This document provides an overview of carbohydrate digestion, absorption, and metabolism. It discusses: - The breakdown of ingested carbohydrates (starch, lactose, sucrose) into monomers like glucose, fructose, and galactose by enzymes in the digestive tract. - The transport of these monomers across intestinal cells and into the bloodstream, facilitated by sodium-dependent and facilitative glucose transporters. - The major pathways of glucose metabolism, including glycolysis. Glycolysis converts glucose to pyruvate, generating a small amount of ATP both aerobically and anaerobically in most cells. - Tissues and cell types rely heavily on glycolysis, like red blood

1. MEDICAL BIOCHEMISTRY LECTURE
CBD 200 LEVEL MBBS
CARBOHYDRATES: DIGESTION,
ABSORPTION AND GLYCOLYSIS

2. OUTLINE
• Introduction
• Sources of monomers
• Metabolisms of glucose
- Primary
- Secondary
• Clinical correlates

3. LEARNING OBJECTIVES
• Describe the breakdown of ingested
carbohydrates meal
• Describe the glycolytic pathway and its control
• Explain how glycolysis operate under
anaerobic conditions
• Identify the cells/organs that substantially
dependent on glycolysis

4. Introduction
• Carbohydrates are the largest source of dietary calories
for most of the world’s population
• The major carbohydrates diet are rich in starch, lactose
and sucrose
• The starches amylose and amylopectin are
polysaccharides composed of hundreds to millions of
glucosyl units linked together through α-1,4- and
α-1,6-glycosidic bonds
• Lactose is a disaccharide composed of glucose and
galactose, linked together through a β-1,4-glycosidic
bond
• Sucrose is a disaccharide composed of glucose and
fructose, linked through an α-1,2-glycosidic bond

5. Introduction cont’d
• The major monomers: glucose, fructose and
galactose
• Glucose is the universal fuel for human cells.
• Every cell type in humans is able to generate
adenosine triphosphate (ATP) from glycolysis, the
pathway in which glucose is oxidized and cleaved to
form pyruvate
• Glucose is the major sugar in our diet and the sugar
that circulates in the blood to ensure that all cells
have a continuous fuel supply
• The brain uses glucose almost exclusively as a fuel
• RBCs is wholly dependent on glucose as a metabolic
fuel

6. Digestion of dietary carbohydrates
(primary metabolism)
Sources of monomers
1. Dietary : The digestive processes convert all of the dietary
carbohydrates to their constituent monosaccharides by
hydrolyzing glycosidic bonds between the sugars
2. The digestion is rapid and catalyze by enzymes-
glycosidases-hydrolyze the glycosidic bonds
3. The enzymes are primarily endoglycosidases that hydrolyze
polysaccharides and oligosaccharides=reducing sugar
4. Disaccharidases –hydrolyze trisaccharides & disaccharides
• The major dietary polysaccharides are plants (starch-
composed of amylose and amylopectin)& animal(glycogen)
origin

7. Digestion of dietary carbohydrates
(primary metabolism) cont’d
• The digestion of starch begins in the mouth
• The salivary gland releases α-amylase, (acted on α
1,4 glycosidic bond)converts starch to smaller
polysaccharides and α- dextrins (mixture of short
branched & unbranched (maltose, maltotriose )
oligosaccharides
• Salivary α-amylase is inactivated by the acidity (HCl-
pH 1.5-3.5) of the stomach
• Pancreatic α-amylase and bicarbonate are secreted
by the exocrine portion of pancreas into the lumen
of the small intestine(duodenum), where
bicarbonate neutralizes the gastric secretions

8. Digestion of dietary carbohydrates
(primary metabolism) cont’d
• Pancreatic α-amylase continues the digestion of α-
dextrins, converting them to disaccharides (maltose)
trisaccharides (maltotriose), and oligosaccharides
called limit dextrins
• Limit dextrins usually contain four to nine glucosyl
residues and an isomaltose branch (two glucosyl
residues attached through an α-1,6-glycosidic bond)
• The digestion of the disaccharides lactose and sucrose,
as well as further digestion of maltose, maltotriose,
and limit dextrins, occurs through disaccharidases, in
the brush border (microvilli) of intestinal epithelial
cells

9. • Glucoamylase hydrolyzes the α-1,4- bonds of dextrins
• The sucrase–isomaltase complex hydrolyzes sucrose,
most of maltose, and almost all of the isomaltose
formed by glucoamylase from limit dextrins.
• Lactase-glycosylceramidase (β-glycosidase)
hydrolyzes the β- glycosidic bonds in lactose and
glycolipids.
• Trehalase, hydrolyzes the bond (an α-1,1-glycosidic
bond) between two glucosyl units in the sugar
trehalose
• The monosaccharides produced by these hydrolases
(glucose, fructose, and galactose) are then
transported into the intestinal epithelial cells
Digestion of dietary carbohydrates
(primary metabolism) cont’d

10. Figure 1: Structures of major constituents of dietary carbohydrates
a
b
c
d

11. Sources of Glucose cont’d
2. Endogeneous source
a. Stored glycogen via glycogenolysis
b. Salvage from non carbohydrate substrates e.g
fatty acids; amino acids, nucleic acid through
gluconeogenesis

12. Digestion of carbohydrate: Primary metabolism
• In the digestive tract, dietary
polysaccharides and disaccharides are
converted to monosaccharides by
glycosidases, enzymes that hydrolyze
the glycosidic bonds between the
sugars.
• All of these enzymes exhibit some
specificity for the sugar, the glycosidic
bond (α or β), and the number of
saccharide units in the chain.
• The monosaccharides formed by
glycosidases are transported across the
intestinal mucosal cells into the
interstitial fluid and subsequently enter
the bloodstream.
• Undigested carbohydrates enter the
colon, where bacteria may ferment
them to short chain fatty acids.
Figure 2. Digestive system

13. Transport and Absorption of monomers/glucose
• Once the carbohydrates have been split into monosaccharides, the
sugars are transported across the intestinal epithelial cells and into
the blood for distribution to all tissues.
• Glucose, galactose, and fructose formed by the digestive enzymes
are transported into the absorptive epithelial cells of the small
intestine by Na⁺-dependent glucose transporters and the facilitative
glucose transporters
• Monosaccharides are transported from these cells into the blood and
circulate to the liver and peripheral tissues, where they are taken up
by facilitative transporters
• Facilitative transport of glucose across epithelial cells and other cell
membranes is mediated by a family of tissue-specific glucose
transport proteins (GLUT 1 to GLUT 5). The type of transporter
found in each cell reflects the role of glucose metabolism in that cell.

14. Transport and Absorption of Glucose cont’d
1. Na+-Dependent Transporters
• Na⁺-dependent glucose transporters, are located on the luminal side
of the absorptive cells, enable these cells to concentrate glucose
from the intestinal lumen.
• A low intracellular Na⁺ concentration is maintained by a Na⁺-K⁺
ATPase on the serosal side (visceral layer of peritoneum)of the cell
that uses the energy from adenosine triphosphate (ATP) cleavage, to
pump Na+ out of the cell into the blood.
• Thus, the transport of glucose from a low concentration in the lumen
to a high concentration in the cell is promoted by the co-transport of
Na⁺ from a high concentration in the lumen to a low concentration in
the cell (secondary active transport).
• Similar transporters are found in the epithelial cells of the kidney,
which are thus able to transport glucose against its concentration
gradient

15. Figure 3. structure of Gastrointestinal tract(adventitia=serosal lining)

16. 2. Facilitative Glucose Transporters
• Facilitative glucose transporters, which do not
bind Na⁺, are located on the serosal side of the
cells
• Glucose moves via the facilitative transporters
from the high concentration inside the cell to the
lower concentration in the blood without the
expenditure of energy
• In addition to the Na⁺-dependent glucose
transporters, facilitative transporters for glucose
also exist on the luminal side of the absorptive
cells.

17. Glucose transporters and Distribution
Transporter Tissue Distribution Remark
GLUT 1 Human erythrocytes
Barriers:
Blood-brain ,Blood-retinal
Blood-testis
Blood-placental
High affinity glucose
transport system. Expressed
in cell types with barrier
functions
GLUT 2 Liver
Kidney
Pancreatic β cell
Intestinal mucosal cells
(serosal surface)
A high capacity, low affinity
transporter. May be used as
glucose sensor in the
pancreas
GLUT 3 Brain (neurons) A high affinity system; major
glucose transporter in the
central nervous system
GLUT 4 Adipose tissue
Skeletal muscle
Heart tissue
Insulin-sensitive system; in
the presence high affinity
GLUT 5 Intestinal epithelium
spermatozoa
This is actually, a fructose
transporter

18. Metabolism of glucose
• Glucose metaboilsms
Glycolysis
Glycogenesis
Glycogenolysis
Pentose phosphate pathway
Gluconeogenesis

19. Glycolysis
• Glycolysis is the main pathway of glucose (& other
carbohydrate) metabolism
• It occurs in the cytosol of all cells
• Can function under aerobic or anaerobic condition
depending on the availability of oxygen and the electron
transport chain (the presence of mitochondria)
• The erythrocytes(RBCs) lacks mitochondria, completely
depend on glucose as their metabolic fuel and
metabolize it by anaerobic glycolysis
• Its ability to provide ATP in the absence of O₂ allows
skeletal muscle to perform at very high levels of work
output when O₂ supply is insufficient and allows tissues
to survive anoxic episodes

20. • GLYCOLYSIS: 2 phases
• 1. preparatory phase
• 2. Payoff phase

21. Glycolysis : Secondary metabolism
Preparatory phase
1. Conversion of Glucose to Glucose 6-Phosphate
• Glucose metabolism begins with transfer of a phosphate from ATP to
glucose to form glucose 6-P.
• Phosphorylation of glucose commits it to metabolism within the cell
because glucose 6-P cannot be transported back across the plasma
membrane.
• The phosphorylation reaction is irreversible under physiologic conditions
because the reaction has a high negative ΔG0′.
• Phosphorylation does not, however, commit glucose to glycolysis
• Phosphorylation of glucose is catalyze by Hexokinases, are enzyme
family of tissue-specific isoenzymes that differ in their kinetic properties.
• The isoenzyme found in liver and β-cells of the pancreas has a much higher
Km than other hexokinases and is called glucokinase.
• In many cells, some of the hexokinase is bound to porins in the outer
mitochondrial membrane (voltage dependent anion channels), which gives
these enzymes first access to newly synthesized ATP as it exits the
mitochondria

23. 2. Conversion of Glucose 6-Phosphate to the Triose Phosphates
• In the remainder of the preparative phase of glycolysis, glucose 6-P is
isomerized to fructose 6-phosphate (fructose 6-P), again phosphorylated,
and subsequently cleaved into two three-carbon fragments
• The isomerization, which positions a keto group next to carbon 3, is
essential for the subsequent cleavage of the bond between carbons 3 and
4
• Phosphorylation of fructose 6-P to fructose 1,6- bisphosphate by PFK-1, is
generally considered the first committed step of the pathway.
• This phosphorylation requires ATP and is thermodynamically and
kinetically irreversible
• Therefore, PFK-1 irrevocably commits glucose to the glycolytic pathway
PFK-1 is a regulated enzyme in cells, and its regulation controls the entry
of glucose into glycolysis.
• Like hexokinase, it exists as tissue-specific isoenzymes whose regulatory
properties match variations in the role of glycolysis in different tissues
• Fructose 1,6-bisphoshate is cleaved into two phosphorylated three-
carbon compounds (triose phosphates) by aldolase
i. Dihydroxyacetone phosphate (DHAP) and
ii. Glyceraldehyde 3-P

25. • DHAP is isomerized to glyceraldehyde 3-Phosphate by
triose phosphate isomerase
• Aldolase is named for the mechanism of the forward
reaction, which is an aldol cleavage, and the mechanism
of the reverse reaction, which is an aldol condensation.
• The aldolase exists as tissue-specific isoenzymes, which
all catalyze the cleavage of fructose 1,6-bisphosphate
but differ in their specificities for fructose 1-Phosphate
• The enzyme uses a lysine residue at the active site to
form a covalent bond with the substrate during the
course of the reaction
• Inability to form this covalent linkage inactivates the
enzyme
• Thus, at this point in glycolysis, for every mole of glucose
that enters the pathway, 2 mol of glyceraldehyde 3-(P)
are produced and continue through the pathway

26. The preparatory phase

27. Significance of preparatory phase
• Uptake of glucose into the cell
• Phosphorylated glucose, feeder for other pathways e.g
glycogen synthesis, in the skeletal muscle and liver;
pentose phosphate pathway-a source of reducing
equivalents( NADPH) for FAs synthesis
• 2 molecules of 3 carbon compound-triose phosphate -
aldose & ketose
• The triose phosphate intermediates give rise to glycerol
moiety of triacylglycerols
• Precursor for fatty acids and ketone formation
• Energy investment phase

28. PAYOFF PHASE OF GLYCOLYSIS
• Oxidative and Substrate-level phosphorylation

29. Oxidation and Substrate-Level Phosphorylation(payoff phase)
In this part of glycolytic pathway, glyceraldehyde 3-P is oxidized and
phosphorylated so that subsequent intermediates of glycolysis can donate
phosphate to ADP to generate ATP.
• Step 6: glyceraldehyde-3-P dehydrogenase catalyzed the conversion of
glyceraldehyde 3-P to 1,3-bisphosphoglycerate (1,3-BPG)
• (is really the key to the pathway).
• This enzyme oxidizes the aldehyde group of glyceraldehyde 3-P to an
enzyme-bound carboxyl group and transfers the electrons to NAD⁺ to form
NADH as a reducing equivalent.
• The oxidation step is dependent on a cysteine residue at the active site of
the enzyme, which forms a high-energy thioester bond during the course of
the reaction
• The high-energy intermediate immediately accepts a Pi to form the high-
energy acyl phosphate bond in 1,3-bisphosphoglycerate (1,3-BPG), releasing
the product from the cysteine residue on the enzyme.
• This high-energy phosphate bond is the start of substrate-level
phosphorylation (the formation of a high-energy phosphate bond where
none previously existed, without the use of oxygen).

31. Oxidative and Substrate-level
phosphorylation

32. Step 7: In the next reaction, the phosphate in this bond is
transferred to ADP to form ATP by 3-phosphoglycerate kinase
• The energy of the acyl phosphate bond is high enough (~10
kcal/mol) so that transfer to ADP is an energetically favorable
process. 3-Phosphoglycerate is also a product of this reaction
• Step 8: To transfer the remaining low-energy phosphoester
on 3-phosphoglycerate to ADP, it must be converted into a
high-energy bond;This conversion is accomplished by moving
the phosphate to the second carbon (forming 2-
phosphoglycerate) ; catalyze by phosphoglycerate mutase
• Step 9: and then removing water to form
phosphoenolpyruvate (PEP) catalyze by enolase
• Step 10: The enolphosphate bond is a high-energy bond (its
hydrolysis releases ~14 kcal/mol of energy), so the transfer of
phosphate to ADP by pyruvate kinase is energetically
favorable and not reversible
• This final reaction converts PEP to pyruvate, catalyze by
pyruvate kinase

33. Overview of glycolytic reaction

34. Bioenergetic of glycolysis
Reaction catalyzed by Method of ATP formation ATP/mol of Glucose
Glyceraladhyde -3-
phosphate dehydrogenase
Respiratory chain oxidation
of 2 NADH
5
Phosphoglycerate kinase Substrate-level
phosphorylation
2
Pyruvate kinase Substrate-level
phosphorylation
2
Consumption of ATP for
interaction of hexokinase
and phosphofructokinase
-2
Net 7

35. Regulation of glycolysis
• In each cell, glycolysis is regulated to ensure that ATP homeostasis is
maintained, without using more glucose than necessary.
• In most cell types, hexokinase, the first enzyme of glycolysis, is
inhibited by glucose 6-P (step 1)
• Thus, glucose is not taken up and phosphorylated by a cell unless
glucose 6-P enters a metabolic pathway, such as glycolysis or glycogen
synthesis.
• The control of glucose 6-P entry into glycolysis occurs at
phosphofructokinase-1 (PFK-1), the rate-limiting enzyme of the
pathway. PFK-1 is allosterically inhibited by ATP and allosterically
activated by adenosine monophosphate (AMP).
• AMP increases in the cytosol as ATP is hydrolyzed by energy-requiring
• Glucokinase: indirectly inhibited by fructose 6-phosphate
• Fructose 1,6-bisphosphate activates pyruvate kinase
• Phosphorylation of PK by cAMP-dependent protein kinase leads to
inactivation in the liver

36. • In tissues other than liver, and pancreatic β-islet cells) glucose
availability for cellular metabolism is controlled by transport into cell,
inturn by insulin
• Phosphoglycerate kinase (3-phosphoglycerate and ATP)-inhibited by
arsenate, competitive inhibition of inorganic phosphate (Pi) forming 1-
arseno-3-phosphoglycerate=spontaneous hydrolysis to 3-
phosphoglycerate without yielding ATP
• The glycolytic breakdown of glucose is the sole source of
metabolic energy in some mammalian tissues and cell types
(erythrocytes, renal medulla, brain, and sperm, for example)
• Enolase is inhibited by fluoride; depends on the presence of
either Mg²⁺ or Mn ²⁺
• Reaction of PK ,essentially irreversible under physiological
condition
• During glycolysis some of the energy of the glucose molecule is
conserved in ATP, while much remains in the product, pyruvate

37. Anaerobic glycolysis
• Availability of the O2 now determines which of the 2 pathways to
follow. Under anaerobic conditions, the NADH cannot be reoxidized
through the respiratory chain and pyruvate is reduced to lactate by
lactate dehydrogenase
• This permits the oxidation of NADH, allowing another molecule to
undergo glycolysis
• Under aerobic state, pyruvate is transported into mitochondria and
undergoes oxidative decarboxylation to acetyl CoA then oxidation
to CO2 in the CAC
• The reducing equivalent from the NADH formed in glycolysis are
taken up into mitochodria for oxidation via either malate-aspartate
shuttle or the glycerophophate shuttle
• Tissues: normally derived their energy from glycolysis and
produced lactate largely depend on erythrocytes, brain, GIT, renal
medulla, the lens and cornea of the eyes, and skin

38.  The overall equation for glycolysis is
• Glucose + 2NAD + 2ADP + 2Pi → 2 pyruvate + 2NADH + 2H ⁺ + 2ATP
+2H₂O
• Generation of energy mostly inform of ATP
• As an intermediate of TCA when pyruvate is oxidised to acetyl CoA
under aerobic condition
• Glycolysis is an anabolic pathway that provides biosynthetic
precursors. For example, in liver and adipose tissue, this pathway
generates pyruvate as a precursor for fatty acid biosynthesis,
gluconeogenesis
• Glycolysis also provides precursors for the synthesis of
compounds such as amino acids and of nucleotides
• Glucose 6-phosphate :Branched intermediate for PPP; nucleosides;
glycogenesis etc
• Pyruvate and intermediates of the CAC provide carbon skeletons for
the synthesis of nonessential amino acids
• Pyruvate, provide acetyl CoA-precursor of Fas and cholesterol
Clinical importance of glycolysis

39. Clinical importance of glycolysis cont’d

40. Assignments
1. Distinguish between aerobic and anaerobic glycolysis
2. Describe the glycolytic pathways
3. Explain the clinical importance of preparatory phase
of glycolysis
4. Catabolism of one molecule of glucose produced 2
molecules of pyruvate, discuss
5. Describe the regulation of glycolysis and its
biomedical implications
6. Enumerate functions of glycolysis
7. Describe the glycolysis in the erythrocytes

41. QUIZ
Choose ONEbest Answer
• 1. Which one of the following statements concerning
glycolysis is correct?
A. The conversion of glucose to lactate requires the
presence of oxygen.
B. Hexokinase is important in hepatic glucose metabolism only in the absorptive period following consumption of a
carbohydrate-containing meal.
C. Fructose 2,6-bisphosphate is a potent inhibitor of
phosphofructokinase.
D. The regulated reactions are also the irreversible
reactions.
E. The conversion of glucose to lactate yields two ATP
and two NADH.
The reaction catalyzed by phosphofructokinase-1:
2. A. is activated by high concentrations of ATP and
citrate.
B. uses fructose 1-phosphate as substrate.
C. is the rate-limiting reaction of the glycolytic pathway.
D. is near equilibrium in most tissues.
E. is inhibited by fructose 2,6-bisphosphate.
• 3.Compared with the resting state, vigorously contracting skeletal muscle shows:
A. an increased conversion of pyruvate to lactate.
B. decreased oxidation of pyruvate to CO2 and water.
C. a decreased NADH/NAD+ ratio.
D. a decreased concentration of AMP.
E. decreased levels of fructose 2,6-bisphosphate.
4. A 43-year-old man presented with symptoms of weakness, fatigue, shortness of breath, and dizziness. His
hemoglobin level was less than 7 g/dl (normal for a male being greater than 13.5 g/dl). Red blood cells
isolated from the patient showed abnormally low level of lactate production. A deficiency of which one of the
following enzymes would be the most likely cause of this patient’s anemia?
A. Phosphoglucose isomerase
B. Phosphofructokinase
C. Pyruvate kinase
D. Hexokinase
E. Lactate dehydrogenase

42. THANK YOU