During the second half of the twentieth century, great strides were made in revealing the molecular details of oxygen-generating photosynthesis, the basis of almost all life on Earth. Horst Witt was one of the prime movers behind this revolution in understanding. His obituary appears in Nature 2007, vol 448, p425
Witt was born in 1922 in Bremen, Germany.
Witt chose oxygenic photosynthesis as his lifelong research topic. Inspired by the methods pioneered by George Porter and Ronald Norrish, he embarked on work with the technique of flash spectrophotometry. Using algae, in 1955 he discovered reactions of chlorophylls, carotenoids and cytochromes that occurred in microseconds. By 1961,he along with Lou Duysens and of Bessel Kok, demonstrated a scheme with two photochemical reaction centres in series
At photosystem II, electrons are removed from water, generating a strong oxidant, oxygen. At photosystem I, the electrons are used to produce a strong reductant, NADPH (and thence sugars). The energy difference between the strong oxidant and the strong reductant powers all oxygen-based life.
During the ensuing years,his team of investigators marked off milestones in photosynthesis research. They found that excess light energy is disposed of harmlessly as heat through protective carotenoids. They discovered that the reactive pigment of photosystem II is chlorophyll a: its cationic form extracts electrons from the active site of water oxidation, with kinetics that depends on the electrostatics of charge accumulation. And they identified plastoquinone as an electron acceptor in photosystem II, where it functions as a one-to-two electron gate and as a mobile carrier between the two photosystems.
A further success was to demonstrate reactions with associated proton uptake and release at opposite sides of the membrane, as shown earlier by Peter Mitchell, which generated the proton-motive force required for the synthesis of ATP.
Rut it was the mechanism of water oxidation that remained Witt’s career-long preoccupation. The manganese cluster In photosystem 11 is charged up with four oxidizing equivalents before it reacts with two water molecules, releasing dioxygen. Witt and his co-workers contributed valuable information on many aspects of the mechanism, including the valence changes of manganese, associated electrostatic changes and effects of extrinsic reductants. Despite these and other contributions from many laboratories around the world, understanding the detailed mechanism has remained a major challenge.
Witt’s early attempts to crystallize photosystem II were fruitless, and it seemed that the game would be lost to others. However, crystals of photosystem I were eventually obtained, a trimeric structure with multiple subunits, diverse electron co-factors, about 100 chlorophyll molecules and many carotenoids. Later photosystem 11 was crystallised.
Insights into a system central to life.
Junge and Rutherford 2007, Nature p425
- Martin Eastwood