The affinity of oxygen (apparent Km) for the photoreduction of molecular oxygen by class II spinach chloroplasts increased with an increase in light intensity. Using a reaction scheme in which the photoreduction of oxygen occurs via the spontaneous oxidation of an electron acceptor of photosystem I, P430, we inferred that the “Km” value is a ratio of the first-order rate constants of photoreduction of P430 plus the internal back decay of the reduced P430 to the second-order rate constant for the oxidation of reduced P430 with oxygen.
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Takahashi et al. (1982) studied this question.