Structure and Function of Biological Membranes

Lawrence I. Rothfield

• Publisher: Academic Press
• Year: 2014
• Print ISBN: 9780125986502
• eBook ISBN: 9781483281964
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

Structure and Function of Biological Membranes explains the membrane phenomena at the molecular level through the use of biochemical and biophysical approaches. The book is an in-depth study of the structure and function of membranes. It is divided into three main parts. The first part provides an overview of the study of the biological membrane at the molecular level. Part II focuses on the detailed description of the overall molecular organization of membranes. The third part covers the relationship of the molecular organization of membranes to specific membrane functions; discusses catalytic membrane proteins; presents the role of membranes in important cellular functions; and looks at the membrane systems in eukaryotic cells. Biochemists, cell physiologists, biologists, researchers, and graduate and postdoctoral students in the field of biology will find the text a good reference material.

STRUCTURE-FUNCTION RELATIONSHIPS IN BIOLOGICAL MEMBRANES

Outline

Chapter 6: ENZYME REACTIONS IN BIOLOGICAL MEMBRANES

Chapter 7: THE MOLECULAR BASIS OF MEMBRANE TRANSPORT SYSTEMS

Chapter 8: THE ROLE OF MEMBRANES IN THE SYNTHESIS OF MACROMOLECULES

Chapter 9: STRUCTURAL AND FUNCTIONAL ORGANIZATION OF MITOCHONDRIAL MEMBRANES

Chapter 10: STRUCTURE AND FUNCTION OF SURFACE COMPONENTS OF MAMMALIAN CELLS

6

ENZYME REACTIONS IN BIOLOGICAL MEMBRANES

L. ROTHFIELD and D. ROMEO

Publisher Summary

This chapter discusses enzyme reactions in biological membranes. A large number of cellular enzymes are located in membranes. Indeed, it is likely that most particulate enzymes are in truth membrane-bound enzymes whose particulate nature reflects their association with the lipid matrix of biological membranes. Included in this category are enzymes located in plasma membranes, mitochondria, microsomes, and in other sub-cellular organelles. In fact, if one is willing to extend the definition to proteins that catalyze the physical translocation of substrates, the class of membrane enzymes can be extended to include proteins responsible for transmembrane transport, such as bacterial permeases. Membrane lipids can also participate in enzyme-catalyzed reactions by forming covalently linked intermediates in the reaction sequence. So far, this has been shown to occur in the synthesis of macromolecules in bacteria and in all cases examined, the carrier lipids are polyiso-prenoid compounds. In general, the solubilization of membrane proteins requires the disruption of the organized membrane structure. Nonpolar bonds (lipid–lipid, lipid–protein, and protein–protein) are believed to play a major role in stabilizing the structure of membranes and it is, therefore, not surprising that agents that disrupt such bonds are often effective in solubilizing membrane-bound enzymes.

Abbreviations

I Introduction

II Participation of Lipids in Membrane Enzyme Reactions

A Lipids as Physical Cofactors

B Lipids as Covalently Linked Intermediates (“Carrier Lipids”)

III Studies of Individual Enzymes

A Introduction

B Methods of Isolation

C Bacterial Enzymes

D Microsomal Enzymes: Lysophosphatides and Membrane Enzymes

E Mitochondrial Enzymes

IV Regulation of the Activity of Membrane Enzymes: Some Speculations

References

ABBREVIATIONS

PE Phosphatidylethanolamine

LPS Lipopolysaccharide

I INTRODUCTION

A large number of cellular enzymes are located in membranes. Indeed, it is likely that most“particulate” enzymes are in truth membrane-bound enzymes whose particulate nature reflects their association with the lipid matrix of biological membranes. Included in this category are enzymes located in plasma membranes, mitochondria, microsomes, and in other subcellular organelles. In fact, if one is willing to extend the definition to proteins which catalyze the physical translocation of substrates, the class of membrane enzymes can be extended to include proteins responsible for transmembrane transport, such as bacterial permeases.

The existence of membrane-bound enzymes has been known for many years, but until recently these enzymes were thought to be refractory to solubilization and hence were considered unsuitable for purification and detailed study. In the past ten years, however, it has become clear that many membrane-bound enzymes require the lipid components of membranes for activity and that previous difficulties In “solubilization” were due to the failure to recognize this fact.

Fleischer and his co-workers (Fleischer and Klouwen, 1961; Fleischer et al., 1962) were the first to show clearly a lipid requirement in enzyme reactions by demonstrating that lipid extraction resulted in a marked decrease in activity of several reactions of the mitochondrial electron transport system and that activity was restored by the addition of phospholipids. Since then a large number of membrane-bound enzymes from mitochondria, microsomes, and bacteria have been studied and have been shown to be affected by phospholipids.

These observations suggest that it should be possible to dissociate membrane-bound enzyme systems into phospholipid and protein components and to restore the biological activity by recombining the purified components, thus achieving the reconstitution of a functional portion of the original membrane. Thus far this has been convincingly achieved in only a few cases, as discussed below.

The significance of this type of experiment is severalfold. First, it permits study of a new parameter of enzyme catalysis, namely the role of lipids as cofactors in enzyme reactions. Second, it provides a model system for study of the physical and chemical aspects of lipid–protein interactions of biological significance. Third, it permits detailed study of perhaps the most important unanswered question of membrane structure—the nature of the interaction of lipids and proteins in membranes. Therefore, a discussion of enzyme reactions in membranes must focus chiefly on the several roles of membrane lipids in these reactions. They can be divided into two general categories: (a) lipids as physical cofactors in membrane enzyme reactions and (b) lipids as covalently linked intermediates in membrane enzyme reactions.

II PARTICIPATION OF LIPIDS IN MEMBRANE ENZYME REACTIONS

A Lipids as Physical Cofactors

A requirement for phospholipids in the activity of many membrane-bound enzymes has now been shown. In all cases the basic experimental scheme is similar—enzyme activity is lost when lipid is removed from an enzyme system and is restored when lipid is added back. In most cases the lipids do not form covalently linked intermediates in the reaction sequence, and they therefore play the role of physical cofactors, activating the enzyme system but not themselves participating in the reaction.

Several possible mechanisms can account for this type of effect. (a) The lipid can activate the substrate. This has been demonstrated clearly with the enzymes that glycosylate bacterial lipopolysaccharides (see below). (b) The lipid can directly activate the enzyme, perhaps by inducing a conformational change in the p...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• LIST OF CONTRIBUTORS
• PREFACE
• INTRODUCTION
• NEWER APPROACHES TO THE STUDY OF MEMBRANE STRUCTURE
• STRUCTURE-FUNCTION RELATIONSHIPS IN BIOLOGICAL MEMBRANES
• AUTHOR INDEX
• SUBJECT INDEX
• Inside Back Cover