Maurizio Marta

Physostigmine analogs anticholinesterases: effects of the lengthening of the N-carbamic chain of the inhibition kinetics

Data are presented about the inhibitor power of new carbamates against acetylcholinesterase. The study was carried out on two series of physostigmine analogs, N-alkyl and N-methyl,N-alkylphysostigmines. For these inhibitors, the second-order rate constants for inhibition, ki, and the first-order rate constants of reactivation, k3, have been determined. From the reported results, electronic, hydrophobic and steric effects, due to the enhancement of the alkyl chain, may have influenced all kinetics parameters discussed. In comparison to physostigmine, both the new N-methyl,N-alkylphysostigmines and the N-alkylphysostigmines showed a non-linear decrease in the values of ki and k3. This study presents the hydrophobic interactions as an important factor not only in determining carbamylation but also decarbamylation rates constants.

A revised structure for the triterpene rigidenol

Abstract The structure of the triterpene rigidenol has been revised to 11α-hydroxy-lup-20(30)-en-3-one.

Bovine hemoglobin cross-linked through the beta chains: functional and structural aspects.

2-Nor-2-formylpyridoxal (NFPLP) has been synthesized and coupled to bovine Hb according to the procedure developed by Benesch and Benesch(1). The reaction of bovine Hb with NFPLP leads to a cross-linkage between the β subunits, which greatly stabilizes the low affinity T state of the molecule and simultaneously abolishes the tendency of the tetramer to dissociate into αβ dimers. The functional properties, examined from both the equilibrium and kinetic points of view, indicate that the chemical modification affects the O affinity, abolishes cooperativity, and induces a slight decrease of the Bohr effect. From modeling studies we are confronted with two different structural alternatives; the cross-link of β chains may be formed between lysine 82 of β and the N terminus of methionine 2 of β or between the two lysine 82 residues of both β chains. Digestion of modified β globin chains and isolation of the cross-linked peptide have showed that NFPLP cross-links Met-β2 and Lys-β82. This allowed discussion in some detail of the molecular basis of the Bohr effect of the modified bovine hemoglobin. On the whole, NFPLP-modified bovine Hb could be considered as a first step toward the synthesis of a potential blood substitute.

POLAR BEAR HEMOGLOBIN AND HUMAN Hb A0: SAME 2,3-DIPHOSPHOGLYCERATE BINDING SITE BUT ASYMMETRY OF THE BINDING?

Polar bear (Ursus maritimus) hemoglobin (Hb) shows a low response to 2,3-diphosphoglycerate (2,3-DPG), compared to human Hb A0, even though these proteins have the same 2,3-DPG-binding site. In addition, polar bear Hb shows a high response to chloride and an alkaline Bohr effect (Δlog P50/ΔpH) that is significantly greater than that of human Hb A0. The difference in sequence Pro (Hb A0)→Gly (polar bear Hb) at position A2 in the A helix seems to be critical for reduced binding of 2,3-DPG. Our results also show that the A2 position may influence not only the flexibility of the A helix, but that differences in flexibility of the first turn of the A helix may affect the unloading of oxygen for the intrinsic ligand affinities of the α and β chains. However, preferential binding to either chain can only take place if there is appreciable asymmetric binding of the phosphoric effector. Regarding this point, 31P NMR data suggest a loss of symmetry of the 2,3-DPG-binding site in the deoxyHb–2,3-DPG complex.