52 BIOCHEMICAL E D U C A T I O N S u m m e r 1973 Vol . 1 No . 3

Some segments o! the curriculum are so laden with bio- chemistry that the committee responsible for the system can be placed under the chairmanship of a biochemist. A prominent example is the Metabolic-Endocrine system. Other systems have no direct participation from members of the Biochemistry department, for example, Infectious Diseases, because the bio- chemical aspects of the course are easily handled by the micro- biologists and immunologists.

Recently, American biochemists in medical schools were jolted by pressures to eliminate biochemistry from the medical curriculum and make undergraduate biochemistry a prerequisite for admission to Medical School. The interweaving of biochemistry with the rest of the medical curriculum in an integrated program illustrates the need for biochemistry as an essential part of medical training. A free-standing biochemistry course in the first half of the first year of a traditional medical curriculum is much more vulnerable to displacement into the pre-medical years.

Finally, a few words about the laboratory exercises; there aren't any. In a 3-year curriculum there isn't enough time available. Moreover, to keep in tune with the rest of the curriculum, the laboratory should be integrated. Perhaps it may be possible in the future to provide the time and facilities to use the methods of all experimental disciplines in a concerted exercise to illustrate an important principle. An example would be a laboratory exercise directed at the thorough investigation of a new drug. The resources and methods of the anatomist, biochemist, biometrician, microbiologist, pathologist, pharmacist, pharmacologist and psychologist could be brought to bear on the problems of the action, metabolism and side-effects of a single drug.

.I.

GLYCOLY,IS

II¢~RODUCT I ~ ,'R EN DIG

Yeir 1 II~.o~egy H e = r t able-s=

H ~ T

RBP ~ LOC NS~

. . . . . - I ...... II .....

Chg ICSHIPS

I ' I

INF CLERI~$HI PS

G KAD~bAT ION INTERNSNIP

1 I .... I I I I

Fig. 5 Opportunities to review glycolysis in the curriculum.

L E T T E R S TO THE E D I T O R S

From Professor Dr. P. Karlson

Dear Sirs,

The contribution of Donald E. Nicholson on the pentose phosphate cycle in Biochemical Education (Vol. 1 No. 1) has, in my opinion, some shortcomings.

I certainly agree with the aim of the author that a presentation should make sense and not appear as a meaningless complex of reactions. However, according to the scheme drawn by Dr. Nicholson, the student may get the impression that this cycle is used to produce CO2 from glucose 6-phosphate. Most workers in the field agree that this is not the case. The bio- chemical "sense" of the pentose phosphate shunt is two-fold: f'trstly, to produce NADPH for other reactions, and secondly to produce pentose phosphate from hexose phosphates whenever pentose phosphate is needed - e.g. for the biosynthesis of nucleic acids. Indeed, in some tissues hexose is converted into pentose via a non-oxidative pathway resulting in the formation of 3 molecules of ribose 5-phosphate from 2 molecules of fructose 6-phosphate and 1 molecule of glyceraldehyde 3-phosphate. This reaction is not borne out in the presentation of the pentose phosphate cycle as given in the scheme of Dr. Nicholson.

With these aims in mind, I have constructed the following scheme (Fig. 1) for the interconversions involved. It is on purpose not given as a cycle but rather as a switch or "shunt". In this form, it can also be used to explain the regeneration of ribulose 5-phosphate and ribulose 1, 5-diphosphate from triose phosphate in photosynthesis, bringing another important metabolic sequence into play.

With respect to the presentation of the transketolase reaction in Dr. Nicholson's Fig. 1 and 2, 1 fully agree with him. However,

Gluo~t~ure -6- ( ~

0,2Jo ® /

[ ] "N]"

:-ktxosen in P e n n Pent~en in

Fig. 1 Scheme of pentose phosphate shunt, from P. Kartson, "Kurzes Lehrbuch der Biochemm f~Jr Mediziner und Naturwissenschaftler", 8th edit on, Heorg Thieme Verlag, Stuttgart 1972.

the argument about the name transaldolase is misleading. Trans- aldolase as well as aldolase have received their names from the fact that they catalyse an aldol re, action. Moreover, the name "aldolase" has been in use for a long time, ever since the trans- aldolase reaction was discovered and it was natural to name the enzyme in such a way that the close relationship to aldolase was apparent.

As far as transketolase goes, the derivation of this name is not so obvious, and the criticism more lustified. The reaction is

BIOCHEMICAL E D U C A T I O N Summer 1973 Vol . 1 No . 3 53

formally an acyloin addition that, starting with two aldehydes, yields a ketol. This was referred to as a "ketol reaction" and the enzyme therefore termed transketolase (c.f.E. Racker et al, J. Amer. Chem. Soc. (1951), 75, 1010). There is no doubt that the substrate molecule is a ketol as all ketose are; the final product is also a ketol. But the fragment transferred is an alde- hyde, as Dr. Nicholson properly points out. However, the proposal to call the enzymes "transtricase" and "transdicase" respectively, would never meet with the approval of the Enzyme Commission. Even ff the - C - is taken as an indication of the number of carbon atoms transferred (which is not obvious), the names do not suggest the chemical nature of the moieties transferred. Thus, they seem to me to be even worse than the old ones, which at least give some indication about the class of reaction and compounds involved.

Peter Karlson Institut f/it Physiologische Chemic Philipp s-Univer sit,it 355 Marburg (Lahn) Germany

From Professor F. Dickens

Dear Sirs,

Thank you for allowing me to see the correspondence arising from the article by D.E. Nicholson in your Autumn 1972 issue, pp 6 -7 . Although I am a grateful user of the Metabolic Pathways Chart, I also think that the uncritical use of such charts is not without its dangers. In my view, the present slightly artificial discussion on the detailed formulation of the transaldolase and transketolase reactions may be taken as an example of this. Fig. 2 on p. 7 of Dr Nicholson's article shows the transfer by transketolase of a molecule of glycolaldehyde 'which includes a hydrogen atom which is derived in effect from the -OH group on the adjacent carbon', i.e. C -3 of the ketose. This presentation 'shows that an aldehyde is left behind and not an outrageously unlikely free radical!'. This seems to me pure Papierbiochemie. The detailed mechanism of this reaction has been most satisfyingly worked out by Horecker and his colleagues, as summarized by B.L. Horecker (1968) in Carbohydrate Metabolism and its Disorders (F. Dickens, P.J. Randle and W.J. Whelan, eds. Vol. 1, pp. 139-167 especially pp. 144 et seq., Academic Press, London and New York.) The protein-bound coenzyme, thiamin pyre- phosphate, combines at C - 2 of the thiazole ring to give a glycolaldehyde-ThPP complex, as has been confirmed by its synthesis and also by isolation from the enzymic reaction. This is 'active glycolaldehyde' in the older terminology, and from it the enzyme transfers the two-carbon unit to an accepter aldehyde, e.g. to ribose 5-phosphate thus yielding sedoheptulose 7-phosphate. &study of the mechanism of this reaction (see Horecker, loc. cit.) shows that no 'outrageously unlikely free radical' is involved. Moreover, there seems to be no justification to show .the H-atom of the hydroxyl at C-3 of the donor ketose as that which is transferred, thereby becoming the hydroxyl-H atom at C-3 of the product ketose (e.g. sedoheptulose 7-phos- phate). In fact, the proposed mechanism based on experimental evidence shows that protons are taken up from the medium at several stages of the reaction, and that one of these finally becomes the hydroxyl-H atom under consideration. The point could presumably be decided also experimentally by the use of heavy water as medium, but in the meantime there seems no objection to the usual way of conventional representation.

Similar considerations apply, I think, to the transaldolase reaction. In this instance the stable complex formed by the dihydroxyacetone moiety is with the lysine e-amino group o f the enzyme protein, no coenzyme being involved. The Schiff base formed with the ketonic function is here 'active dihydro- oxyacetone'. Otherwise, similar seasoning seems to apply.

Since on this reckoning the two enzymes discussed do in fact transfer, to an accepter aldehyde, either a ketoi group or a

dihydroxyacetone moiety (the latter analogous to the aldolase reaction), the names at present in use - transketolase or transaldolase - seem quite appropriate.

With Dr Nicholson's views on the overall formulation of the pentose phosphate cycle (better called the pentose phosphate pathway, since it can and does also function either non-cyclically or non-oxidatively), I am more in agreement, and consider that one of the best (of numerous possible) diagrams is that of G.E. Glock and P. McLean (Proc.Roy.Soc.Ser.B, 1958, 149, 355 based on a still earlier one by H.A. Krebs and H.L. Kornberg), But such diagrams should not be too literally interpreted, being merely useful formalizations. Nor do we expect the fates of all the intermediates to be shown in such a diagram, as long as we remember that the provision of intermediary metabolites is often the main function of the whole pathway.

Frank Dickens 15 Hazelhurst Crescent, Findon Valley, Worthing, Sussex BN14 OHW.

From Dr. D.E. Nicholson

Dear Sirs,

You asked for a provocative article. I seem to have succeeded! It is with some trepidation that I reply to the criticism of such eminent biochemists as Professors Karlson and Dickens, but in some important respects I belive each to be wrong. Professor Karlson criticises my presentation of the pentose phosphate pathway on the grounds that it suggests that its function is the production of CO2. This may be a valid criticism but I would not have thought that many students would have interpreted it in this way since it clearly shows how and where the reducea NADP arises and this is driven home by a balanced equation. It also shows that the purpose of the cyclical part of the pathway is to regenerate five molecules of hexose from six of pentose. The main purpose in illustrating this pathway at all was to plead for clarity and accuracy. It was to insist that in any cycle, or indeed in any pathway, what goes in must come out - usually in a different form. The validity of this insistence can be seen by reference to Professor Karlson's own diagram which was constructed to illustrate the following interconversions:- 2 Fructose 6-phosphate+l glyceraldehyde phosphate ~- 3 Ribose 5-phosphate The diagram clearly shows the triose and one of the hexose molecules feeding into the system, but the origin of the second molecule of hexose is obscure and there is no obvious product. Furthermore, the diagram which is explicitly intended to illustrate the interconvertibflity of hexoses and pentoses is drawn in such a way as to suggest a complete irreversibility. The arrows all point towards pentoses and none in the reverse direction.*

! ~ 2~:b ~ l :: ~t2~'i i ~°....i ~ co :

~ . - - W + ? o HOOH HCOH HCOH HCOH

I

r -I . . . . ~

. ~ +

~ol ~ 2°H HCC~ CO

~ . . . . . . J

~ . + ~ ,

HCOH

*Comment by Professor Karlson:

This is true; it was done for graphical reasons. In the text, attention is drawn to the reversibility of the pathway.