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24.1Catabolism 7rl
24.I Cotobolism
AIMS: To write an equotionfor the cotobolismof one mole of
glucose.Tonome the three stogesof oerobiccatobolism
of glucose.
Aerobic catabolism ofglucose is
accomplished in three stages.
One tomato and one potato each
contain about 3.5 g of sugar;an
orange contains about 12.5g, and
a banana,about lB g.
Glycolysis
glylcos(Greek): sugar or sweet
lysis(Greek): splitting
The glucose produced by digestion of the carbohydrate.sa person eats
enters the bloodstream. In a normal individual, this glucose is extracted
from the bloodstream by body cells. Once inside the cells, the glucoseis
completely oxidized to carbon dioxide and water.
c6H12o6+ 02 +
Glucose oxygen
6co2 +6H2o
water
*?:
The paths that cells use to oxidize glucose completely to carbon dioxide
involve many individual chemical reactions. But all aerobic cells have
adopted essentially the same three-stagemaster plan to do the job. Briefly
these three stages,sho'*min Figure 24.1, consist of initial breakdor.min glycolysis, further degradation to acetyl coenzymeA, and, flnally complete
oxidation in the citric acid cycle.
Glycolysis
StageI
Glucose
CCCCCO
H
&
Glycolysis
C
I
(--t\
u-v
CHt
*
*
Pyruvic acid
Pyruvate
ccc
ooH
\,/
I
g
_g
Glycolysis is the sequeryce
of chemical reactionsby which glucose,a six-carbon sugar,is cleauedto two moleculesof pyruuate, a three-carbonacid. Biochemists usually call organic acids produced in metabolism by the names
of their dissociatedforms, since these are the forms that exist at pH 7.0.
P1'ruvateis simply the anion of pyruvic acid.
ccc
oo-
\,/
C
I
I
C:O
+ H'
CHu
Plruvate
Glycolysis is sometimes called the glycolytic pathway or the Embden-Meyerhof pathway after two German biochemists, Gustav Embden and Otto
Meyerhof, who proposed it in the latter part of the nineteenth century. .
stasell
=p
Acetyl coenzymeA
AceryI coenzynneA
@CO
StageIII
CC@^
&
g
The second stageof glucosecatabolism occurs when p1'ruvateions lose car- .
bon dioxide.The remaining two-carbon fragments end up attached to coen- \
zymeAto form acerylcoenzymeA (Fig.za.D.Coenz5rmeA,usuallyabbreuiated CoA, is a thiol; acetyl CoA ls the thioesterof acetic acid and CoA.The
structure of CoA contains pantothenic acid, one of the B-complexvitamins.
Citric acid cycle
@@
@@
Figure24.1(left)
Theoxidationof glucose,
a six-carbon
by
sugar(thecarbonatomsarerepresented
yellowcircles),
to sixmolecules
of carbondioxideoccursin threestages.
Twomoleculesof carbondioxide(purplecircles)areproducedin the secondstage,and
fourareproducedin the thirdstage.
7T2
CHAPTER
24 Carbohydrates
in Livingorganisms
N
.:*S
j-t
-lN.-e\
-r\t.\O-
S
Sr.rO
s", ^\
\
\
p a n L o r h e nacid
From
F r o mpantothenic
aic i
iq- \\
L*ffi-
NH,
S
a.N',n\*' '-i
-,,t a$"
*'$*
s
f \'tr-.
\
.df
,/-\,-"--\
ooH9H,
cHe- cH,- r,r- C- cH,
- - cHe-N
- t - J- J- J -.*r,
|
Il
HHHOCH3
CH
N
o
tl
-o-l-o-iof"i
-o
-o
(a) CoenzymeA (CoA)
ooH
Figure24.2
(a) The molecularstructure
of coenzymeA, Noticethe
portionof the moleculethat
comesfrom pantothenicacid,
a B-complexvitamin. (b) A
block diagramof coenzymeA.
(c) Acetyl coenzymeA is the
thioesterof aceticacid and
coenzymeA.
I
O:P-O-
\F'P'
(b) Block diagram of coenzymeA
I
o-
(c) Block diagram of acetyl coenzymeA
Like ATB acetyl CoA is often considered an energy-rich compound. Just
as the transfer of a phosphoryl group of AIP to some other molecule is
made easier by its attachment to ADB the transfer of an acetyl group is
made easierby its attachment to CoA.
Citric
acid cycle
The acetyl group of aceryl CoA enters the citric acid cycle. The citric acid,
rycle ls the pathway used by most organisms to oxidize completely to carbon
dioxide the acetyl carbons of acetyl CoAformed in the breakdown of sugars,
fats, and amino acids. Prodrtction of two molecules of carbon dioxide and
two ofwater for eachmolecule of acetylCoAentering the cyclecompletesthe
third stageof glucosecatabolism.The citric acid cycle is also called the Krebs
cyclnafter Sir Hans Krebs,the English biochemist who proposed it in 1937.
24.2 Glucoseoxidation
AtM: To stote the entry pointsof sugors,fotty ocids,ond onino\
acids into the three stogesof the aerobiccatobolismof
glucose.
Focus
The oxidation ofglucose is the
core of catabolism.
Most biochemists consider the oxidation of glucose to be the core of catabolism because the cells of virtually all organisms rely on glucose as their
main source of energy.Human brain cells,for example,will accept no other
nutrient to obtain energy except under dire circumstances such as starva-
24.3 Glycolysis 7T'
Sugars
Fatty acids
t
t
t
t
t
tffi
Glucose
Amino acids
tion. Many other cells do oxidize other sugars, fatty acids, and amino
acids to obtain energy, however. Certain enz)rrnescontained in such cells
degradethesesubstancesto compounds that eventually enter catabolism
through the central core of glucose metabolism. Figure 24.3 shows the
entry points.
Many organisms have the enzymes necessaryto convert galactose,
fructose,and other hexosesinto glucose.Thesesugarsthereforeenter glycolysis as glucose.Fatty acids are oxidized and enter the central core of
glucose catabolism as acetyl CoA. Becauseof the variety of amino acid
structures, their degradation products enter the central core at several
different points: at the tail end of glycolysis as pyruvate, as acetyl CoA, or
as chemical intermediates of the citric acid cycle.
PlTuvate
AcerylCoA
Citric acid cycle
Figure24.5
Proteins
to amino
arehydrolyzed
to fatty
acids;lipidsarehydrolyzed
acidsandglycerol.
Above,we see
thatthe aminoacidsandfatty
acidsareconverted
to simpler
compounds
thatenterthe main
pathways
of glucosemetabolism.
24.t Glycolysis
AIM: To list the stepsin the oerobicglycolysisof one moleculeof
glucose.
The enzymesthat catalyzethe steps of glycolysisare found in the cytoplasm
of cells.This is where glycolysisoccurs. Glycolysisbegins with the phosphorylation of glucoseto glucose6-phosphate.
HO
\/
C
C
I
I
H-C-OH
H-C-OH
I
--z----
I
ATP
HO-C-H
H-C-OH
I
H-C-OH
I
CH2OH
Glucose
I
HO-C-H
ADP
I
H-C-OH
I
H-C-OH
t^
cH2o-{B)
Glucose6-phosphate
o
(Thesymbol@ representsthe phosphoryl group -P-O-.;
Focus
One glucose molecule
produtes two ATP and two
NADH molecules in glycolysis.
The phosphorylation of glucose to
form glucose6-phosphateis so
energetically favorable that essentially all the glucose that enters
the cell is immediately phosphorylated.
d
The names and structures of some of the intermediate compounds in
metabolism are complex. You do not need to memorize them, but they wi[
be used in the text to make it easierto followwhat is happening. Remembei
also that all the steps ofthese reactions are catalyzedby enzyrnes.
The phosphoryl group of glucose 6-phosphate comes from ATP This
may seem a little surprising. Since glycolysisis a pathway of catabolism, we
might expect it to produce NlP,not to zseit!The important point here is that
the cell is investingAlB just asyou might invest money in the stock market.
Your investment-if vou are luckv-will be returned with dividends of more