Search

UNLABELLED Besides aiding digestion, bile salts are important signal molecules exhibiting a regulatory role in metabolic processes. Human ileal bile acid binding protein (I-BABP) is an intracellular carrier of bile salts in the epithelial cells of the distal small intestine and has a key role in the enterohepatic circulation of bile salts. Positive binding cooperativity combined with site selectivity of glycocholate and glycochenodeoxycholate, the two most abundant bile salts in the human body, make human I-BABP a unique member of the family of intracellular lipid binding proteins. Solution NMR structure of the ternary complex of human I-BABP with glycocholate and glycochenodeoxycholate reveals an extensive network of hydrogen bonds and hydrophobic interactions stabilizing the bound bile salts. Conformational changes accompanying bile salt binding affects four major regions in the protein including the C/D, E/F and G/H loops as well as the helical segment. Most of these protein regions coincide with a previously described network of millisecond time scale fluctuations in the apo protein, a motion absent in the bound state. Comparison of the heterotypic doubly ligated complex with the unligated form provides further evidence of a conformation selection mechanism of ligand entry. Structural and dynamic aspects of human I-BABP-bile salt interaction are discussed and compared with characteristics of ligand binding in other members of the intracellular lipid binding protein family. PROTEIN DATA BANK ACCESSION NUMBERS The coordinates of the 10 lowest energy structures of the human I-BABP : GCDA : GCA complex as well as the distance restraints used to calculate the final ensemble have been deposited in the Brookhaven Protein Data Bank with accession number 2MM3.

Molecular dynamics was used to analzye energy transfer rates between matrix and guest molecules in matrix-assisted laser desorption allowing for a large number of internal degrees of freedom. The effect of initial matrix temperature jump on internal energy equilibration times and on guest limiting temperatures has been studied on a model system comprising a pyridine-3-carboxylic acid matrix and leucine enkephalin (Tyr-Gly-Gly-Phe-Leu) guest molecule. It appears that the energy transfer rates between matrix and guest molecules depend on the initial matrix temperature jump. The initial matrix temperature of 900 K leads to incomplete desorption, whereas at 1500 K complete desorption and the formation of an energy-transfer bottleneck was observed. Following the guest center-of-mass at different initial matrix temperatures indicates that in the case of 1500 K and 3000 K the desorption process is complete, whereas at 900 K the guest molecule stays near the matrix surface. In the case of complete desorption deeper embedding of the guest molecules leads to somewhat lower guest limiting temperatures. Uniformly higher limiting temperatures are observed for higher laser irradiance and the increase in burial depth leads to more complete equilibration between the two species. © 1997 by John Wiley & Sons, Ltd.

Rate constants for the unimolecular decomposition of CH3CO have been obtained as a function of temperature (420 - 500 K) and helium density (3 - 18 · 1016 atom cm−3), conditions which are in the second order region of the fall-off curve. An Arrhenius expression for the low-pressure limit unimolecular rate constant was obtained from the results, k1±(He) = (6.7 ± 1.8) · 10−9 exp[(-6921 ± 126 K)/T] cm3 molecule−1 s−1. Using a Master Equation formalism to calculate values of k1, a set of the two energy parameters needed in the calculations, E± and ΔE)down (including its temperature dependence), was found that is within the range of expected values (including a temperature dependence in the case of ΔEdown), which, when incorporated into the Master Equation, provides calculated rate constants which agree well with the measured ones. They are E± = 65.3 ± 4.0 kJ mol−1 and ΔEdown = 65.6 + 0.271 T cm−1 (the latter, a parameterized expression, is valid only in the temperature range of this study). A transition state model for the unimolecular decomposition of CH3CO was produced which provides high-pressure limit rate constants for this reaction (k1±(CH3CO ± CH3 + CO) = 2.50 · 1013 exp(-8244 K/T) s−1 and k−1±(CH3 + CO ± CH3CO) = 7.64 · 10−13 exp(-3073 K/T) cm3 molecule−1 s−1) and k(E) values for solving the Master Equation for reaction conditions that are in the fall-off region. Fall-off behavior of k1 and k−1 reported by others for several different bath gases was reproduced within the uncertainty limits of the experimental results using the Master Equation formalism incorporating the transition state model, the energy parameter E± given above, and reasonable values for ΔEdown for the different bath gases used. This Master Equation formalism and transition state model should provide unimolecular rate constants for reaction (1, - 1) in the fall-off region for additional bath gases using reasonable estimates of ΔEdown (e. g., values obtained for this energy-transfer parameter for collisions between other polyatomic radicals and the bath gases of interest).