93
7plummer, D T, Elliot, B A, Cooke, K B and Wilkinson, J H (1963) Biochem J 87,416-422 8Zubay, G (1993) Biochemistry, Third Edition, p 663, Brown', Dubuque, Iowa '~Rawn, J D (1983) Biochemistry, p 675. Harper and Row, New York "~Horton, H R, Moran, L A, Ochs, R S, Rawn, J D and Scrimgeour, K G (1993) Principles of Biochemistry. pp 12.20-21, Neil Patterson Publishers/Prentice Hall, Englewood Cliffs, New Jersey I I Werner, R (1992) Essential Biochemistry and Molecular Biology, Second Edition, pp 38-39, Elsevier, Amsterdam ~2Perham, R N (1993) Trends Biochem Sci 18, 270-271 ~3Bohinski, R C (1987) Modern Concepts in Biochemistry, Fifth Edition, p 5(19, Allyn and Bacon, Newton, MA ~4Conn, E E, Stumpf, P K, Bruening, G and Doi, R H (1987) Outlines of Biochemistry, Fifth Edition, p 354, John Wiley, New York I~Mathews, C K and van Holde, K E (1990) Biochemistry, p 449, Benjamin/Cummings, Redwood City, CA ~t'Voet, D and Voet, J G (1990) Biochemistry, p 444, John Wiley, New York ~7Wuntch, T, Chen, R F and Vesell, E S (1970) Science 167, 63-65 ~SVesell, E S (1975) In Isozymes edited by C L Markert, Vol 2, pp 1-28, Academic Press, New York 1"Burgher, J W, Ainslie, G R, Cleland, W W and Ray, W J (1978) Biochemistry 17, 1646-1653 -'°Cornish-Bowden, A and Cfirdenas, M L (1991) Trends Biochem Sci 16, 281-282 2~Blackstock, J C (1989) Guide to Biochemistry, pp 76-77, Wright/ Butterworth, London -~-'Newsholme, E A and Leech, A R (1983) Biochemistry for the Medical Sciences, pp 206-207, John Wiley, Chichester 23Scott, T and Eagleson, M (1988) Concise Encyclopedia: Biochemistry, Second Edition, p 331, de Gruyter, New York 24Kitamura, M and Nishina, T (1975) in lsozymes edited by Markert, C L, Vol 2, pp 97-111, Academic Press, New York 25Campbell, M K (1991) Biochemistry, p 311, Saunders College Publishing, Orlando, FL 2~'Cooper, J A, Reiss, N A, Schwartz, R J and Hunter, T (1983) Nature 302, 218-223

be a prosthetic group - - the tightly bound coenzyme, or a cofactor - - the loosely bound small organic or inorganic molecule.
The cofactors are those small organic or inorganic molecules that the enzyme requires for a c t i v i t y . . . The prosthetic group is similar to the cofactor but is tightly bound to the apoenzyme,. Devlin I Most coenzymes are linked to enzymes by noncovalent forces. Those which form covalent bonds to enzymes may also be termed prosthetic groups. Murray et al 2 A coenzyme may be defined as a molecule that possesses physicochemical properties not found in the polypeptide chain o f the enzyme and that acts together with the enzyme to catalyze a biochemical rea c t i o n . . . Tightly bound coenzymes are sometimes referred to as prosthetic groups. Zubay 3

Controversial Terminology
Whilst it is generally clear that the prosthetic group is a strongly bonded non-polypeptide helper component of a holoenzyme (as well as a non-catalytic protein) as described in most major biochemistry textbooks, the question of whether it could also be termed a coenzyme or a cofactor is a matter for debate. Textbooks by Lehninger et al 4 and Voet and Voet 5 describe the cofactor as the universal enzyme-associated non-protein factor which may either be a prosthetic group - - a metal ion essentially permanently associated with its protein, or a coenzyme - a small organic molecule transiently associated with enzyme. However, unlike Voet and Voet, who classify coenzyme and prosthetic group separately, Lehninger et al further asserts that a coenzyme that is strongly bound to the enzyme is also considered a prosthetic group.
The cofactor may be either one or more inorganic i o n s . . , or complex organic or metalloorganic molecule called a coenzyme . . . A coenzyme or metal ion that is covalently bound to the enzyme protein is called a prosthetic group. Lehninger et al 4 Cofactors may be metal i o n s . . , or organic molecules known as coenzymes . . . Some cofactors . . . are but transiently associated with a given enzyme molecule... Other cofactors, known as prosthetic groups, are essentially permanently associated with their p r o t e i n . . . Voet and Voefl

0307-4412(94) E0018-J Coenzyme, Cofactor and Prosthetic Ambiguous Biochemical Jargon ONN H HASHIM and NOR AZILA ADNAN
Department o f Biochemistry Faculty o f Medicine University o f Malaya 59100 Kuala Lumpur, Malaysia

Group

Introduction
For enzymes which require non-protein components to be functionally active, it is generally accepted that the term holoenzyme refers to the complete catalytic entity which consists of a protein part known as the apoenzyme and a non-protein constituent. The terms used for the functionally required non-protein constituents however, are ambiguous and differ from one commonly used biochemistry textbook to another. In certain textbooks, any associative non-protein factor required for the enzymatic function is termed coenzyme. The coenzyme may either

In most textbooks, the coenzyme is also said to be chemically changed by enzymatic reaction. Regeneration of the molecule may be catalysed by a different enzyme. As such, the coenzyme essentially functions as a cosubstrate. Conforming to this latter description of coenzyme, Montgomery et al 6 further defined the terms coenzyme and cofactor according to their roles in catalytic reaction. Whilst coenzyme is described as an organic

BIOCHEMICAL EDUCATION 22(2) 1994

94 molecule directly involved in catalysis, a cofactor (organic molecule or metal ion) does not directly participate in the catalytic event.

References
~Devlin. T M 11990) Textbook o! Biochemistry with Clinical Correlations. Second edition. John Wiley and Sons, New York 2Murray, R K, Granner, D K, Maycs, P A and Rodwell. V W (19931 Harper's Biochemisto', Twenty-third edition, Prentice Hall, London ~Zubay, G (1989) Biochemisto', Second edition, Macmillan, New York 4Lehninger, A L, Nelson, D L and Cox, M M (1993) Principles Of Biochemistry. Second edition, Worth. New York ~Voet, D and Voet, J G (1990) Biochemistry,John Wiley and Sons, New' York ~'Montgomery, R, Conway, T W and Spector, A A (19911)Biochemisto'a Case Oriented Approach. Fifth edition, C V Mosby, St Louis 7Stryer, L (1988) Biochemisto,. Third edition, W H Freeman, New York

Small organic m o l e c u l e s . . , essential to maintain certain enzyme proteins in a conformation suitable for c a t a l y s i s . . , are regarded as cofactors even though they do not directly participate in the catalytic e v e n t . . . The term coenzyme applies to organic' m o l e c u l e s . . , which are essential for activity of numerous enzymes. M o n t g o m e r y et al ~
C o n t r a r y to the initial terminology however, a cofactor in this case is not necessarily by definition a coenzyme. To avoid controversy, it is a p p a r e n t that several textbooks of biochemistry have uncannily refrained using the term cofactor. T e x t b o o k s by Stryer, 7 M u r r a y et al 2 and Z u b a y 3 for instance, considered any associative enzymatic nonprotein factor as coenzyme, with no mention of cofactor.

Book Review
0307-4412(941 E0017-I

The Cause
P e r h a p s one o f the contributing factors that lead to the confusion in terminology is the lack of elaborative definition o f terms. T h e terms, cofactor, coenzyme and prosthetic group have been defined t h r o u g h different perspectives. A prosthetic group is generally defined based on its covalent association with e n z y m e or proteins. Cofactor and coenzyme however, are considered through their roles in e n z y m e catalysed reaction (or protein function). D u e to this reason, any restrictive measure confining certain substances strictly to certain terms would only result in controversial and antagonistic terminology.

Methods in Molecular Biology, Volume 27: Biomembrane Protocols: II. Architecture and Function
Edited by J G r a h a m and J Higgins. pp 362. H u m a n a Press, New Jersey. 1994. $59.50 ISBN 0 - 8 9 6 0 3 - 2 5 0 - 7 The structures of biological membranes and the various functions that are carried out by membrane systems are areas of research which are essential to the work not only of biochemists and cell biologists but also immunologists, parasitologists, virologists and molecular biologists. Thus numerous books are currently available, and no doubt many more will appear over the coming years, which attempt to cover some of the techniques currently used to explore this important and multidisciplinary area of research. Biomembrane Protocols: 11 Architecture and Function is a companion to an earlier volume in the Methods in Molecular Biology series Biomernbrane Protocols: 1 Isolation and Analysis. This new volume is concerned exclusively with the organisation of membrane components and how this organisation controls function. Of the 24 chapters, the earlier ones cover topics such as crystallisation of membrane proteins for Xray analysis, determination of cell surface polarity and topography, the use of antipeptide and monoclonal antibodies for the isolation and characterisation of membrane proteins, and the reconstitution of proteins into lipid vesicles. Several chapters then deal with a variety of techniques used to measure proteinprotein and protein-lipid interactions, phospholipid topography, asymmetry and movement. The later chapters deal with more specific membrane functions such as G-protein analysis, assay of protein kinases, phosphoinositide analysis and calcium measurements. The last four chapters cover topics as diverse as membrane permeabilisation with bacterial toxins, measurement of ion fluxes and pH gradients across membranes, and ligand binding and processing. Each topic is briefly introduced and then the experimental protocols are presented in a step-by-step, easy to follow style. Numerous explanatory notes are included to help you avoid the not-so-obvious pitfalls and benefit from the authors' own experience. Thus, at least in theory, 'the reader will be able to perform the techniques without the need to consult other texts'. All in all, a useful book which covers a very wide range of topics used to study the structure and function of biological membranes. N M Hooper

Prosthetic group, coenzyme and cofactor are all generally defined. Whilst a cofactor is any factor essentially required for e n z y m e activity or protein function, a coenzyme is the cofactor which is directly involved in e n z y m e catalysed reaction. A cofactor which is not directly involved in e n z y m e catalysis, or is associated with the function of a non-catalytic protein is not a coenzyme. A prosthetic group (covalently associated non-protein constituent required for a particular function), on the o t h e r hand, m a y also be t e r m e d a coenzyme if it is directly involved in catalytic reaction. A prosthetic group which is not involved in e n z y m e catalysed reaction but functionally essential to the e n z y m e or a non-catalytic protein is also called a cofactor. Coenzyme and cofactor weakly b o u n d to e n z y m e or protein are however, not classified as prosthetic group.

The Solution
As teachers o f biochemistry to science and medical u n d e r g r a d u a t e s , we often e n c o u n t e r questions pertaining to these controversial terms. T o avoid confusion and p e r m a n e n t l y resolve the prevalent controversy, we strongly feel that m o r e emphasis and elaboration should be p r e s e n t e d in defining each of the terms in all textbooks of biochemistry.

B I O C H E M I C A L E D U C A T I O N 22(2) 1994