Complexation which facilitates rejoining of horse cytochrome c apofragment [Homoser-lactone65](1-65) or [Homoser-lactone65] (23-65) to apofragment (66-104).

We have shown that two CNBr fragments of horse apocytochrome c, [Homoser-lactone65](1-65) and (66-104), bind to the ferric heme fragment (1-25)H to form a non-productive three-fragment complex, and that when the heme of this complex has been kept reduced for 48 h at 25 degrees, the peptide bond between residues 65 and 66 is restored with a yield of 24% or more. We have also shown that another CNBr fragment [Homoser-lactone65](23-65), but not [Homoser-lactone65](39-65), similarly rejoins to fragment (66-104) in the presence of the ferrous heme fragment with 25% or more yield. For complex of ferro-heme fragment [Hse-lacton65](1-65)H and apofragment (66-104) of horse cytochrome c, which restores the peptide bond between residues 65 and 66 (located on the left side of the heme) (cf. Harbury, H.A. (1978) in Semisynthetic Peptides and Proteins (Offord, R.E. & DiBello, C., eds.), pp. 73-89, Academic Press, New York). Corradin & Harbury have suggested that axial ligation of methionine 80 to the heme (on the left side) is important. Consistent with their idea, fragment [Hse80](66-104) was found not to link to [Hse-lactone65](1-65) in the presence of ferro(1-25)H. Furthermore, the present studies indicate that the interaction involving residues 26 to 38 (on the right side) is also important for such a conformation which assists in the rejoining of the two apofragments. Combining these two ideas, we suggest that restoration of the peptide bond between residues 65 and 66 reflects the structural integrity of these complexes in the reduced form. Thus, the present reaction system can be used not only for chemical synthesis of [Homoser65] apocytochrome c but also to extend amino acid substitution studies of cytochrome c to residues 1 to 64.

Extremely fast hydrogen exchange of ribonuclease-(1-118) as compared with native RNase A and its implication for the conformational energy state.

RNase-(1-118) containing native disulfide bonds is similar in fold to native RNase A but not of lowest Gibbs energy as compared with the isomers containing non-native disulfide bonds. The present n.m.r. studies have indicated a dramatic increase in the exchange rate of all of the 'protected' amide protons of RNase-(1-118) over RNase A. A calculation shows a large increase in the rate of 'opening' of the structure. The exchange rate of the protected amide protons of RNase-(1-120) is slower than RNase-(1-118) but much faster than RNase A. Binding with a synthetic complementing fragment (114-124) markedly reduces the exchange rate of 20 to 25 amide protons of RNase-(1-118). It has previously been shown that binding with a complementing fragment of RNase-(1-118) generates a lowest Gibbs energy state. Thus, using available thermodynamic information for interpretation, we suggest that a) removal of six carboxy terminal residues of RNase A would disrupt coupling between these residues and those distant in the structure (loss of extra stabilizing energy), b) this would, in turn, alter the enthalpy-entropy compensation in such a way that the magnitude of Gibbs energy change favoring folding is significantly reduced without a large change of fold and c) in this activated state the molecule would be highly motile.

Carbon-13 nuclear magnetic resonance studies of the binding of selectively 13C-enriched oxytocins to the neurophypophyseal protein, bovine neurophysin II.

Complex formation between bovine neurophysin II and oxytocin molecules containing 85% 13C enrichment in specific amino acid residues was studied using 13C nuclear magnetic resonance spectroscopy. Chemical shift and relaxation time values of the analogue [13C-Leu3]oxytocin, [13C-Gly9]oxytocin, and the doubly labeled [13C-Ile3 Gly9]oxytocin were obtained for the hormones in the absence and presence of neurophysin. The results showed that certain 13C nuclear magnetic resonance parameters of residue 3 but not of residue 9 of oxytocin are altered upon binding to neurophysin. These observations suggest that residue 3 but not residue 9 is involved in the protein-hormone interaction and they demonstrate the general applicability of selective 13C enrichment for the study of peptide-protein interactions.

Crystalline semisynthetic ribonuclease-S'.

Crystals of solid phase-derived semisynthetic ribonuclease-S' were prepared and compared with those for native ribonuclease-S' and -S. The semisynthetic species used was the noncovalent complex of synthetic fragment-(1-20), corresponding to residues 1 through 20 of bovine pancreatic ribonuclease-A (ribonucleate 3'-pyrimidino-oligonucleotidohydrolase, EC 3.1.4.22), and native ribonuclease-S-(21-124); the fragment containing residues 21 through 124 of ribonuclease-A. This semisynthetic complex was completely active enzymatically, was homogeneous as judged by polyacrylamide gel electrophoresis, and had no greater than trace amounts of excess ribonuclease-s(21-124) as judged by affinity chromatography. Crystallization of both semisynthetic and native ribonuclease-s' at pH 5.3 resulted in well-formed crystallseater than trace amounts of excess ribonuclease-S-T21-124) as judged by affinity chromatography. Crystallization of both semisynthetic and native ribonuclease-S' at pH 5.3 resulted in well-formed crystals with the symmetry of space group P3121 and unit cell dimensions a=b=44.82, c=97.3 A. This crystal form corresponds to the Y form of native ribonuclease-S previously reported [Wyckoff et al. (1967) J. Biol. Chem. 242, 3749-3753]. X-ray diffraction patterns of the crystals were indistinguishable, indicative of the structural identity of semisynthetic and native ribonuclease-S'.