Siegward Knof

Crystal structure determination, refinement and molecular model of creatine amidinohydrolase from Pseudomonas putida.

The three-dimensional crystal structure of creatine amidinohydrolase (creatinase EC 3.5.3.3) from Pseudomonas putida, a dimeric enzyme with a molecular weight of 97,000, has been determined by multiple isomorphous replacement, averaging over the local dyad and restrained crystallographic refinement at 1.9 A with a crystallographic R-value of 17.7%. The asymmetric unit contains a dimer. The two chemically identical subunits consist of 403 residues each. A subunit is built up of two domains, a small N-terminal and a larger C-terminal domain. The small domain has a central seven-stranded beta pleated sheet with short helices on the outside. The large domain forms a six-stranded antiparallel beta half-barrel with helices on the outside. The two domains are connected by a segment that links two helices. The binding site of the competitive inhibitor carbamoyl sarcosine, a close analog of the substrate creatine, is located in the center of the large domain and partly covered by the small domain of the other subunit. The carbamoyl group is tightly co-ordinated to a water molecule, which presumably represents the nucleophile involved in hydrolysis of creatine. A catalytic mechanism is proposed on the basis of this structure.

Enzymatic mechanism of creatine amidinohydrolase as deduced from crystal structures.

Crystal structures of the enzyme creatine amidinohydrolase (creatinase, EC 3.5.3.3) with two different inhibitors, the reaction product sarcosine and the substrate creatine, bound have been analyzed by X-ray diffraction methods. With the inhibitor carbamoyl sarcosine, two different crystal forms at different pH values have been determined. An enzymatic mechanism is proposed on the basis of the eight structures analyzed. The enzyme binds substrate and inhibitor in a distorted geometry where the urea resonance is broken. His232 is the general base and acid, and acts as a proton shuttle. It withdraws a proton from water 377 and donates it to the N(3) atom of the guanidinium group. OH- 377 adds to the C(1) atom of the guanidinium group to form a urea hydrate. Proton withdrawal by His232 leads to products. The reaction product sarcosine binds to the active site in a reverse orientation. The free enzyme was found to have a bicarbonate bound to the active site.

[Side-reactions in peptide synthesis, IV. Charactertzation of C- and C,N-teri-butylated tryptophan derivatives (author's transl)].

Besides Nin-tert-butylated tryptophan derivatives, other tert-butyl-tryptophan analoga are also found as a result of side-reactions accurring during the acidolytic cleavage of protecting groups being based on a tert-butyl moiety. C-mono-substitution could be detected as well as di- and tri-substitution. All these tert-butylated tryptophans could be isolated in a pure form from the tert-butylation mixture either free or as derivatives and could unequivocally be identified in particular by means of mass spectrometry and nuclear magnetic resonance spectroscopy.

Structure-function studies on gastrointestinal hormones: I. Synthesis of secretin analogs and their biological and immunological properties

Abstract Syntheses by conventional procedures of the three analogs corresponding to the porcine secretin sequence crossed at position 6 by the N-terminal hexapeptide sequences of VIP, GIP, and glucagon are described, viz., Ala 4 ,Val 5 -, Tyr 1 ,Ala 2 ,Glu 3 -, and Gln 3 -secretin (VIP-SN, GIP-SN, and GLU-SN). The analog Phe 1 ,Phe 2 ,Trp 3 ,Lys 4 -secretin (SOMA-SN), designed on the basis of the surprising homology of the sequence portions 10–13 of somatostatin and 5–8 of secretin, was also prepared. Finally, the synthesis of N α -3-(4-hydroxyphenyl)propionyl-β-alanyl-secretin (DATA-SN), a tracer suitable for secretin radioimmunoassay and as an N-terminus modified secretin analog, is reported. The analogs are compared, in terms of their biological and immunological properties in different assay systems, with pure synthetic secretin.

Interaction of metal ions with gastrointestinal hormones: Binding of Ca2+ to Nle11-minigastrin I☆

The biological action of the gastrin hormones is confined to the C-terminal tetrapeptide amide TrpMet-Asp-Phe-NH2 [2-51. The physiological role of the remaining peptide sequences is, however, still obscure, as no clear evidecce for a specific function has been found. Among the various possibilities the -(G~u)~sequence common to big gastrin, little gastrin and minigastrin could be suited for the binding of metal ions such as calcium or magnesium. To test this hypothesis we studied the interaction of Nle”-minigastrin I (Nle”-HG-13) with Ca”. The investigation was carried out in trifluoroethanol as the solvent, which mimics the lipophilic environment of a membrane [6], where the binding process could occur.

Zur Synthese von Human-Big-Gastrin I und seinem 32-Leucin-Analogon

The preparation of the pure tetratriacontapeptide amide <Glu-Leu-Gly-Pro-Gln-Gly-His-Pro-Ser-Leu-Val-Ala-Asp-Pro-Ser-Lys-Lys-Gln-Gly-Pro-Trp-Leu-Glu-Glu-Glu-Glu-Glu-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH2 (human big gastrin I) and the analogue Leu32-human big gastrin I from the crude synthetic materials obtained after deblocking of the overall protected tetratriacontapeptide amide derivatives by means of trifluoroacetic acid is described. The criteria for homogeneity obtained by chromatographic, electrophoretic, enzymatic and spectroscopic methods are reported.