December 2005 Molecule of the Month by David Goodsell
Keywords: synthase activity, rotational mechanism, APT biosynthetic process, hydrogen ion transmembrane transporter activity, proton transport, ATP synthesis coupled proton transport, molecular motor protein
ATP synthase is one of the wonders of the molecular world. ATP synthase is an enzyme, a molecular motor, an ion pump, and another molecular motor all wrapped together in one amazing nanoscale machine. It plays an indispensable role in our cells, building most of the ATP that powers our cellular processes. The mechanism by which it performs this task is a real surprise.
ATP synthesis is composed of two rotary motors, each powered by a different fuel. The motor at the top, termed F0, an electric motor. It is embedded in a membrane (shown schematically as a gray stripe here), and is powered by the flow of hydrogen ions across the membrane. As the protons flow through the motor, they turn a circular rotor (shown in blue). This rotor is connected to the second motor, termed F1. The F1 motor is a chemical motor, powered by ATP. The two motors are connected together by a stator, shown on the right, so that when F0 turns, F1 turns too.
Motor to Generator
So why have two motors connected together? The trick is that one motor can force the other motor to turn, and in this way, change the motor into a generator. This is what happens in our cells: the F0 motor uses the power from a proton gradient to force the F1 motor to generate ATP. In our cells, food is broken down and used to pump hydrogen ions across the mitochondrial membrane. The F0 portion of ATP synthase allows these ions to flow back, turning the rotor in the process. As the rotor turns, it turns the axle and the F1 motor becomes a generator, creating ATP as it turns.
Large, complex molecular machines like ATP synthase pose
difficult problems for structural scientists, so the structures of these
machines are often determined in parts. The picture shown here is a composite of
four different structures, combining structures determined by X-ray
crystallography and NMR spectroscopy. The F0 motor is included in PDB file 1c17. The F1 motor
and the axle that connects the two are included in PDB file 1e79. The stator has
proven to be the most elusive part--the two pieces shown here are from PDB files
2a7u and 1l2p.
For information on ATP synthase from a genomics perspective, take a look at the Protein of the Month at the European Bioinformatics Institute.