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INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY INORGANIC CHEMISTRY DIVISION in conjunction with the
ROYAL IRISH ACADEMY THE CHEMICAL SOCIETY (LONDON) THE INSTITUTE OF CHEMISTRY OF IRELAND
COORDINATION CHEMISTRY-XVI Plenary lectures presented at the XVIth INTERNATIONAL CONFERENCE ON COORDINATION CHEMISTRY held at Dublin, Ireland 19-24 August 1974 Symposium Editors D. A. BROWN (University College, Dublin) and W. J. DAVIS (University of Dublin)
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The contents of this book appear in
Pure and Applied Chemistry, Vol. 44, No. 1 (1975) All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including photocopying and recording, without the written permission of the copyright holder, application for which should be addressed to the publisher. Such written permission must also be obtained before any part of this publication is stored in a retrieval system of any nature.
International Union of Pure and Applied Chemistry 1975 ISBN 0408 70726 7
Printed in Great Britain by Page Bros (Norwich) Ltd., Norwich
EXECUTIVE COMMITTEE D. A. BROWN (Chairman) W. J. DAVIS (Secretary) B. HATHAWAY J. G. JONES S. M. NELSON S. O CINNEIDE J. TEEGAN
VI
FUNCTIONAL AND CONFORMATIONAL PROPERTIES OF CARBOXYPEPTIDASE A*f BERT L. VALLÉE
The Biophysics Research Laboratory, Department of Biological Chemistry, Harvard Medical School and Division of Medical Biology, Peter Bent Brigham Hospital, Boston, Massachusetts, USA ABSTRACT The conformations of carboxypeptidase Αα> γ have been investigated by comparing chemical, spectral and kinetic data in solution and crystals with the results of x-ray structural analysis of the enzyme crystals. The data indicate that the conformation(s) in the two physical states differ and the consequent limitations of mechanistic interpretations of structural analysis are discussed. The chemical, spectral and kinetic results show that the physical state of an enzyme is a significant experimental variable that likely induces changes in function-related-conformation. Since the kinetic properties of carboxypeptidase crystals differ markedly from those of solutions, enzyme substrate model building, based on kinetics in solutions, would seem of questionable value in solving mechanistic problems. INTRODUCTION Three-dimensional structures of enzymes derived from x-ray analysis of crystals have served increasingly to deduce their mechanisms of action in solution, based on the hypothesis that the conformations in the two physical states are the same. This assumption is critical in assigning functional significance to the structural details of active sites of enzymes. It would be difficult to define the precise roles of amino-acid sidechains thought to be involved in catalytic mechanisms if their positions were to be a function of the physical states of a protein. That proteins can assume multiple and readily interconvertible but closely related conformations has long been suspected1 and is supported now by x-ray studies of proteins and enzymes indicating that sidechains and segments of peptide chains may be motile 2-5 . Since protein structure analysis is often carried out under con* This research was supported by Grant-in-Aid GM-15003 from the National Institutes of Hea