carbohydrate metabolism Flashcards
glycolysis definition
Oxidation of glucose or glycogen to pyruvate
and lactate is called glycolysis. It occurs virtually in all tissues. Erythrocytes and
nervous tissues derive its energy mainly from glycolysis.
This pathway is unique in the sense that it can utilise O2 if available (aerobic) and it can function in absence of O2
also (anaerobic).
two phases of glycolysis
Aerobic phase: Oxidation is carried out by dehydro-
genation and reducing equivalent is transferred to
NAD+. Reduced NAD in presence of O2 is oxidised
in electron-transport chain producing ATP.
* Anaerobic phase: NADH cannot be oxidised in
electron transport chain, so no ATP is produced in
electron transport chain. But the NADH is oxidised
to NAD+ by conversion of pyruvate to lactate, without
producing ATP. Anaerobic phase limits the amount
of energy per mol. of glucose oxidised. Hence, to
provide a given amount of energy, more glucose must
undergo glycolysis under anaerobic as compared to
aerobic.
stage 1 of glysolysis
Stage I
This is a preparatory stage. Before the glucose molecule
can be split, the rather asymmetric glucose molecule is
converted to almost symmetrical form fructose 1,6-
biphosphate by donation of 2 PO4 groups from ATP.
1.1 -
- Uptake of glucose by cells and its phosphorylation:
Glucose is freely permeable to Liver cells. Insulin
facilitates the uptake of glucose in skeletal muscles,
cardiac muscle, diaphragm and adipose tissue.
Glucose is then phosphorylated to form glucose-6-P.
The reaction is catalysed by the specific enzyme
glucokinase in liver cells and by non-specific hexokinase in liver and extrahepatic tissues (Refer
second box in right hand side this page).
-irreversible step
1-2
Conversion of fructose-6-P to fructose-1, 6-bi-P: The
above reaction is followed by another phosphory-
lation. Fructose-6-P is phosphorylated with ATP at
1-position catalysed by the enzyme phospho-
fructokinase-1 to produce the symmetrical molecule
fructose-1,6-bi-phosphate.
stage 2
Actual Splitting of Symmetrical Fructose-1-6-bi-P.
Fructose-1,6-bi-P is split by the enzyme aldolase into two molecules of triose-phosphates, an aldotriose–glyceral-
dehyde-3-P and one ketotriose, Dihydroxy acetone-P.
-reversible
inhibitors -1
Bromohydroxyacetone-P: It resembles structurally
to dihydroxyacetone-P. Hence it binds covalently
with the γ-COOH group of a glutamate residue of the
enzyme phosphotriose isomerase at the active site of
the enzyme molecule. Thus the enzyme becomes
inactive and cannot catalyse the reaction. It blocks
glycolysis at the stage of dihydroxyacetone-P and
leads to accumulation of dihydroxyacetone-P and
fructose-1,6-bi-phosphate.
stage 3
It is the energy-yielding reaction. Reactions of this type
in which an aldehyde group is oxidised to an acid are
accompanied by liberation of large amounts of potentially
useful energy.
This stage consists of the following two reactions:
3.1
- Oxidation of glyceraldehyde-3-P to 1,3-bi-phospho-
glycerate: Glycolysis proceeds by the oxidation of
glyceraldehyde-3-P to form 1,3-bi-phosphoglycer`ate.
Dihydroxyacetone-P also form 1,3-bi-phospho-glycerate via glyceraldehyde-3-P. Enzyme responsible
is Glyceraldehyde-3-P dehydrogenase which is NAD+
dependant.
3-2
- Conversion of 1,3-Biphosphoglycerate to 3-Phos-
phoglycerate The reaction is catalysed by the enzyme phospho-
glycerate kinase. The high energy PO4 bond at position-
1 can donate the PO4 to ADP and forms ATP molecule.
stage 4
It is the recovery of the PO4 group from 3-Phospho-
glycerate. The two molecules of 3-phosphoglycerate, the
end-product of the previous stage, still retains the PO4
group originally derived from ATP in stage 1. Body wants
back the two ATP spent in first stage for two phosphory-
lations. This is achieved by the following three reactions:
4.1
- Conversion of 3-Phosphoglycerate to 2-Phospho-glycerate
3-phosphoglycerate formed by the above reaction is
converted to 2-phosphoglycerate, catalysed by the
enzyme Phosphoglycerate mutase. It is likely that
2, 3-bi phosphoglycerate is an intermediate in the
reaction and probably acts catalytically.
4.2
- Conversion of 2-Phosphoglycerate to Phosphoenol
Pyruvate
The reaction is catalysed by the enzyme Enolase, the
enzyme requires the presence of either Mg++ or Mn++ for
activity. The reaction involves dehydration and redistribution of energy within the molecule raising the PO4 in position 2 to a “high-energy state”.
4.3
Conversion of Phosphoenol Pyruvate to Pyruvate
Phosphoenol pyruvate is converted to ‘Enol’ pyruvate,
the reaction is catalysed by the enzyme Pyruvate
kinase. The high energy PO4 of phosphoenol pyruvate
is directly transferred to ADP producing ATP (Refer
box).
-irreversible
glycolysis produces in presence of O2
Stage I
1. Hexokinase/
Glucokinase
reaction (for phosphorylation) – 1 ATP
2. Phosphofructokinase-1
(for phosphorylation) – 1 ATP
Stage III
3. Glyceraldehyde-3-P dehydrogenase
(oxidation of 2 NADH in
electron transport chain) + 6 ATP’
4. Phosphoglycerate kinase
(substrate level
phosphorylation) + 2 ATP
Stage IV
5. Pyruvate kinase
(substrate level phosphorylation) + 2 ATP
Net gain = 10–2
= 8 ATP
glysolysis in absence of O2
- In absence of O2, reoxidation of NADH at glyceralde-
hyde-3-P-dehydrogenase stage cannot take place in
electron-transport chain.
- But the cells have limited coenzyme. Hence to conti-
nue the glycolytic cycle NADH must be oxidised to
NAD+. This is achieved by reoxidation of NADH by
conversion of pyruvate to lactate (without producing
ATP) by the enzyme lactate dehydrogenase.
It is to be noted that in the reaction catalysed by gly-
ceraldehyde-3-P-dehydrogenase, therefore, no ATP is
produced.
In anaerobic phase per molecule of glucose oxidation
4 – 2 = 2 ATP will be produced.
+ 2 ATP
regulation of glycolysis
by 3 mechanism:
a-induction and repression of key enzymes
b-covalent modification by reversible phopshorylation
c-allosteric modification
induction and repression of key enzymes
This is not
rapid and takes several hours to come into operation.
* Glucose: When there is increased substrate, i.e.
glucose, the enzymes involved in utilisation of
glucose are activated. On the other hand, enzymes responsible for producing glucose (gluconeogenesis) are inhibited. Glucose also increases the
activity of the key enzymes glucokinase, phospho-
fructokinase-1 and pyruvate kinase.
* Insulin: The secretion of insulin which is res-
ponsive to blood glucose concentration enhances
the synthesis of the key enzymes responsible for
glycolysis. On the other hand, it antagonises the
effects of glucocorticoids and glucagon-stimulated
c-AMP in stimulating the key enzymes responsible
for gluconeogenesis.
covalent modification by reversible phosphorylation
Hormones like epinephrine and glucagon which increase cAMP level activate cAMP-dependant Protein kinase which can phosphorylate and inactivate the Key enzyme Pyruvate kinase and, thus, inhibit
glycolysis. This is a rapid process and occurs quickly.
allosteric regulation
Phosphofructokinase-1 is the
Key regulatory enzyme and is subject to “feedback”
control.
* Inhibition of the enzyme: The enzyme is inhibited
by citrate and by ATP.
* Activator of the enzyme: The enzyme is activated
by AMP.
* AMP acts as the indicator of energy status of the
cell: When ATP is used in energy requiring proces-
ses resulting in formation of ADP, the
concentration of AMP increases. Normally ATP
concentration may be fifty times that of AMP
concentration at equilibrium, a small decrease in
ATP concentration will cause a several fold rise in
AMP concentration. Thus a large change in AMP
concentration acts as a metabolic amplifier of a
small change in ATP concentration.
The above mechanism allows the activity of the
enzyme phosphofructokinase-1 to be highly sensitive to
even small changes of energy status of the cell and hence
it controls the amount of glucose that should undergo
glycolysis prior to its entry as acetyl-CoA in TCA cycle.
converstion of pyruvate acid to lactic acid
It is an important reaction, because it occurs in skeletal
muscles working under conditions of absolute or
relative lack of O2. In anaerobic glycolysis, Pyruvate
acts as a temporary H-store. It dehydrogenates
(oxidises), the reduced NADH + H+ back to oxidised
NAD+, so that glycolysis can continue even in absence
of O2. Pyruvate is thus reduced to Lactic acid. In
presence of O2, Lactic acid can be oxidised to pyruvic
acid again.
* Reversible reaction
* Oxidation-reduction
* Same enzyme and co-enzyme required.
convertion of pyruvate to OAA (CO2 fixation reaction)
Pyruvic acid can be converted to oxaloacetate by the
enzyme Pyruvate carboxylase. The enzyme requires:
* ‘Biotin’ as a prosthetic group which brings CO2
* ATP and Mg++
* Requires ‘acetyl-CoA’
Acetyl-CoA does not enter into the reaction but may
by combination with the enzyme maintains it in “active”
conformation (+ve modifier). The generation of “acetyl-
CoA” in metabolic reactions activates the enzyme and
promote the formation of oxaloacetic acid (OAA) required
for oxidation of acetyl-CoA in the TCA Cycle.
convertion of pyruvate to OAA through malic acid formation
The other anaplerotic reaction is formation of malic
acid by Malic enzyme in presence of CO2 and NADPH.
The ‘malate’ is converted to OAA by dehydro-
genation by the enzyme Malate dehydrogenase in
presence of NAD+.
covertion of pyruvate to acetyl coA
In presence of O2, Pyruvate undergoes oxidative
decarboxylation to form 2-C compound ‘acetyl-CoA’
Pyruvate formed in cytosol is transported to
mitochondrion by a ‘transport’ protein. Since the overall reaction involves both oxidation and loss of
CO2 (decarboxylation) it is termed oxidative
decarboxylation. The mechanism of the reaction is one
of the most complex involved in metabolism of carbohydrates. The reaction is catalysed by a multi-enzyme complex called pyruvate dehydro-
genase complex, which can exist both as “inactive form”
and the “active” form
mechanism of formation of acetyl coA
- ‘Acetyl’ moiety of PA is transferred to CoA –SH.
- Carbon of COOH group is liberated as CO2 (decar-
boxylation). - Remaining two H atoms: One from –COOH group of
PA and another from CoA –SH (–SH group) are
transferred to NAD+, by way of a mechanism
involving Lipoic acid and FAD.
energteics of production of acetyl coA
One molecule of glucose produces two molecules of PA
which inturn by oxidative decarboxylation produces
2 molecules of Acetyl-CoA and 2 NADH. Two molecules
of NADH will be oxidized to 2 molecules of NAD+
producing 6 ATP molecules in respiratory chain.
+ 6 A T P
citric acid cycle
It is a cyclic process.
* The cycle involves a sequence of compounds interrelated
by oxidation-reduction and other reactions which finally
produces CO2 and H2O.
* It is the final common pathway of break down/catabolism
of carbohydrates, fats and proteins. (Phase III of
metabolism).
* Acetyl-CoA derived mainly from oxidation of either glucose
or β-oxidation of FA and partly from certain amino acids
combines with oxaloacetic acid (OAA) to form citrate the
first reaction of citric acid cycle. In this reaction acetyl-CoA
transfers its ‘acetyl-group’ (2-C) to OAA.
stage 1.1 of CAC
Formation of Citric Acid from Acetyl-CoA and OAA
- An irreversible reaction and an exergonic reaction-
gives out 7.8 Kcal. - Acetyl group of acetyl-CoA is transferred to OAA,
no oxidation or decarboxylation is involved. - A molecule of H2O is required to hydrolyse the “high
energy” bond linkage between the acetyl group and
CoA, the energy released is used for citrate
condensation. No ATP is required. - COA-SH released is reutilised for oxidative de-
carboxylation of PA.
1.2-CAC
Formation of cis-aconitic acid and isocitric acid from
citric acid: Citric acid is converted to isocitric acid by
the enzyme aconitase. This conversion takes place in
two steps:
* Formation of cis-aconitic acid from citric acid as
a result of asymmetric dehydration, and
* Formation of isocitric acid from cis-aconitic acid
as a result of stereospecific rehydration. Both processes are catalysed by the same enzyme
Aconitase which requires Fe++.
stage 2 of citric acid cycle
Stage II: The six-carbon isocitric acid is converted to a
derivative of the four carbon succinyl-CoA. The isocitric
acid undergoes oxidation followed by decarboxylation
to give α-oxoglutarate (5 C) (α-ketoglutarate).
2.1 CAC
- Formation of oxalosuccinic acid and α-oxo-glutarate
from isocitric acid: Since it is not possible to separate
the dehydrogenase from the decarboxylase activity,
it is concluded that these two reactions are catalyzed
by a single enzyme. It is believed that oxalo-succinate
is not a free intermediate but rather exists bound to
the enzyme.
Respiratory chain-linked oxidation of isocitrate
proceeds almost completely through the NAD+ depen-
dant ICD in mitochondrion.
2.1 CAC
- Oxidative decarboxylation of α-oxoglutarate to
succinyl-CoA: This reaction is analogous to oxidative
decarboxylation of Pyruvic acid to acetyl-CoA.
Enzyme is α-Ketoglutarate dehydrogenase complex,
stage 3 of cac
The product of preceding stage succinyl-CoA is converted
to succinic acid to continue the cycle. Enzyme catalysing
this reaction is succinate thiokinase (also called as
succinyl-CoA synthase).
