Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 49

In the 1930s, Robert (Robin) Hill (1899–1991), working at Cambridge University, was able to separate and investigate individually the oxidation and reduction reactions of photosynthesis. This taking apart of a complex system into individual parts, and investigation of them in detail in the absence of the complexities of a living cell is a cornerstone of biochemistry, often called reductionism. Hill established that it was possible to restore high rates of oxygen evolution to chloroplast suspensions if the latter were supplied with any of a number of artificial electron acceptors (Hill, 1939). Initially, he used ferric salts, and the Fe³⁺ was reduced to Fe²⁺ through the action of light, at the same time producing O₂. The reduction in artificial acceptors with concomitant O₂ production is today known as the Hill reaction. An example of the Hill reaction is given in Eq. (3.6):

(3.6)

Hill measured the O₂ in an ingenious way, which is worth relating if only to give an idea of the remarkable advances made by many of the pioneers of the field despite their primitive instrumentation. Hill obtained whole blood from a slaughterhouse, which has a dark blue color when deoxygenated and bright red color when oxygenated. He combined this with his chloroplast preparation and illuminated the mixture, monitoring the degree of oxygenation of the blood using a hand‐held spectroscope. At first, the results were disappointing, because the sample produced little oxygen. It is now clear that this was because the outer chloroplast envelope membranes were broken during the preparation, and the enzymes needed for CO₂ assimilation were lost. In searching for the factors needed to restore the lost activity, Hill made a fundamental discovery: namely, that it was possible to replace the reduction of CO₂ with the reduction of artificial electron acceptors, thereby restoring high rates of O₂ production. The physiological compound that acts as the light‐driven electron acceptor facilitating CO₂ production is NADP⁺, the oxidized form of nicotinamide adenine dinucleotide phosphate. The reduced form of this compound, NADPH, then serves as the reductant for CO₂ assimilation.

Hill did not set out to discover the reaction that bears his name. Instead, he was trying to establish whether an isolated chloroplast was capable of the complete process of photosynthesis, which was an important issue at the time. In fact, it is quite difficult to isolate chloroplasts with the envelope membranes still intact, and this was not routinely achieved until the mid‐1960s. This is a good example of Louis Pasteur's famous saying that “fortune favors the prepared mind.”