Structure and Function of Chloroplast Proteins*

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Abstract
Kinetic studies were carried out on the inactivation of spinach leaf ribulose– 1, 5– diphos-phate carboxylase [3– phospho– D– glycerate carboxylyase (dimerizing), EC 4. 1.1. 39] (18 S) y urea. Prior incubation of the enzyme protein with substrate effectively prevented the subsequent time-dependent enzyme inactivation by 4. 0 M urea at 25° C, whereas the very rapid activity decline occurred at 0° C. Analytical ultracentrifugal experiments have shown that by the 4. 0 M urea treatment the enzyme molecule of 18 S can be dissociated into two components with different sedimentation rates (15 S and 2 S), and sucrose density gradient centrifugation was employed to separate these fractions. The enzyme activity was detected only in the fast-sedimenting (large) component. It was also found that SH-groups related to the enzyme activity were specifically localized in the large component. However, the restoration of the enzyme activity after dilution of urea was accompanied by the structural reassociation of the 18 S component. A definite answer was not gained from the present experiment in support of the formation of the catalytically active component by treating the sub-strate-preincubated enzyme molecule with urea. Results are discussed in the light of dissociation-association phenomenon of the macromolecular organization of RuDP carboxylase.

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