TIBS - March 1979 N 53

the calculation as thermodynamically meaningless. It is only meaningless in this sense because the wrong figures for free energy have been used. It is not meaning- less (in a thermodynamic sense) to define the efficiency of the creatine kinase reaction as the ratio of the free energy change of ATP hydrolysis to the free energy change of creatine phosphate hydrolysis; it is merely incorrect to define it that way and to calculate it using values for standard changes in free energy. A more useful criticism of this procedure would be to de- nounce it as biochemically meaningless.

No metabolic process has yet been illumin- ated by referring to its efficiency - still less by comparing the efficiency with that of a petrol engine.

Banks and Vernon are right that even modern biochemistry textbooks do not explain sufficiently clearly the way that thermodynamic constants have been and are helpful in understanding metabolic biochemistry. But most modern books are not as bad as they imply. Furthermore, future books are more likely to be im- proved by constructive and not destructive criticism of existing books.

Thermodynamic concepts are abused The hydrolysis of ATP (and other and the approach to equilibrium com- polyphosphates) and of acyl phosphates mences, albeit at a finite rate, slowly in entails the production of a proton the grave, rapidly in the crematorium.

[eqn WI. We are sorry that Dr Garratt finds

ATP*- + H,O --f ADP3- + Pr2- + H+ this a difficult concept.

(1) It might well be that certain reactions

in intermediary metabolism are close

whereas the hydrolysis of

ROP- + H,O + ROH + Pi2-

‘simple’

(2)

phosphates does not [eqn (2)].

At neutral pH, the equilibrium for eqn (1) will, all other things being equal, be more favourable to products than will that for eqn (2). This is equivalent to the statement (agreed by Garratt) that the standard free energy changes AGXe (pH dependent, please, not ‘bio- chemical’) differs from the value of AGe by approx. 48 kJ mol-’ for re- actions in which a proton is formed. Where is the error? We repeat the main reason that the value of AGre for the hydrolysis of ATP is numerically larger than that for the hydrolysis of ‘simple’ phosphates is that a proton is formed at neutral pH.

the presence of glucose, oxygen and carbon dioxide in aqueous solution in

to equilibrium, presumably catalysed

the blood indicates that the total living system is well away from equilibrium.

by enzymes working well within their maximum capacity. Overall, however,

To proceed to the second alleged ‘error’ in our original article. Living systems are open systems exchanging both matter and energy with their surroundings. The continuous flow of matter keeps the system in steady state (hence homeostasis) away from equi- librium. After death matter flow ceases

We are glad that Dr Garratt agrees that it is meaningless to calculate ‘efficiencies’ in terms of values of AC’ but are much saddened by the suggestion that the calculation becomes meaningful in terms of actual changes in Gibbs function. It is curious that a reaction for which AC = 0 is said to be 100% efficient when, by definition, from such a reaction no external work can be derived. The statement ‘where AC is effectively zero, the efficiency with which energy is transferred from one set of chemical bonds to another is close to 100%’ typifies the abuse of thermodynamic and chemical concepts to which we originally took exception. The age of caloric we had thought to be dead, but is it? A change in Gibbs function, whether actual or standard, is not a quantity of ‘energy’ and should not be referred to as if it were.

BARBARA E. c. BANKS and CHARLES A. VERNON

Department of Physiology, Department of Chemistry, University College, University College, London University, London University, London, U.K. London, U.K.

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