. Predictions were then made as to the heparln-blnding domains In endothellal cell growth factor, purpurln, and antithrombln-lll. Many of the natural sequences conforming to these consensus motifs show prominent amphlpathlc periodicities having both a-hellcal and 0-strand conformations as determined by predictive algorithms and circular dlchroism studies. The heparln-blnding domain of vttronectln was modeled and formed a hydrophlllc pocket that wrapped around and folded over a heparln octasaccharide, yielding a complementary structure. We suggest that these consensus sequence elements form potential nucleation sites for the recognition of potyanions In proteins and may provide a useful guide In identifying heparln-blnding regions In other proteins. 1112 However, the structural heterogeneity of the acidic mucopolysaccharides with respect to size, carbohydrate composition and charge, 13 and the variety of proteins that are known to bind to these molecules have complicated a detailed molecular analysis of protein-GAG interactions. The most studied system, binding of heparin to a specific domain or site on human antithrombin-lll (AT-III), involves a unique carbohydrate structure representing only a fraction of the total heparin. 14 The exact amino acid residues on AT-III or other proteins which comprise their heparin-binding domains have not been fully elucidated.To understand the molecular recognition of heparin by proteins, we recently determined the structure of the heparin-binding regions of apolipoprotein (apo) B-100, the major protein constituent of human plasma low density Received June 10, 1988; revision accepted September 14, 1988. lipoproteins (LDL). The structure of these regions was of particular interest as we hypothesized that they could mediate the interaction of LDL with acidic mucopolysaccharides of arterial tissue. We showed that LDL contains five to seven heparin-binding sites on the lipoprotein surface. 15 The heparin-binding cyanogen bromide peptides of apo B-100 were isolated, and their amino acid sequences were determined. 16 These peptides account for five domains of high positive charge density that we suggest mediate the lipoprotein's interaction with glycosaminoglycans. These same regions were also identified by Weisgraber and Rail. 17 We noticed 16 that these domains in apo B-100 have a sequence similarity to the heparinbinding regions of apo E 1819 and human vitronectin (Vn) 20 with respect to the organization of basic and hydropathic residues. The present study extends our initial observations to other proteins whose functions are modified by heparin. We show that all these proteins contain unique consensus sequences of amino acid residues that are potentially involved in heparin binding. MethodsSecondary structure predictions of Vn peptide conformations were performed by the MSEQ program 21 based on Chou-Fasman algorithms 22 and by the method of Gamier et al. 23 Both methods gave comparable predictions of predominant /3-strand and 0-tum structures, and circular dichroism studies on...

No abstract

No abstract

No abstract

The role of stereoelectronic effects in controlling the reaction specificity of biological reactions involving pyridoxal phosphate-amino acid Schiff's bases was tested with nonenzymatic models. The rates of racemization and Halpha exchange of a series of pyridoxal-amino acid Schiff's bases were determined. The order of these rates does not parallel the predictions based solely on electronic or steric effect, but parallels the proportions of the reactive conformers (e.g., conformers with the Calpha--Halpha bond orthogonal to the pi system) estimated by CPK models. The special reactivity of the phenylalanine Schiff's base was consistent with a special conformation in which some type of pi--pi interaction increases the proportion of exchangeable conformers, thus further substantiating the role of conformation in governing the reactivity of the Calpha--Halpha bond. Furthermore, semiempirical calculations of the conformation about the Calpha00N bond were performed using the CAMSEA conformational analysis program. The results of conformational calculations are consistent with the results of conformational analysis by nuclear magnetic resonance. The order of reactivity of the the Calpha--Halpha bond of the SB dianion, pH 12.0, predicted by calculation based on stereoelectronic effects, though not quantatively parallel to the observed rate constants, is qualitatively in agreement with the experimental results.

No abstract

No abstract

1-(2-Thienylalkyl)imidazole-2(3H)-thiones (5a-k) are competitive inhibitors of dopamine beta-hydroxylase (DBH) and demonstrate the utility of thiophene in the design of potent competitive inhibitors of this enzyme. The structure-activity relationships for these compounds are discussed and compared with those of 1-phenylalkyl-imidazole-2(3H)-thiones (1). With the aid of molecular modeling, an idealized active-site conformer is proposed and an explanation for the difference in activity between the phenyl (1) and thienyl (5) DBH inhibitors is presented. The difference in activity is consistent with our proposal that thiophene may not always be a bioisostere for phenyl. The inhibitor of most interest, 1-[2-(2-thienyl)ethyl]imidazole-2(3H)-thione (5g), was selected for study in the spontaneously hypertensive rat. The changes in dopamine and norepinephrine levels that resulted from oral administration of 5g correlated with the reduction of blood pressure.