Structural studies of two different crystal forms of mitochondrial malate dehydrogenase are being carried out by x-ray diffraction analysis. For both forms, two heavy atom derivatives have been identified. In one instance, it has now been possible to correlate the relative y-coordinate of the heavy atom sites by using a double derivative and the final stages of phase refinement at low resolution will be carried out during the next grant period. For the second crystal form, heavy atom locations will be studied using Patterson methods. NMR studies of the lipid domain within a soluble lipoprotein, lipovitellin will be completed. The remaining NMR studies, in collaboration with Seeling at the Biozentrum in Basel, Switzerland, will consist of an analysis of 31P relaxation times of both phosphoserine and phospholipid components. The relaxation times, T1, will be compared with similar data obtained from the crystalline lipoprotein and from other lipid:protein systems. The comparison with data measured from membrane protein systems, should make it possible to understand dynamic lipid organization in micro-domains containing as few as 30 phospholipid molecules. An attempt will also be made to test the exchangeability of triglycerides in this lipoprotein system. The NMR studies of the phospholipid micro-domain suggest that the role of neutral lipid may be in forming the hydrophobic protein wall for the lipid domain, a factor previously assigned to phospholipid. Such a hydrophobic region may then serve as the boundary for insertion of the micro-domain of bilayer-like phospholipid. If it is possible to exchange 2H labelled triglycerides into the lipid domain of lipovitellin, NMR methods would be used to study their physical properties within the lipoprotein.
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