catalyzed by FoF1, Paul Boyer proposed a rotational catalysis mechanism in which the three active sites of F1 take turns catalyzing ATP synthesis 19.2 ATP Synthesis 711 (e)       b2 c10 H+ a N side P side ADP + Pi ATP (f) 8885d_c19_690-750 3/1/04 11:32 AM Page 711 mac76 mac76:385_reb: