CITRATE SYNTHASE FROM A MESOPHILE, THERMOPHILE AND HYPERTHERMOPHILE. G.L.Taylor, R.J.M.Russell, D.W.Hough & M.J.Danson., School of Biology and Biochemistry, University of Bath, BA2 7AY, U.K.

We have determined the crystal structures of dimeric citrate synthase from two Archaea: Thermoplasma acidophilum, whose habitat is around 55C, and Pyrococcus furiosus, whose habitat can be up to 110C. Comparison of these structures with citrate synthase from pig, a mesophile, reveals certain structural trends which correlate with increasing thermostability. These include: (i) a reduction in thermolabile residues and an increase in isoleucines and glutamic acids, (ii) an increase in compactness, achieved by shortening of loops and a reduction in internal cavities, (iii) an increase in complex ion-pair networks, and (iv) variations in the nature of dimer formation. Some of these features are being analyzed by site-directed mutagenesis, to gain an insight into their relative contributions to overall thermostability. The structures of several other thermostable proteins have appeared recently, some of which point to the importance of ion-pairs in conferring hyperthermostability and some of which point to compactness. We will compare and contrast our findings on citrate synthase with these studies in an attempt to discover how many roads there are to thermostability. Can we hope to emulate what Nature has spent several million years perfecting ?

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