Citrate synthase domain structure

Fig. 5. Rigid-body analysis of citrate synthase, using two X-ray structures (after Hayward and Berendsen, Proteins 30 (1998) 144). The decomposition of the protein into two domains (dark gray and white) and two interconnecting regions (light gray) is shown, together with the hinge axis for the closing/opening motion between them.

The relatively unhindered domain motion in immunoglobulins may be contrasted with that observed in some enzymes, in which the domain motion occurs upon substrate binding during the catalytic cycle. The phenomenon has been established by crystal structure analysis of the different forms of yeast hexokinase [11], liver alcohol dehydrogenase [12] and citrate synthase [13], and appears to occur also in glyceraldehyde-3-phosphate-dehydrogenase. 

In the large domain of citrate synthase there are small movements. In the small domain a large change occurs, which can be described as a rigid body movement in a first approximation. However, on detailed inspection one finds that there are small structural changes all over the molecule, particularly in the small domain, but also some in the large domain. So it is only to a first approximation a two state model. One must postulate this because the environment of the small domain in the open state faces the solvent much more than in the closed state, where it is in contact with the large domain. This must be reflected in the internal conformation, and this is actually what we see. Nevertheless, I think it is useful to talk of the two state model. 

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