Using X-ray diffraction techniques we intend to determine the structures of biological macromolecules involved in metabolic control. Structures of these molecules and their complexes with substrates, activators and inhibitors should provide insights into the molecular mechanisms of cellular regulation. Particularly amenable to the crystallographic techniques and of special interest to us are the detailed mechanisms and roles of structural changes to the regulation of enzyme activity and to enzymatic specificity. Our major efforts will be directed initially towards calculation of high resolution electron density maps and the building of atomic models of yeast hexokinase and its substrate complexes. This 104,000 m.w. dimeric protein displays the characteristics of a histeretic enzyme showing cooperative behavior with respect to both glucose and ATP. Following on our low resultion structure determination, which showed each subunit to interact with the other in a heterologous fashion and to be folded into two lobes, we expect to calculate and interprete a 3 A resolution electron density map. Substrates bind to a second crystal form producing such large changes in diffraction pattern as to suggest extensive structural changes. The structures of substrate complexes will not only elucidate the mechanism of cooperativity in this enzyme but also the mechanism of this kinase. Our preliminary x-ray diffraction and electron microscopic studies of small crystals of the E. coli lac repressor protein indicate this 150,000 m.w. tetramer has an axial ratio of about 3 to 1. We aim to obtain larger single crystals of the repressor and its complex with specific lengths of oligo-d (AT) and to solve their structure in order to understand the specificity of repressor-operator DNA0interaction and the nature of the allosteric transition produced by inducer binding. Attempts will be initiated to crystallize other proteins involved in transcription and translation: T7 specified RNA0polymerase (107,000), seryl-tRNA synthetase (100,000; small crystals in hand), protein synthesis elongation factors EF-Tu and EF-Ts.