(CIT): Special 1:00 PM Director's Wednesday Afternoon Lecture Modern oxygen evolving photosynthesis requires more than 200 proteins. Many of them are organised in highly complex structures in the chloroplasts of green plants (and also in cyanobacteria) where incident light energy from the sun is entrapped in the carbohydrates and fats that provide our food with calorific value. We release the energy by respiration (controlled burning), consuming in the process most of the oxygen that we have breathed in. Of the 1000 or so proteins that are involved, about 100 of them are organised into the respiratory enzyme complexes in mitochondria that function as molecular machines to convert the redox energy derived from food-stuffs into adenosine triphosphate (ATP), the energy currency of biology. The final synthetic step is achieved by a remarkable molecular machine that has a mechanical rotary action. Its closest man-made analogue is the Wankel rotary engine. Its workings will be described and how such complex machines evolve will be discussed. Professor John Walker FRS is Director of the Medical Research Council Dunn Human Nutrition Unit in Cambridge, UK, since 1998. In 1974 he was recruited by Fred Sanger to the Laboratory of Molecular Biology in Cambridge, UK, and early on he helped to uncover details of the modified genetic code of mitochondrial DNA. In 1978 he began studying the ATP synthase from mitochondria and from bacteria, resulting in a complete sequence analysis of this complex enzyme from several species. This was the ouverture to his work leading, in 1994, to the resolution of the 3D structure of the headpiece and stalk of this remarkable energy transducer by X-ray crystallography. Most significantly, the crystal structure was obtained in conditions where three nominally identical units (alpha 3 beta 3) of the catalytic headgroup (F1), were each in a different configuration due to differential binding of adenine nucleotides, which at once pointed t...