Laser Photolysis Investigations of Ligand Binding With Models of the Active Site of Respiratory Hemoproteins: Kinetic and Thermodynamic Aspects

C Reed,J M Lhoste,C Tetreau,G E Wuenschell,J Mispelter,D Lavalette,M Momenteau

doi:10.1155/1990/24768

C Reed, J M Lhoste + Show 5 more

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https://doi.org/10.1155/1990/24768

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Abstract

During the past 15 years, laser photolysis has been the method of choice for probing the complex reaction kinetics of respiratory proteins. In an attempt to determine the structural parameters which govern their reactivity, synthetic heme model compounds capable of simulating particular aspects of the reactivity of the active site of hemoproteins have been successively proposed. Laser photolysis of heme compounds merely induces a reversible photodissociation of one ligand at a time. This is equivalent to performing a fast concentration jump "in situ" and provides a powerful, fast and "clean" chemical relaxation technique. To gather association and dissociation rate constants of various ligands (O2, CO, nitrogenous bases) special methods have been developed or adapted. The problem of comparing and classifying a large number of collected data has been greatly simplified by introducing a Linear Free Energy Relationships formalism. In the first part of this paper, some of the methods and concepts which have emerged from several years of investigations of heme proteins and heme models and which are of a sufficient generality to be useful in other fields of chemical kinetics are reviewed. In the second part of the paper we present the application of the preceding methods to a kinetic study of a series of heme models which were specifically designed to investigate the important problem of H-bonding as a stabilizing factor of the oxygenated heme model and hemoprotein complexes.

Highlights

Kinetic studies of the reaction of oxygen and other ligands with iron(II) porphyrins are directly relevant to the understanding of the mechanism by which hemoglobinD
During the past 15 years, flash and laser photolysis have been the methods of choice for probing the complex reaction kinetics of respiratory proteins
In a previous work[4] we have shown that Linear Free Energy Relationships (LFERs) provide a simple and adequate formalism to rationalize a large amount of data and to classify structurally related molecules into reacting families

Summary

INTRODUCTION

Kinetic studies of the reaction of oxygen and other ligands with iron(II) porphyrins are directly relevant to the understanding of the mechanism by which hemoglobin. There are a few minimum requirements that any realistic model must satisfy in order to reproduce the most pertinent features of the active site of hemoproteins The latter consists essentially of a heme group (iron(II) protoporphyrin IX) embedded in the protein "pocket" and chelated by a histidine residue (the "proximal" histidine F8) (see Figure 1). These concepts and techniques are applied in Part II of the paper to the specific problem of the stabilization of the oxygenated complex by hydrogen bonding in a series of newly synthesized heme-model compounds

GENERAL CONSIDERATIONS

I-2. Determination of Rate Constants using Laser Phototriggered Reactions

I-3. Linear Free Energy Relationships

I-4. Information Theory and Chemical Kinetics

STABILIZATION OF OXYGENATED HEME MODELS BY INTERNAL HYDROGEN BONDING

II-1. Materials and Methods

II-2. Results and Discussion