SJSM

1. Dr. Vysochyn, MD, PhD.
Course: Biochemistry
Prepared/Modified by Dr. A. Chebotarova, MD, PhD
Modified by Dr. M. Vysochyn, MD, PhD
Summer 2020
CARBOHYDRATES (I)
CHEMISTRY, DIGESTION,
ABSORPTION

2. Dr. Vysochyn, MD, PhD.
INTENDED LEARNING OBJECTIVES
To the end of the lecture students must be able to:
1. Identify the major monosaccharides, disaccharides, and polysaccharides found in
the human body and diet
2. Explain why ingested disaccharides and polysaccharides must be broken down into
monosaccharides and describe how this is accomplished
3. Draw a diagram of how glucose is transported across intestinal epithelial cells and
into the bloodstream and describe the proteins involved
4. Describe the role of glucose transporters (GLUTs) in the transport of glucose into
and out of cells, and tissue specific differences in the expression and regulation of
GLUTs
5. Explain the biochemical basis for the symptoms seen in lactose intolerance
6. Describe the steps of Digestion of Carbohydrates
7. Describe Absorption of Carbohydrates

3. Dr. Vysochyn, MD, PhD.
CARBOHYDRATES
Most abundant organic molecules in nature with empiric formula:
(CH2O)n
FUNCTIONS OF CARBOHYDRATES:
• significant fraction of energy in the diet of most organisms
• storage form of energy in the body
• cell membrane components that mediate some forms of intercellular
communication
• structural component of many organisms
CLASSIFICATION:
• Monosaccharides - cannot be hydrolyzed into simpler carbohydrates
• Disaccharides - composed of 2 molecules of monosaccharides
• Oligosaccharides - composed of 3–10 monosaccharide units
• Polysaccharides - composed of >10 monosaccharide units

4. Dr. Vysochyn, MD, PhD.
MONOSACCHARIDES
Subdivided depending upon:
A. Number of carbon atoms
Triose (3). Tetrose (4). Pentose (5)
Hexose (6). Heptose (7). Octose (8)
B. Whether the most oxidized functional group is
an aldehyde or a ketone group
• Aldose
• Ketose
Pentoses and hexoses are the most
abundant monosaccharides in living
cells

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6. Dr. Vysochyn, MD, PhD.
ISOMERISM: TYPES
The presence of asymmetric carbon atoms in a compound gives rise to the formation of
isomers of that compound. Such compounds which are identical in composition and
differs only in spatial configuration are called isomers.
1. Stereoisomers & Epimers
2. Enantiomers ( D- and L- isomerism)
3. Optical isomers [ (+) and (-) ]
4. Anomers

7. Dr. Vysochyn, MD, PhD.
Stereoisomers: alternate configurations around chiral carbons, but not
mirror images.
Epimers: a special case of isomers that differ in configuration about only one asymmetric
carbon (that is not the penultimate carbon-the carbon atom just adjacent to the terminal
primary alcohol carbon)
Examples : glucose and galactose (C4) glucose and mannose (C2)
1. STEREOISOMERS & EPIMERS

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2. ENANTIOMERS
Compounds that are mirror-images of each other designated as D- or L-sugars
Form depends on the orientation of the –H and –OH groups around the penultimate
carbon (carbon atom adjacent to the terminal primary alcohol carbon).
When the – OH group on this carbon is on the
right, it belongs to D-series, when the – OH
group is on the left, it is a member of L-series
The vast majority of the sugars in
humans are D-sugars.

9. Dr. Vysochyn, MD, PhD.
3. ANOMERS
As the two reacting groups aldehyde and alcoholic group belong to the same
molecule, a cyclic structure takes place.
Carbonyl carbon - becomes a new chiral center as cyclization occurs
It is called the anomeric carbon.

10. Dr. Vysochyn, MD, PhD.
ANOMERS
Cyclic structure has 2 possible forms (called anomers), that based on the
position of the -OH group attached to the anomeric C.
Simply put:
When –OH group is below the ring = -anomeric form
When –OH group is above the ring = -anomeric form
Mutarotation -  and  forms are spontaneously interconverted when dissolved in water.

11. Dr. Vysochyn, MD, PhD.
Pyranose = 6-membered ring
Furanose = 5-membered ring
Glucose in solution - more than 99% is in the pyranose form
Fructose in solution – primarily in the furanose form
TAUTOMERISM

12. Dr. Vysochyn, MD, PhD.
REDUCING PROPERTIES OF SUGARS
The anomeric carbon
of a reducing sugar
becomes oxidized in
the presence of
Benedict’s solution.
A positive Benedict’s
test is the formation of
a red precipitate
Benedicts reagent – a colourimetric reagent used to detect reducing sugars.
Can be used to detect reducing sugars in urine – indicative of some pathology
since sugars are not normally present in urine. Followed up by specific tests
to identify the sugar.

13. Dr. Vysochyn, MD, PhD.
DERIVED MONOSACCHARIDES
1. DEOXYSUGARS -H has replaced an -OH
2. AMINO SUGARS – a hydroxyl group is replaced by an amino group.

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DERIVED MONOSACCHARIDES
3. SUGAR ALCOHOLS – reduction of the carbonyl carbon (aldehyde or
keto group).
4. PHOSPHORYLATED
AND SULFATED SUGARS

15. Dr. Vysochyn, MD, PhD.
Disaccharides = 2 monosaccharide - formed from the condensation between the
hydroxyl group of the anomeric carbon of a monosaccharide and a hydroxyl group of
another monosaccharide.
B. DISACCHARIDES

16. Dr. Vysochyn, MD, PhD.
LACTOSE – milk sugar.
Synthesized by forming a glycosidic bond
between carbon 1 of a -galactose and
carbon 4 of glucose  (1→4) glycosidic bond
DISACCHARIDES
• Free anomeric carbon of glucose can exist
in either the  - or  - configuration.
• A reducing sugar because the anomeric
end of the glucose residue is free (not
involved in a glycosidic linkage).

17. Dr. Vysochyn, MD, PhD.
SUCROSE – cane sugar or beet sugar.
Synthesized by forming a glycosidic bond
between carbon 1 of -glucose and
carbon 2 (anomeric carbon) of -
fructose. Linkage is an (1  2)
glycosidic bond.
Not a reducing sugar because both
anomeric carbons are not free (both
are involved in the glycosidic linkage).
DISACCHARIDES

18. Dr. Vysochyn, MD, PhD.
MALTOSE – malt sugar
(intermediate product of starch
hydrolysis)
Does not exist freely in nature.
Synthesized by forming  (1
4) glycosidic linkage between 2
D-glucose molecules.
Free anomeric carbon
undergoes mutarotation in
solution which results in an
equilibrium mixture of - and -
maltose.
DISACCHARIDES

19. Dr. Vysochyn, MD, PhD.
CELLOBIOSE – a degradation product of cellulose.
Does not exist freely in nature.
Composed of 2 molecules of glucose linked by a 
(1,4) glycosidic bond.
DISACCHARIDES

20. Dr. Vysochyn, MD, PhD.
C. OLIGOSACCHARIDES
3-10 monosaccharides most often found attached to polypeptides in glycoproteins
and some glycolipids
- with 2 broad classes:
A. N-linked oligosaccharides - attached to
polypeptides by an N-glycosidic bond with the side
chain amide group of the amino acid asparagine
B. O-linked oligosaccharides - attached to the side
chain hydroxyl group of amino acid serine or
threonine in polypeptide chains or the hydroxyl
group of membrane lipids

21. Dr. Vysochyn, MD, PhD.
D. POLYSACCHARIDES
Used as storage forms of energy or
structural materials
Most contain from hundreds to
thousands of sugar units
May have linear structure ( eg
cellulose, amylose) or may have
branched structure (eg glycogen,
amylopectin)
• May be divided in 2 classes:
- homopolysaccharides – composed of 1 type of monosaccharides
- heteropolysaccharides – contain 2 or more types of monosaccharides

22. Dr. Vysochyn, MD, PhD.
HOMOPOLYSACCHARIDES:
STARCH – most important food source of carbohydrate and energy reservoir of plant cells.
With 2 chief constituents:
1. Amylose (15-20%) – composed of long unbranched chains of D-glucose residues that
are linked with  (1,4) glycosidic bonds.
- typically contains several thousand glucose residues
- form long tight helices  compact shape is ideal for storage function
2. Amylopectin (80-85%) - consists of branched chains composed of 24-30 glucose
residues united by  (1 4) linkages in the chains and by  (1 6) linkages at the
branch points.
- number of glucose units may vary from a few thousand to a million
- each molecule has only one reducing end

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24. Dr. Vysochyn, MD, PhD.
GLYCOGEN (often called animal starch)
The carbohydrate storage molecule in
vertebrates.
Structure is similar to amylopectin except that it
has more branch points. Presence of many non-
reducing ends allow for stored glucose to be
rapidly mobilized when the animal demands
energy

25. Dr. Vysochyn, MD, PhD.
CELLULOSE – most important structural polysaccharide of plants.
Consists of -D-glucopyranose units linked by (14) bonds to form long, straight chains
strengthened by cross-linked hydrogen bonds
Cannot be digested by humans because of absence of a hydrolase that attacks the  linkage
Important source of dietary fiber/“bulk” in the diet
Can be digested only by microorganisms that possess the enzyme cellulose.

26. Dr. Vysochyn, MD, PhD.
HETEROPOLYSACCHARIDES
High-molecular-weight carbohydrate polymers that contain more than 1 kind of
monosaccharide.
Many of the sugar residues are amino derivatives. Often referred to as glycosaminoglycans
(GAGs).

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DIGESTION AND ABSORPTION OF CARBOHYDRATES
DIGESTION IN THE MOUTH – salivary -amylase (ptyalin).
• Acts briefly on dietary starch in a random manner during mastication
an endoglycosidase.
• Hydrolyzes some internal  (1 4) bonds between glucosyl residues within
amylopectin, amylose, and glycogen.
• Products: primarily -limit dextrins.
DIGESTION IN THE STOMACH AND THE DUODENUM
• High acidity inactivates salivary amylase and carbohydrate digestion halts
temporarily.
• In the duodenum acidic stomach contents are neutralized by bicarbonate secreted by
the pancreas.
• Acted on by pancreatic a-amylase

29. Dr. Vysochyn, MD, PhD.
DIGESTION BY PANCREATIC AMYLASE
Pancreatic amylase – continues to hydrolyze -1,4 bonds
Products:
glucose
disaccharides (maltose and isomaltose)
trisaccharides (maltotriose)
oligosaccharides (limit dextrins): containing an average of about 8 glucosyl residues,
including  1,6 glucosidic bond
DIGESTION BY OLIGOSACCHARIDASES AND DISACCHARIDASES
Responsible for final conversion of carbohydrates into monosaccharides
Disaccharidases are bound to the gut epithelium protruding into the intestinal lumen.
Disaccharidase may cleave -glycosidic linkages.
Examples include: Lactase, sucrase, maltase, isomaltase.
Glucoamylase is grouped with disaccharidases even though it is technically an
oligosaccharidase that hydrolyzes α 1-4 linkages at reducing ends of oligosaccharides.

30. Dr. Vysochyn, MD, PhD.

31. Dr. Vysochyn, MD, PhD.
ALL CARBOHYDRATES ARE ABSORBED ONLY
AS MONOSACCHARIDES
Major monosaccharides that result from digestion
of di- and polysaccharides:
•GLUCOSE
•GALACTOSE
•FRUCTOSE
* D-fructose

32. Dr. Vysochyn, MD, PhD.
ABSORPTION OF CARBOHYDRATES
2 major mechanisms responsible for the absorption of monosaccharides from the intestinal
lumen:
1. Active transport against a concentration gradient by a sodium-dependent transporter
(SGLT1-sodium glucose linked transporter). Has high specificity for D-glucose and D-galactose,
occurs with Na+ symport.
2. Facilitative transport with the concentration gradient
Via a Na+-independent facilitated diffusion type of monosaccharide transport system using
GLUT-5 transporters. Transports mainly fructose, also glucose and galactose.

33. Dr. Vysochyn, MD, PhD.
Phlorizin is a competitive inhibitor of SGLT1 and SGLT2 because it competes with D-
glucose for binding to the carrier; this reduces renal glucose transport, lowering
the amount of glucose in the blood. Phlorizin was studied as a potential
pharmaceutical treatment for type 2 diabetes, but has since been superseded by
more selective and more promising synthetic analogs, such as empagliflozin,
canagliflozin and dapagliflozin.

34. Dr. Vysochyn, MD, PhD.
GLUT transporters: TISSUE SPECIFIC EXPRESSION
There are actually 14 GLUTs with differing tissue specificity.
Exist in 2 conformational states – state 1) binds glucose outside cell, this induces
conformational change to state 2) glucose is released inside cell; GLUT returns to original
conformation, ready to bind extracellular glucose again. All GLUTs are bidirectional and
energy independent
GLUT4 – Insulin dependent – muscle and adipose
GLUT2 – (liver, pancrease, kidney, serosal surface of intestinal mucosal cells) – low
affinity. Allows liver and beta cells to respond to fluctuations in blood glucose.
GLUT1 – (erythrocytes, blood-brain barrier endothelial cells) – high affinity. Allows RBCs
to import glucose even when plasma levels are very low.
GLUT3 – (brain neurons) – high affinity.
GLUT5

35. Dr. Vysochyn, MD, PhD.
Brain – transport through GLUT1 and GLUT3 is required for glucose entry into neurons.
Both are high affinity transporters enabling glucose transport to brain even when
plasma glucose concentrations drop postpriandally. Nevertheless, partly due to the
extremely tight junctions in the blood-brain barrier glucose diffusion is limited and the
overall Km for transport into the brain is 7-11mM (postpriandal glucose levels).
The Vmax for glucose transport into the brain is only slightly higher than the rate of
glucose utilization by the brain – thus the brain is very sensitive to hypoglycemia (1-3
mM glucose) – light headedness, dizzyness, coma.
BRAIN

36. Dr. Vysochyn, MD, PhD.
Elevated concentrations of glucose in blood stimulate
release of insulin from β-cells of the pancreas
Glucokinase is most important in liver and islet cells of pancreas. Both organs respond
to fluctuations in plasma glucose levels. Glucokinase higher Km, lower affinity than
hexokinase in most other tissues.
1. Glucose enters beta cells via GLUT2 (low affinity)
2. Phosphorylated to glucose-6-phosphate by glucokinase (trapped in cell)
3. Glycolysis/TCA Cycle/Oxidative Phosphorylation – increased ATP levels
4. ATP-dependent Potassium channel closes
5. Membrane depolarization
6. Activation of a voltage-gated calcium channel
7. Ca2+ increases in the cell
8. Ca2+ stimulates fusion of insulin-containing exocytotic vesicles with the plasma
membrane
9. Insulin is secreted through the plasma membrane

37. Dr. Vysochyn, MD, PhD.
Elevated concentrations of glucose in blood stimulate release of
insulin from -cells of the pancreas
GLUCOKINASE

38. Dr. Vysochyn, MD, PhD.
Insulin signaling leads to:
1.Auto cross phosphorylations.
2.Phosphorylation of insulin receptor substrate (IRS).
3.Recruitment of SH2-containing proteins (grb2, PLC and PI3Kinase)
4.PIP signaling pathways.
5.PI3Kinase activates PKB (Akt) which is necessary for GLUT4 recruitment to membranes.

39. Dr. Vysochyn, MD, PhD.
GLUT 4 RECRUITMENT: INSULIN AND EXERCISE

40. Dr. Vysochyn, MD, PhD.
INSULIN AND UPTAKE OF GLUCOSE
Insulin acts on cells throughout the body to stimulate uptake, utilization and storage of
glucose.
Insulin Independent glucose transport:
The BRAIN and LIVER do not require insulin for efficient uptake of glucose.
Other cells include Erythrocytes, cells of the intestinal mucosa, renal tubules, cornea.
These cells don't use GLUT4 for importing glucose, but rather, another transporter that is
not insulin-dependent.
• liver, kidneys, pancreas – GLUT2
• erythrocytes, endothelial cells of blood-brain barrier – GLUT1
• neurons – GLUT3

41. Dr. Vysochyn, MD, PhD.
1.Robert K. Murray, David A. Bender et al. Harpers
Illustrated Biochemistry, 30th Edition, 2015. McGraw-Hill Medical. P. 152-160, 538-
539.
2. JW Baynes and MH Dominiczak, Medical Biochemistry, 4th edition, 2009, Elsevier
Mosby. P. `104-113, 369-383,

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