Scientists reveal fine detail of cell`s energy machinery

A molecular pump that helps to keep cells flush with energy has been visualised by scientists at Imperial College, London.

The structure of the pump, a key enzyme in bacterial respiration, reveals for the first time one of the molecular mechanisms that underpins cellular respiration, and confirms a Nobel Prize-winning theory proposed over 40 years ago by Briton Peter Mitchell.

Professor So Iwata and colleagues from the Laboratory of Membrane Protein Crystallography, Imperial College Centre for Structural Biology describe in Science (8 March) what the enzyme formate dehydrogenase-N looks like to a resolution of 1.6 angstroms – or one hundred millionth of a centimetre *1.

“From bacteria to humans, the mechanism of energy conversion is shared by a wide range of organisms, and solving this enzyme`s structure provides a valuable insight into the molecular machinery of life,” said Professor Iwata.

Formate dehydrogenase-N, a bacterial enzyme involved in nitrate respiration, lies in the membranes of cells. Iwata`s Laboratory of Membrane Protein Crystallography is one of a small number around the world that focuses on solving membrane protein structures using X-ray crystallography.

Membrane proteins are technically difficult targets for structural biologists to solve. Fewer than 30 membrane protein structures are presently known, compared with over 10,000 soluble protein structures, estimates Professor Iwata.

“Many genetic disorders such as cystic fibrosis are directly related to membrane proteins, and as many as 70 per cent of drugs currently available act through membrane proteins.

“Solving the structure of membrane proteins is essential to facilitate the rational design of effective drugs and to develop new therapies for genetic diseases,” he said.

Their work with the bacteria E. coli provides the first real evidence for the `chemiosmotic` theory proposed by Dr Peter Mitchell in 1961. Initially dismissed by mainstream science, Mitchell`s theory on energy conversion is now accepted as a fundamental principle in the field of `bioenergetics` *2.

To stay alive organisms must be able to release energy in a controlled and useable form. Cells do this by converting metabolic energy derived from respiration into a compound called adenosine triphosphate (ATP).

“In all cells, metabolites are converted via a series of respiratory enzymes into an electric potential or `proton motive force` across the cell membrane. This proton motive force drives the generation of ATP,” said Professor Iwata.

Professor Iwata and his team are the first to solve the structure of a respiratory enzyme that produces the proton motive force by the “redox-loop mechanism” originally proposed by Peter Mitchell.

“Forty years on, this is the first enzyme structure to be determined that shows Peter Mitchell`s original hypothesis of how cells convert energy into a usable form is correct,” said Professor Iwata.

Professor Paul Freemont, Director of the Centre for Structural Biology said: “Membrane proteins constitute almost 30 per cent of all gene products yet we have so few structures of them. This is a serious limitation in terms of applying the results of the human genome project to our understanding of human disease, and makes Professor Iwata`s contribution even more significant.”

The research team, originally based at Uppsala University in Sweden, joined the Laboratory of Membrane Protein Crystallography, Centre for Structural Biology, Imperial College in 2000. The UK Biotechnology and Biological Sciences Research Council (BBSRC) funded this research project and are core funders of the Centre.