Possible CO2 concentrating mechanism in chloroplasts of C3 plants. Role of carbonic anhydrase.

The possibility of a specific CO2 concentrating mechanism present in chloroplasts of C3 plants is analyzed. Proton gradient between thylakoids and the stroma is assumed to be the driving force for this process. The possible CO2 concentrating mechanisms are: 1. HCO3- permeation into thylakoids, its dehydration there and diffusion of CO2 formed into the stroma; 2. Dehydration of HCO3- present in the stroma at the thylakoid surface in a reaction with H+ leaving the thylakoids through: a) channels of membrane-bound carbonic anhydrase; b) channels of the ATPase complex. A system of equations describing CO3- and CO2 diffusion as well as CO2 assimilation and formation was used. The increase in photosynthesis rate, upon CO2 diffusion being facilitated in the presence of carbonic anhydrase, and due to the action of CO2 concentrating mechanisms, was numerically estimated. The CO2 concentrating mechanism was shown to function effectively only with the entire chloroplast being the CO2 concentrating zone. This is the case when the bulk of stromal carbonic anhydrase is localized near the inner chloroplast envelope. The existence of CO2 concentrating mechanisms around a single granum or around thylakoids is hardly possible. Approaches enabling the detection of similar concentrating mechanisms are discussed.

[Modelling the effect of photosynthetic enhancement and chromatic transitions]. / Modelirovanie éffekta usileniia i khromaticheskikh perekhodov fotosinteza.

A simple method is proposed and analysed which explains characteristic manifestation of the enchancement effect (2nd Emerson effect) and of chromatic transients effect (Blinks effect). The model is based on the hypothesis concerning the ATP/NADP concentrations ratio with a change of falling light wavelength. The model is suggested to reflect the behaviour of the reduction pentosophosphate cycle (Calvin cycle) under low illumination intensity, when the photosynthesis rate is determined only by ATP and NADP.H concentrations formed in the course of different photochemical reactions.

[Possible mechanism of cooperative control in polyenzyme systems]. / Vozmozhnyi mekhanizm kooperativnogo upravleniia v polifermentnykh sistemakh.

A mathematical model of an anabolic multienzyme system and its function mechanism is considered. The first substrate in this kind of biochemical system may be binded to its active site, provided the last product was released out of the system. The functional control is assumed to be exercised by cooperative rearrangement of the protein structure. Any intermediate or product molecule if present within the system, induces conformational changes in the entrance center of the multienzyme system, preventing the initial substrate binding. The steady-state product synthesis rate in such a system is determined by joint influence of rate constants at all stages of the reaction chain. The model corresponds closely to the multienzyme system of chlorophyll biosynthesis in higher plants. Essential advantages of cooperative controlled multienzyme systems in regulation of biochemical processes are discussed.