carbohydrate metabolism

Question 1

glycolysis definition

Answer 1

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).

Question 2

two phases of glycolysis

Answer 2

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.

Question 3

stage 1 of glysolysis

Answer 3

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.

Question 4

1.1 -

Answer 4

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

Question 5

1-2

Answer 5

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.

Question 6

stage 2

Answer 6

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

Question 7

inhibitors -1

Answer 7

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.

Question 8

stage 3

Answer 8

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:

Question 9

3.1

Answer 9

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.

Question 10

3-2

Answer 10

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.

Question 11

stage 4

Answer 11

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:

Question 12

4.1

Answer 12

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.

Question 13

4.2

Answer 13

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”.

Question 14

4.3

Answer 14

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

Question 15

glycolysis produces in presence of O2

Answer 15

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

Question 16

glysolysis in absence of O2

Answer 16

• 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

Question 17

regulation of glycolysis

Answer 17

by 3 mechanism:
a-induction and repression of key enzymes
b-covalent modification by reversible phopshorylation
c-allosteric modification

Question 18

induction and repression of key enzymes

Answer 18

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.

Question 19

covalent modification by reversible phosphorylation

Answer 19

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.

Question 20

allosteric regulation

Answer 20

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.

Question 21

converstion of pyruvate acid to lactic acid

Answer 21

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.

Question 22

convertion of pyruvate to OAA (CO2 fixation reaction)

Answer 22

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.

Question 23

convertion of pyruvate to OAA through malic acid formation

Answer 23

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+.

Question 24

covertion of pyruvate to acetyl coA

Answer 24

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

Question 25

mechanism of formation of acetyl coA

Answer 25

1. ‘Acetyl’ moiety of PA is transferred to CoA –SH.
2. Carbon of COOH group is liberated as CO2 (decar-
   boxylation).
3. 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.

Question 26

energteics of production of acetyl coA

Answer 26

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

Question 27

citric acid cycle

Answer 27

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.

Question 28

stage 1.1 of CAC

Answer 28

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.

Question 29

1.2-CAC

Answer 29

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++.

Question 30

stage 2 of citric acid cycle

Answer 30

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).

Question 31

2.1 CAC

Answer 31

1. 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.

Question 32

2.1 CAC

Answer 32

2. 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,

Question 33

stage 3 of cac

Answer 33

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).