Kinetic and proteolytic identification of heat‐induced conformational changes in the urea herbicide binding site of isolated Phaseolus vulgaris chloroplast thylakoids

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Abstract
Kinetic parameters of 3‐(3, 4‐dichlorophenyl)‐1, 1‐dimethyl urea (DCMU)‐induced inhibition of electron transport in chloroplast thylakoids isolated from Phaseolus vulgaris L. cv. Oregon 1604 were determined from analysis of a convergent, parallel electrical circuit. Through this analogue, the apparent affinity of the purported binding site for DCMU (K1) and the relative amount of DCMU‐insensitive electron transport (vmax1/vo) were obtained using a reiterative non‐linear least squares curve‐fitting procedure. Exposure of thylakoids to heat caused a gradual increase in K1 (or decrease in the affinity of the thylakoid for DCMU) with an apparent activation energy of 134 kJ mol−1. Tryptic susceptibility of a protein region regulating K1 also decreased gradually with exposure to 45°C, suggesting that the heat‐induced increase in K1 might be due to a protein conformational change. On the other hand, thylakoid exposure to 45°C resulted in a rapid (maxI/vo, which was also the apparent result of a conformational change in a region of the protein which regulates this function. These results are suggestive of the existence of differential thermal sensitivities of proteins within the thylakoids and, perhaps, of different regions within a single membrane protein.