W. Grady Smith and J. Lyndal York University of Arkansas Medical Center Little Rock, 72201
Stereochemistry of the Citric Acid Cycle
M o s t standard textbooks1 of biochemistry contain a d~ngramshowing the reactions of the citric acid with the chemical formulas written in a Fischer Projection. They also refer to the "Ogston Concept" of three point attachment2 between citrate and aconitase to explain the enzymatic differentiation of the two carboxymethyl groups of citrate. Often, however, the obvious requirement that the reaction forming citrate must also be stereospecificsuch that the acetate derived carboxymethyl group always occupies the same relative position in the citrate formed is not emphasized. Possibly as a result, the Fischer Projection representations of the citric cycle reactions often show the wrong stereochemistry. I n essence, what most authors have done is to explain the enzymatic non-identity of the citrate carboxyl groups but then have failed in the logical extension of the concept when they show the resulting aconitate and isocitrate in the wrong stereochemical configuration. The absolute configuration of citrate synthesized in mammalian and most microbial systems is known to be as follows3 COOH
700
Textbook Errors,
COOH HA-H Ho-LooH
-
H-LH AOOH citric acid
a-COOH IN
HOOC-C
H-A-HI
AOOH isocitric acid
-
COOH I
H-LH
C-H
e Hooc-
I
H-c-H
HA - H 4OOH citric acid
*&OOH cis-aconitic acid
COOH I
H-LOH I
es HOOC-C-H
Hd-H *hOOH isocitric acid (2R:3S)
or the alternative 0
II
C-COOH HA-H
I
O
O
COOH H& -O -H
I
acetyl CoA
I
I n addition, the isocitrate resulting from the aconitase reaction should be shown as COOH
H-A-cooH H-C-H
d S c o A
COOH
~
-
Hod-H
II
I ~I
H-6~
+H
0 oxaloacetic acid
COOK
"H I
I
HOOCA
Thus, the cis-aconitate, even though i t may only be an enzyme bound inter~nediate,~ should be shown as: either I or I1 rather than the trans structure (111) which is usually shown in most standard texts.
II
C-COOH
0
I
I
H-bH
II
COOH H-c-H
oxaloaeet~tederived, aconitase active (portion of OOH the molecule modified by aconitase)
COOH
6-H
either with or without designating the top carboxymethyl group as being derived from oxaloacetate, which in any case is incorrect in the absolute stereochemical sense and leads to a formula for isocitrate which is the antipode of biosynthetically formed isocitrate which is known to be 2R:3S i~ocitrate.~ Therefore, using Fischer Projections the first few steps in the citric acid cycle should be shown as follows
acetate derived, aconitase inactive
COOH
COOH
I
AOOH cis-aconitie acid
Hoocd-OH
a
COOH
'
I
H
COOH oxaloacetic acid
8
+ HIc-*ScoA +
acetyl CoA
Suggestions of material suitable for this column and guest columns suitable for publication directly should bk sent with as many details as possible, and particularly with reference to modern textbooks, to W. H. Eberhardt, School of Chemistry, Georgia Institute of Technology, Atlanta, Georgia 30332. ' Since the purpose of this column is t o prevent the spread and continuation of errors and not the evaluation of individual texts, the sources of errors discussed will not be cited. In order to be presented, an error must occur in at least two independent recent standard books. OGSTON, A. G., N c ~ u162, T ~963 , (1948). HANSON, K. R.,AND ROSE, I. A,, PTOC.Nat. Sei., 50, 981
.- .
(19R?> ~ . .. . ,
All too often, what one sees in textbooks is shown as 588
/
Journal of Chernicol Educofion
\Ve realize that or-aronirste may arise a1 a side product d aronitase action and may thrrrfore fnde iwru the I irrir arid cycle in future yrars, hr,wwrr, wxr. now brmg pt~I,IisIterlinclude ir.
HA - H
I
COOH citric acid
H-c
I1
LOOH cis-aonitic acid
Ho-A-H &OOH Isocitric acid (2R:3S)
I n summary, a large percent of the biochemistry texts show the trans structure for aconitate but label it cis. The cis form is indeed the biologically active isomer.' Also, most all of the texts show the structure of an isocitrate that is not the biological active form. I n fact we have found only two texts which correctly depicts MARTIUS, C. Z., Physiol. Chem., 247, 104 (1937).
the stereochemistry of the citric acid cycle. A student who attempted to understand the pathway from a stereochemical viewpoint would be hopelessly bewildered. These errors do not detract significantly, in our opinion, from the students being able to appreciate the biological utility of the cycle or from his understanding of its function. However, in the interest of scientific accuracy as well as helping the student who tries to go beyond the textbook in understanding, future authors should use Fischer Projection formulas only when the correct stereochemistry is known and then should use them accurately. Acknowledgment
We would like to thank Miss Chinan Fan for asking the question which stimulated our investigation of the stereochemistry of the Krebs Cycle as it appears in biochemistry textbooks.
Volume 47, Number 8, August 1970
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