The crystal structure of the catalytic subunit of cyclic adenosine monophosphate-dependent protein kinase complexed with a 20-amino acid substrate analog inhibitor has been solved and partially refined at 2.7 A resolution to an R factor of 0.212. The magnesium adenosine triphosphate (MgATP) binding site was located by difference Fourier synthesis. The enzyme structure is bilobal with a deep cleft between the lobes. The cleft is filled by MgATP and a portion of the inhibitor peptide. The smaller lobe, consisting mostly of amino-terminal sequence, is associated with nucleotide binding, and its largely antiparallel beta sheet architecture constitutes an unusual nucleotide binding motif. The larger lobe is dominated by helical structure with a single beta sheet at the domain interface. This lobe is primarily involved in peptide binding and catalysis. Residues 40 through 280 constitute a conserved catalytic core that is shared by more than 100 protein kinases. Most of the invariant amino acids in this conserved catalytic core are clustered at the sites of nucleotide binding and catalysis.

The 2.3-A crystal structure of recombinant human dihydrofolate reductase (EC 1.5.1.3, DHFR) has been solved as a binary complex with folate (a poor substrate at neutral pH) and also as a binary complex with an inhibitor, 5-deazafolate. The inhibitor appears to be protonated at N8 on binding, whereas folate is not. Rotation of the peptide plane joining I7 and V8 from its position in the folate complex permits hydrogen bonding of 5-deazafolate's protonated N8 to the backbone carbonyl of I7, thus contributing to the enzyme's greater affinity for 5-deazafolate than for folate. In this respect it is likely that bound 5-deazafolate furnishes a model for 7,8-dihydrofolate binding and, in addition, resembles the transition state for folate reduction. A hypothetical transition-state model for folate reduction, generated by superposition of the DHFR binary complexes human.5-deazafolate and chicken liver.NADPH, reveals a 1-A overlap of the binding sites for folate's pteridine ring and the dihydronicotinamide ring of NADPH. It is proposed that this binding-site overlap accelerates the reduction of both folate and 7,8-dihydrofolate by simultaneously binding substrate and cofactor with a sub van der Waals separation that is optimal for hydride transfer.

The 2.2-A X-ray structure for CCP(MI), a plasmid-encoded form of Saccharomyces cerevisiae cytochrome c peroxidase (CCP) expressed in Escherichia coli [Fishel, L.A., Villafranca, J. E., Mauro, J. M., & Kraut, J. (1987) Biochemistry 26, 351-360], has been solved, together with the structures of three specifically designed single-site heme-cleft mutants. The structure of CCP(MI) was solved by using molecular replacement methods, since its crystals grow differently from the crystals of CCP isolated from bakers' yeast used previously for structural solution. Small distal-side differences between CCP(MI) and bakers' yeast CCP are observed, presumably due to a strain-specific Thr-53----Ile substitution in CCP(MI). A Trp-51----Phe mutant remains pentacoordinated and exhibits only minor distal structural adjustments. The observation of a vacant sixth coordination site in this structure differs from the results of solution resonance Raman studies, which predict hexacoordinated high-spin iron [Smulevich, G., Mauro, J.M., Fishel, L. A., English, A. M., Kraut, J., & Spiro, T. G. (1988) Biochemistry 27, 5477-5485]. The coordination behavior of this W51F mutant is apparently altered in the presence of a precipitating agent, 30% 2-methyl-2,4-pentanediol. A proximal Trp-191----Phe mutant that has substantially diminished enzyme activity and altered magnetic properties [Mauro, J. M., Fishel, L. F., Hazzard, J. T., Meyer, T. E., Tollin, G., Cusanovich, M. A., & Kraut, J. (1988) Biochemistry 27, 6243-6256] accommodates the substitution by allowing the side chain of Phe-191, together with the segment of backbone to which it is attached, to move toward the heme. This relatively large (ca. 1 A) local perturbation is accompanied by numerous small adjustments resulting in a slight overall compression of the enzyme's proximal domain; however, the iron coordination sphere is essentially unchanged. This structure rules out a major alteration in protein conformation as a reason for the dramatically decreased activity of the W191F mutant. Changing proximal Asp-235 to Asn results in two significant localized structural changes. First, the heme iron moves toward the porphyrin plane, and distal water 595 now clearly resides in the iron coordination sphere at a distance of 2.0 A. The observation of hexacoordinated iron for the D235N mutant is in accord with previous resonance Raman results. Second, the indole side chain of Trp-191 has flipped over as a result of the mutation; the tryptophan N epsilon takes part in a new hydrogen bond with the backbone carbonyl oxygen of Leu-177.(ABSTRACT TRUNCATED AT 400 WORDS)

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%e have studied the absorption cross section of antiprotons on Al, Cu, and Pb for T = 131. 6 and 193.6 MeV. These results are compared with predictions of an optical model fitted to antiproton elastic scattering data on these nuclei and are in agreement with these predictions. The cross sections have an exponential dependence on the mass number A with an exponent of approximately 0.61,