No abstract

The model of human Fc fragment was refined at 2.9-A resolution. Two different automated procedures for crystallographic refinement were used [Deisenhofer, J., & Steigemann, W. (1975) Acta Crystallogr., Sec. B B31, 238; Jack, A., & Levitt, M. (1978) Acta Crystallogr., Sect. A A34, 931]. The final R value is 0.22. The dimer of CH3 domains closely resembles the CH1-CL aggregate in Fab fragments. There is no contact between CH2 domains. The contact between CH2 and CH3 domains has about one-third of the size of the CH3-CH3 contact. The carbohydrate, a branched chain of nine hexose units, covers parts of the C-contact face of the CH2 domain, shielding hydrophobic residues on this surface. Six atoms of the carbohydrate are within hydrogen-bonding distance of atoms in the CH2 domain. Crystallographic refinement of the complex between Fc fragment and fragment B of protein A from Staphylococcus aureus reduced the R value of the model is 0.24. A major part of the structure of fragment B consists of two alpha helics; the rest of the polypeptide chain is folded irregularly. In the crystal, fragment B forms two contacts with Fc fragment molecules. Contact 1 involves residues from both helices of fragment B, and residues from the CH2 and CH3 domains of FC, and is predominantly hydrophobic. Contact 2 is smaller than contact 1. Residues from the second helix and adjacent residues of fragment B and residues only from the CH3 domain of Fc contribute to contact 2. The nature of contact 2 is mainly polar and includes a sulfate ion. There are strong arguments that contact 1 is the fragment B-Fc contact formed in solution under physiological conditions, while contact 2 is a crystal contact.

HMG-CoA (3-hydroxy-3-methylglutaryl-coenzyme A) reductase (HMGR) catalyzes the committed step in cholesterol biosynthesis. Statins are HMGR inhibitors with inhibition constant values in the nanomolar range that effectively lower serum cholesterol levels and are widely prescribed in the treatment of hypercholesterolemia. We have determined structures of the catalytic portion of human HMGR complexed with six different statins. The statins occupy a portion of the binding site of HMG-CoA, thus blocking access of this substrate to the active site. Near the carboxyl terminus of HMGR, several catalytically relevant residues are disordered in the enzyme-statin complexes. If these residues were not flexible, they would sterically hinder statin binding.

On the basis of x-ray diffraction data to a resolution of 2.9 angstroms, atomic models of most protein components of the bovine cytochrome bc1 complex were built, including core 1, core 2, cytochrome b, subunit 6, subunit 7, a carboxyl-terminal fragment of cytochrome c1, and an amino-terminal fragment of the iron-sulfur protein. The positions of the four iron centers within the bc1 complex and the binding sites of the two specific respiratory inhibitors antimycin A and myxothiazol were identified. The membrane-spanning region of each bc1 complex monomer consists of 13 transmembrane helices, eight of which belong to cytochrome b. Closely interacting monomers are arranged as symmetric dimers and form cavities through which the inhibitor binding pockets can be accessed. The proteins core 1 and core 2 are structurally similar to each other and consist of two domains of roughly equal size and identical folding topology.