Carbohydrates are the sugars, starches and fibers found in fruits, grains, vegetables and milk products. Though often maligned in trendy diets, carbohydrates — one of the basic food groups — are important to a healthy diet.

1. Carbohydrate 2
Md. Saiful Islam
BPharm, MPharm (PCP)
North South University
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2. Glucose can enter into cells:
• a) by facilitative diffusion: About ten glucose transporter (GLUT)
proteins are involves in this pathway but 4 are predominant.
• GLUT 1 – blood-brain barrier, erythrocytes (RBC) and human
pancreatic β-cells
• GLUT 2 – liver, mice β-cells in pancreas (fructose and galactose
also)
• GLUT 3 – neurons
• GLUT 4 – skeletal muscles, heart muscle, adipose tissue
GLUT-4 transporters are insulin sensitive, and are found in
muscle and adipose tissue. As muscle is a principle storage site
for glucose and adipose tissue for triglyceride (glucose can be
converted for storage), GLUT4 is important in post-prandial
uptake of excess glucose from the bloodstream.

3. small intestine, kidneys
Glucose and galactose use a sodium-
glucose symport system where sodium-
dependent glucose transporter (SGLT1)
protein involves, while fructose uses
the glut5
• b) by cotransport with Na+ ion (SGLT-1 and 2)

4. Metabolism of glucose or other hexose molecules
Catabolism: Breakdown of glucose molecules to produce
energy or other products
Anabolism: To synthesis biomolecules
Catabolism of Glucose
Glycolysis
TCA Cycle / Creb Cycle
Electron transport Chain

5. Glucose occupies a central position in the metabolism of plants,
animals, and many microorganisms.
It is relatively rich in potential energy, and thus a good fuel; the
complete oxidation of glucose to carbon dioxide and water proceeds
with a standard free-energy change of 2,840 kJ/mol.
Cell can deposit large quantities of hexose units as starch or
glycogen. When energy demands increase, glucose can be released
from these intracellular storage polymers and used to produce ATP
either aerobically or anaerobically.
Glucose is not only an excellent fuel, it is also a remarkably versatile
precursor, capable of supplying a huge array of metabolic
intermediates for biosynthetic reactions. A bacterium such as
Escherichia coli can utilize carbon skeletons from glucose for amino
acids, nucleotides, coenzyme, fatty acids, or other metabolic
intermediates which needs for growth.

6. Glycolysis:
Glycolysis is the process by which glucose molecule, which has six
carbon atoms, is enzymatically degraded in a sequence of 10 enzyme-
catalysed reactions to yield two molecules of pyruvate, which has
three carbon atoms. During this process much of the free energy
released from glucose is conserved in the form of ATP. It is a
universal biochemical reaction that occurs in every living unicellular
or multicellular organism.
The glycolytic process is a small part of the cellular respiration cycle
and overall body metabolism for the generation of ATP (Adenosine
Triphosphate) which is the energy currency of the body.
The process of glycolysis occurs in the cytoplasm of cells and is
controlled by various enzymes. All the enzymes of glycolysis has been
isolated in pure form from different species and studied in detail.

7. There are two phases in Glyolysis. The
first five steps constitute the
preparatory phase where glucose is
enzymatically phosphorylated by ATP,
first at 6th carbon atom and later at
1st carbon, to yield fructose 1,6
bisphosphate which is then cleaved in
half to yield finally two molecules of
glyceraldehyde 3-phosphate.
Other hexoses like D-fructose, D-
galactose and D-mannose enters in the
preparatory phase of glycolysis
following their phosphorylation.

8. The second phase of glycolysis, in
which the energy released when
two molecules of glyceraldehyde-
3-phosphate are converted into
two molecules of pyruvate. The
released energy is conserved by
the phosphorylation of four
molecules of ADP to ATP. Net
yield 2 molecules of ATP per
molecule of glucose since 2
molecules of ATP were invested
in the first phase.

9. In glycolysis three different types of chemical
transformation takes place:
1) The degradation of carbon skeleton of glucose to yield
pyruvate —the pathway of carbon atoms
2) Phosphorylation of ADP to ATP by high energy phosphate
compounds formed during glycolysis ---- the pathway of
phosphate groups
3) The transfer of a hydride ion to NAD, forming NADH.

10. Importance of Phosphate group:
Each of the nine metabolic intermediates between glucose and
pyruvate is a phosphorylated compound, the phosphate group
serves three functions:
1) PO4 groups are completely ionized at pH 7 which makes the
intermediates of glycolysis a net negative charge and helps not
to escape from the cell. Glucose can enter cells and pyruvate and
lactate can leave only.
2) PO4 groups are essential components in the enzymatic
conservation of metabolic energy since they are ultimately
transferred to ADP to yield ATP.
3) The PO4 groups serve as recognition or binding groups required
for the proper fit of the glycolytic intermediates to the active
sites of their corresponding enzymes.

11. Reaction 1: Phosphorylation of glucose to glucose-6 phosphate
• This reaction requires energy and so it is coupled to the
hydrolysis of ATP to ADP and Pi.
• Enzyme: hexokinase, it has a low Km for glucose; thus, once
glucose enters the cell, it gets phosphorylated. Not only glucose
but other hexoses like D-fructose and D-mannose are also
phosphorylated by this enzyme. Mg2+ is essential because the true
substrate is MgATP2-.
• This step is irreversible. So the glucose gets trapped inside the
cell. (Glucose transporters transport only free glucose, not
phosphorylated glucose)
Hexokinase

12. Regulation of Hexokinase
Hexokinase catalyzed phosphorylation of glucose is the first
regulatory step of glycolysis
• This enzyme is inhibited only by excess glucose-6-phosphate. If Gl-
6-P accumulates in the cell above its normal concentration, there is a
feedback inhibition of hexokinase.
• Gl-6-P is required for other pathways including the pentose
phosphate shunt and glycogen synthesis. So hexokinase step is not
inhibited unless Gl-6-P accumulates.
• liver, the site of glycogen synthesis, and in pancreas, has another
form of hexokinase called glucokinase which differs from hexokinase:
a) Not inhibited by Gl-6-P, but inhibited by Fru-6-P
b) It has much higher Km for glucose (10mM) than hexokinase.
Glucokinase comes into play on the excess blood glucose to convert it
into Gl-6-P for storage as liver glycogen, c) in -cells it acts as
glucose sensor and regulates insulin secretion.

13. Hexokinase vs. glucokinase
KM hexokinase = 0,1 mM
KM glucokinase = 10 mM

14. Reaction 2: Isomerization of glucose-6-phosphate to
fructose 6-phosphate
The aldose sugar is converted into the keto isoform.
Enzyme: Phosphoglucoisomerase
This is a reversible reaction. The fructose-6-phosphate is quickly
consumed and the forward reaction is favored.

15. Reaction 3: is another regulatory step, Phosphorylation of the
hydroxyl group on C1 forming fructose-1,6- bisphosphate.
Enzyme: Phosphofructokinase-1 (Second Regulatory enzyme). This
allosteric enzyme regulates the rate of glycolysis.
Reaction is coupled to the hydrolysis of ATP to ADP.
This is the second irreversible reaction of the glycolytic pathway.

16. Regulation of Phosphofructokinase-1 (PFK-1)
The phosphofructokinase step is rate-limiting step of glycolysis
(major regulatory enzyme in muscle glycolysis).
• Higher levels of AMP or ADP, fructose 1,6-diphosphate and
insulin are activators of this enzyme.
• It is inhibited when the cell has ample ATP, citrate or fatty
acids.
• Fructose 2,6-bisphosphate is the most potent activator of
PFK-1 even at high ATP level,

17. Reaction 4:
Fructose-1,6-bisphosphate is split into two 3-carbon
molecules, one aldehyde and one ketone:
dihyroxyacetone phosphate (DHAP) and glyceraldehyde
3-phosphate (GAP).
The enzyme is aldolase

18. Reaction 5:
DHAP and G-3-P are isomers of each other and can readily inter-
converted by the action of the enzyme triose-phosphate
isomerase, only G-3-P can directly enters into glycolytic pathway.

19. • G-3-P is a substrate for the next step in glycolysis so all of
the DHAP is eventually depleted. So, 2 molecules of G-3-P are
formed from each molecule of glucose. Up to this step, 2
molecules of ATP were required for each molecule of glucose
being oxidized.
• The remaining steps release enough energy to shift the
balance sheet to the positive side. This part of the glycolytic
pathway is called as the payoff or harvest stage.
• Since there are 2 G-3-P molecules generated from each
glucose, each of the remaining reactions occur twice for each
glucose molecule being oxidized.

20. Reaction 6:
GAP is dehydrogenated by the enzyme glyceraldehyde 3-
phosphate dehydrogenase (GAPDH). In the process, NAD+ is
reduced to NADH by the hydrogen in the glyceraldehyde
phosphate dehydrogenase.
Oxidation is coupled to the phosphorylation of the C1 carbon.
The product is 1,3-bisphosphoglycerate.
Aldehyde group is dehydrogenated to produce super high energy
phosphate (anhydride phosphate, acyl phosphate), 1,3-
bisphosphoglycerate
-11.8kcal/mol
-3.2kcal/mol

21. Reaction 7:
BPG has a high energy bond, at C1. This high energy bond is
hydrolyzed to a carboxylic acid and the energy released is used to
generate ATP from ADP and the product is 3-phosphoglycerate.
Enzyme: Phosphoglycerate kinase

22. Reaction 8:
In this reaction there is a reversible shift of the phosphate group
within the substrate molecule, ie, from C3 to C2 to form 2-
phosphoglycerate.
Mg2+ is essential for this reaction which involves the transfer of
the phosphate group from C3 to C2
Enzyme: Phosphoglycerate mutase, mutase is often used to
designate enzymes catalyzing intramolecular shifts of functional
groups.

23. Reaction 9:
Dehydration of 2-phosphoglycerate to phosphoenolpyruvate
catalyzed by enolase (a lyase). A water molecule is removed to form
this high-energy phosphate compound which has a double bond
between C2 and C3.
Although the substrate and product contain nearly the same amount
of total energy, loss of water molecule from the substrate causes a
redistribution of energy within the molecule.
Enolase
-14.8
kcal
-4.2 kcal

24. Reaction 10: Third Regulatory Step
High energy bond of phosphoenolpyruvate is hydrolyzed to form
pyruvate with the transfer of high energy phosphate group to
ADP and produces ATP. This irreversible reaction (3rd in
glycolysis) is catalyzed by the enzyme pyruvate kinase.
Pyruvate kinase

25. Regulation of pyruvate kinase
• Pyruvate kinase activity is inhibited under low glucose
conditions
• fructose 1,6 bisphosphate activates pyruvate kinase
• Other positive effectors are insulin, AMP and ADP while
ATP is a negative effector.
• Alanine, an amino acid derived from pyruvate, is a negative
effector . Glucagon and acetyl CoA are also negative
effector

26. Glycolysis: Energy balance sheet
Hexokinase: - 1 ATP
Phosphofructokinase: -1 ATP
GAPDH: +2 NADH
Phsophoglycerate kinase: +2 ATP
Pyruvate kinase: +2 ATP
Net gain for each molecule of glucose: +2 ATP, +2 NADH

27. Entry of other sugars into glycolysis
D-Fructose Fructose 6- P
Fructose is phosphorylated by fructokinase or hexokinase on the 1
or 6 positions.
• Fructose-6-phosphate is an intermediate of glycolysis.
• Fructose-1-phosphate is acted upon by an aldolase-like enzyme
that gives DHAP and glyceraldehyde.
• DHAP is a glycolysis intermediate and glyceraldehyde can be
phosphorylated to glyceraldehyde-3-P.
Fructose 1,6 di-P
Glyceraldehyde 3-P
ATP, hexokinase
Fr 1 P
DHAP Gleceraldehyde
ATP,
Fructokinase
Fruct-P-aldolase

28. Mannose Mannose 6- P
Fructose 6- P
Fruc 1,6- bisphosphate
Glyceraldehyde 3- P
ATP
Hexokinase
Phoshomannose isomerase
Mannose into glycolytic pathway

29. Galactose into glycolytic pathway
D-Galactose
D-galactose 1-P
Galactokinase, ATP
UDP- D galactose
Galactose 1-P uridylyl transferase
UDP- D glucose
Epimerase
D- glucose 1-P
Glucose pyrophosphorylase, PPi
D-glucose 6-P
Phosphoglucomutase

30. Galactosemia:
It is a genetic defect. Affected persons are unable to metabolize
galactose, derived from lactose. So, concentration of galactose
increases in the blood and this condition is known as galactosemia.
Symptoms: cataract formation, growth failure, mental
retardation, or death from liver damage.
Cause: Galactokinase (mild disorder) or galactose 1- P- uridylyl
transferase (severe disorder) deficiency in the cellular level.
Galactose can be accumulates and reduced to galactitol which
initiates cataract formation in the lens and play a role in the
central nervous system damage.