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Transcript
25
Is there a chain of two or
more hydrocarbon carbons
following carbon number
three (/~ carbon?]
A Summary of Amino Acid Metabolism Based on
Amino Acid Structure
LON J VAN WINKLE
Department of Biochemistry
Chicago College of Osteopathic Medicine
1122 East 53rd Street, Chicago, IL 60615, USA
A difficult pedagogic problem in Biochemistry is teaching
amino acid metabolism. The names of amino acids and the
names of the Krebs-cycle intermediates, pyruvate and/or
acetyl CoA to which the amino acids are first metabolized
are often presented to summarize these catabolic pathways (Fig l). 1-5 However, this approach does not reinforce learning of the structures of the compounds involved
nor does it encourage students to recognize the relationships between structures of compounds in a metabolic
pathway. Thus, students may memorize the names of
amino acids and the names of selected compounds to
which these substances are converted in order to pass an
exam, but it is unlikely that they will retain this information after the examination period.
We have devised a method for summarizing catabolism
of amino acids to intermediates of the Krebs cycle,
pyruvate and/or acetyl CoA based on the structures of
these compounds. This method seems to help students
understand the underlying relationships between biochemicals and remember the structures of related compounds.
The method is based on three basic concepts. First, part
or all of the carbon skeleton of six of the amino acids
coded for in D N A must be metabolized to acetyl CoA in
mammals. These six amino acids can be considered to be
kysine
ICYsteine 1 " ~
I serene I
IPhenyLalanine
~
I ryr°sine
I Oxatoacetate
Krebs
\
~.
]SuccinyL
/
cycle
1
~
CoA~'f
/
GLutamatIe
GLutaminIe
,A,!g)?ine
, I
p'rSi': ne
Methionine I
Threonine
Vat ne
Figure 1 Summary of conversion of amino acids to
intermediates of the Krebs cycle, pyruvate and~or acetyl
CoA
B I O C H E M I C A L E D U C A T I O N 13(1) 1985
/ / / ~ ~ Y e s
~ - ~~ L e t ' :
At Least part of the carbonI
I
I be metnb°°fzethet°°ma~e°tyLidccAUst I
~
(TLe, Leu, Lys, Phe; ~
T-~yr)
J
How Long is the Longest unbrokenI
carbon chain in the amino acid?l
(Except for gLycine, this will I
be the number of carbons in the J
intermediate to which the aminoI
l acid is first metabolized)
I
5 carbons: The a m i
is metabolized to 2_-oxogLutarate
--[(GLu, Gin, Pro, Arg, His)
n ~ o
~
~
~ l ~
n
s
(Two carbons for
IgLycine): The amino acid is
[metabolized to pyruvate
[(ALa, Set, Cys, G-~y)
Are there fewer than four carbon
to hydrogen bonds? (Implies o more
oxidized amino acid which should
produce a more oxidized intermediate
/ ' x
No : The amino acid is /
m-~etaboLized to SuccinyL~"
I Co.__AA(Thr, VaL, M--et~ J
~
Yes The amino acid is
~l metabolized to oxaLoacetate
1(Asp, Asn)
Figure 2 Structural basis of metabolism of the twenty
amino acids coded for in DNA to intermediates of the
Krebs cycle, pyruvate or acetyl CoA via major pathways
found in mammals
partly or entirely 'ketogenic' in that the portion of their
carbon skeleton which must be metabolized to acetyl CoA
cannot be used for net glucose synthesis. The six partly or
wholely 'ketogenic' amino acids are also the amino acids
which are most 'fatty acid-like '5 in that they contain a
chain of three or more hydrocarbon carbons beginning
with the 13 carbon (carbon number 3) (Figs 1, 2 and 3a).
Beta-oxidation of fatty acids, which usually have a
predominately hydrocarbon structure, must, for the most
part, also produce acetyl CoA.
The carbon skeletons of the other 14 amino acids coded
for in DNA can be used to synthesize glucose. Each of
these carbon skeletons can be converted, via major
mammalian metabolic pathways, to a particular intermediate of the Krebs cycle or to pyruvate before being metabolized to other compounds. For these pathways, the intermediate in which the carbon from the amino acid first
appears, contains the same number of carbon atoms as the
longest unbroken chain of carbon atoms present in the
amino acid (Figures 1, 2 and 3b); glycine is the only
exception to this rule. Five amino acids contain five
carbons in a row and are converted to 2-oxoglutarate,
three amino acids contain three carbon atoms and are
metabolized to pyruvate (as is glycine) and five amino
acids contain an unbroken chain of four carbons and can
lead initially to the net production of either succinyl CoA
or oxaloacetate.
26
O-
I
+
H3N--C--H
I
(a) CH2
H
(b)
z c '~c ~ /
[
jc." ~ C - -
]
Hs+N-- C
H
I
2 McGilvery, R W (1983) 'Biochemistry: A Functional Approach', third
edition, p 594, W B Saunders, Philadelphia
3 Montgomery, R, Dryer, R L, Conway, T W and Spector, A A (1983)
'Biochemistry: A Case-Oriented Approach', fourth edition, p 476, C V
Mosby, St Louis
4 Stryer, L (1981) 'Biochemistry', second edition, p 415, W H Freeman,
San Francisco
5 Van Winkle, L J (1980) Biochem Educ 8, 72-75
Recent Books on V i r o l o g y
Comprehensive
Virology,
Volume
18
(Virus-Host
Interactions)
H
(a) Tryptophan
(b) Valine
Figure 3 Examples of the relationship between amino acid
structure and metabolism
(a) Tryptophan has at least three hydrocarbon carbons in a
row beginning with the f5 carbon (carbon 3) and thus must
be converted, at least in part, to acetyl CoA (hydrocarbon
carbons are labeled a, b, c, d, e, f, and g)
(b) Valine has (a) only two hydrocarbon carbons in a row
beginning with the f5 carbon (b) an unbroken chain of four
carbons and (c) a total of more than four carbon to
hydrogen bonds. Thus, it is metabolized to succinyl CoA
before it is converted to any other intermediate of the Krebs
cycle, pyruvate or acetyl CoA
Whether an amino acid with four carbons in a row is
first catabolized to oxaloacetate or succinyl C o A also
relates to the structure of the amino acid. The more
reduced of these amino acids contain more than four
carbon to hydrogen bonds and are catabolized to the more
reduced 'four-carbon' intermediate, succinyl C o A (Figs 1,
2 and 3b). In contrast, the more oxidized of these amino
acids contain fewer than four carbon to hydrogen bonds
and are converted to the more oxidized Krebs cycle
intermediate, oxaloacetate. Because of these logical relationships between the structures of amino acids and the
four-carbon intermediates to which they can be catabolized, the principles of biological oxidation-reduction,
important to understanding the Krebs cycle, electron
transport and oxidative phosphorylation, are reinforced
when summarizing amino acid metabolism.
We believe that summarizing amino acid metabolism in
the manner described above is superior to simply presenting a summary of the names of the amino acids and
selected intermediates to which the amino acids are
metabolized. The method summarized in Fig. 2 not only
presents an overview of amino acid metabolism, but,also
reinforces important biochemical concents and emphasized biochemical relationships. In these ways students are
encouraged to understand rather than memorize biochemistry.
The help and suggestions of Mrs Yvette E Baker, Ms Barb LeBreton,
Dr David Mann and Dr Doris Norwell are gratefully acknowledged.
References
I Lehninger, A L (1982) 'Principles of Biochemistry', p 536, Worth,
New York
B I O C H E M I C A L E D U C A T I O N 13(1) 1985
Edited by H Fraenkel-Conrat and R R Wagner. pp 200.
Plenum Press, New York. 1983. $32.50.
ISBN 0 - 3 0 6 - 4 1 1 5 8 - X
T h e Herpes Viruses, V o l u m e s 1 and 2
Edited by B Roizman. pp 445 and 439. Plenum Press, New
York. 1983. $39.50/$42.50.
ISBN 0 - 3 0 6 - 4 0 9 2 2 4 - 4 and ISBN 0 - 3 0 6 - 4 1 0 8 3 - 4
The writing and publishing of research books in virology has
become a hazardous business. Indeed the dissemination of
virology research in book form has become a victim of virology's (especially molecular virology's) own success. The series
'Comprehensive Virology' is an illustrative case. The present
volume under review (volume 18) is perhaps the penultimate
volume in a series which began in 1974 with the laudable aim
of providing a 'horizontal' treatment of virology, namely to
collect several articles on different virus systems together in
single volumes under a general title. Volume 18, 'Virus-Host
Interactions', contains diverse chapters on cellular receptors
for picomaviruses, persistent infection of mice with lymphocytic choriomeningitis (LCM) virus, and two chapters on 'slow'
virus diseases, subacute sclerosing panencephalitis (SSPE) and
progressive multifocal leukoencephalopathy. I particularly
appreciated the chapters on LCM virus and SSPE. LehmannGrube et al provide a fascinating insight into the complex
interaction of LCM virus with the host immune system and provide an extremely interesting account of a virus from a rarely
considered genus of viruses, the arenaviruses. Ter Meulen et al
provide a clearly written and truly comprehensive chapter on
SSPE viruses and their interesting relationships to measles
virus. My main caveat about this whole series is that of rapid
obsolescence. I think that this worry is less likely to be fulfilled in the case of the present volume, however libraries with
limited funds would do well to consider their purchases of
virology books, especially those in long series, with great care.
Perhaps with such factors in mind, the editors have decided
to initiate a new series of books rather than revising past
volumes in the Comprehensive Virology series. This new series
will adopt a "vertical' approach to virology, focusing on particular virus families. The first two volumes deal with herpes
viruses and look certain to be standard works for researchers
in this rapidly-advancing field. Volume 1 is concerned with primate and avian herpes viruses, preceded by a chapter on taxonomy and classification. Volume 2 deals with cytomegaloviruses, variceUa-zoster virus and a variety of herpes viruses
infecting hosts from reptiles to horses. Departments with a
strong interest in this area will certainly want to have these
volumes in their library. There is a uniformly high standard of
treatment of topics with reference to published work up to
1983. The notable omission is the herpes simplex viruses. No
doubt this will be reserved for a suitably bulky volume in the
future.
G E Blair