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Topics: Monosaccharides, Glucose, Fructose, Mannose, Galactose
• Simplest carbohydrates; cannot be hydrolyzed further.
• General formula: CₙH₂ₙOₙ.
• Trioses – glyceraldehyde
• Tetroses – erythrose
• Pentoses – ribose, deoxyribose
• Hexoses – glucose, fructose, galactose, mannose
• Aldoses → contain aldehyde group (CHO)
• Ketoses → contain keto group (CO)
• D and L forms → based on penultimate carbon orientation
• Epimers → differ at one carbon
• Anomers → differ at hemiacetal carbon (α, β)
• Mutarotation → α ↔ β interconversion in solution
• Aldohexose; pyranose ring form in solution.
• Four chiral centers.
• Oxidation → gluconic acid / glucuronic acid
• Reduction → sorbitol
• Glycosidic bond formation → disaccharides & polysaccharides
• Primary fuel of body.
• Required by RBCs, retina, brain.
• Forms glycogen, lactose, triglycerides, amino acids.
• Hyperglycemia & hypoglycemia disorders
• Sorbitol accumulation → cataract, neuropathy
• Ketohexose; furanose ring form.
• Sweetest natural sugar.
• Rapid hepatic metabolism; bypasses PFK-1 → faster glycolysis
• Fructokinase forms fructose-1-phosphate
• Hereditary fructose intolerance → aldolase B deficiency
• Essential fructosuria → benign fructokinase deficiency
• C-2 epimer of glucose; aldohexose.
• Required for N-linked glycoprotein synthesis
• Mannose-6-phosphate → lysosomal enzyme targeting
• Congenital disorders of glycosylation (CDG)
• C-4 epimer of glucose; aldohexose.
• Leloir pathway → glucose
• UDP-galactose → lactose synthesis
• Classic galactosemia (GALT deficiency)
• Galactokinase deficiency → cataracts
• Galactitol accumulation in lens
• Glucose → major fuel, sorbitol pathway
• Fructose → rapid metabolism, HFI
• Mannose → glycoproteins, mannose-6-phosphate
• Galactose → lactose synthesis, galactosemia
• Contain a free aldehyde or keto group (free anomeric carbon).
• Can reduce Cu²⁺ → Cu⁺ or Ag⁺ → Ag.
• All monosaccharides are reducing sugars.
• Examples: glucose, fructose, galactose, lactose, maltose.
• Do NOT have a free anomeric carbon (both anomeric carbons involved in glycosidic bond).
• Cannot reduce Benedict’s or Fehling’s reagent.
• Examples: sucrose, trehalose.
• Reducing sugar detection in urine → diabetes mellitus, galactosemia, hereditary fructose intolerance.
• Aldehyde → acid
• Examples:
– Glucose → gluconic acid
– Glucose → glucuronic acid (detoxification)
– Diabetic complications via oxidative stress
• Aldoses & ketoses → sugar alcohols
• Examples:
– Glucose → sorbitol
– Galactose → galactitol
• Excess sorbitol → cataract, neuropathy.
• Hemiacetal reacts with alcohol → glycosidic bond.
• Basis of disaccharides (maltose, lactose) & polysaccharides (glycogen).
• Aldose → ketose & vice versa.
• Glucose ↔ fructose ↔ mannose.
• OH groups esterified by phosphates.
• Examples: glucose-6-phosphate, fructose-1-phosphate.
Interconversion of α and β anomers through an open-chain form when dissolved in water.
• Leads to change in optical rotation until equilibrium is reached.
• Occurs in reducing sugars only.
• α-D-glucose ↔ β-D-glucose
• Lactose mutarotates; sucrose does NOT.
Compounds with the same molecular formula but different spatial orientation.
• Determined by configuration at penultimate carbon.
• Most human sugars are D-isomers.
For a sugar with n chiral centers → 2ⁿ stereoisomers.
Monosaccharides that differ only at one specific carbon atom.
• Glucose & Mannose → epimers at C-2
• Glucose & Galactose → epimers at C-4
• Epimerization is used in metabolism (UDP-glucose ↔ UDP-galactose).
• Reducing sugars convert cupric ions (Cu²⁺) to cuprous oxide (Cu⁺) forming a red/orange precipitate.
• Presence of reducing sugar in sample (urine, blood, CSF).
• Diabetes mellitus (glucosuria)
• Galactosemia (galactose in urine)
• Fructosuria
• Inborn errors of carbohydrate metabolism
• Green → trace
• Yellow → moderate
• Orange/brick red → strong positive
Reducing sugars react with phenylhydrazine to form crystalline osazones.
Differentiates sugars based on shape & rate of crystal formation.
• Glucose, fructose, mannose → needle-shaped “broomstick” osazones
• Maltose → sunflower-like crystals
• Lactose → puff-ball crystals
Reaction occurs at C-1 and C-2, so sugars identical at these carbons produce identical osazones.
Helpful in older lab setups for carbohydrate identification.
• All monosaccharides = reducing sugars.
• Sucrose = nonreducing, no mutarotation.
• Mutarotation → α ↔ β anomers interconversion.
• Epimers differ at one carbon (C-2, C-4 important).
• Benedict’s test detects reducing sugars in urine.
• Osazone crystals differentiate common sugars.
• Serve as energy reserves.
• Easily mobilized.
• Examples:
– Glycogen (animals)
– Starch (plants: amylose + amylopectin)
• Provide rigidity and support.
• Not easily digested.
• Examples:
– Cellulose (plant cell wall; β-1,4 link → non-digestible)
– Chitin (exoskeleton; N-acetylglucosamine polymer)
– Hyaluronic acid (ECM component)
Proteins covalently linked with carbohydrate chains (short, branched oligosaccharides).
• N-linked → attached to Asn
• O-linked → attached to Ser/Thr
• Cell recognition
• Hormones (TSH, FSH, LH)
• Antibodies (IgG)
• Receptors
• Enzymes
• Congenital disorders of glycosylation (CDG) → neurological symptoms.
• Blood group antigens are glycoproteins.
Long, unbranched polysaccharides containing repeating acidic disaccharide units.
Amino sugar + uronic acid
• Glucosamine or galactosamine
• Glucuronic or iduronic acid
• Hydration
• Lubrication (synovial fluid)
• ECM structure
• Shock absorption
• Hyaluronic acid
• Chondroitin sulfate
• Dermatan sulfate
• Heparin
• Heparan sulfate
• Keratan sulfate
• Defective degradation → mucopolysaccharidoses (MPS)
– Hurler syndrome (α-L-iduronidase deficiency)
– Hunter syndrome (iduronate sulfatase deficiency)
Carbohydrate linked to a non-sugar moiety (aglycone) through a glycosidic bond.
• Hemiacetal OH of sugar reacts with alcohol/phenol.
• Cardiac glycosides → Digitalis, digoxin
• Anthocyanins (plant pigments)
• Drug metabolism
• Stability of plant toxins
• Formation of glycolipids, nucleosides (adenosine, guanosine)
Sugars where an OH group is replaced by an NH₂ group.
• Glucosamine
• Galactosamine
• N-acetylglucosamine (NAG) → chitin
• Present in GAGs, glycoproteins.
• Building blocks of cartilage and connective tissue.
Sugars in which an OH group is replaced by H.
• Deoxyribose → DNA
• L-fucose → blood group antigens
• Rhamnose → plant products
Disaccharides = two monosaccharides joined by a glycosidic bond.
• Reducing or nonreducing depends on availability of free anomeric carbon.
• Important in diet and digestion.
• Glucose + Fructose
• Linkage: α-1,β-2 glycosidic bond
• Nonreducing sugar
• Cane sugar, beet sugar
• Does not show mutarotation
• No reaction with Benedict’s reagent
• Galactose + Glucose
• Linkage: β-1,4
• Reducing sugar
• Lactose intolerance → lactase deficiency → bloating, diarrhea
• Glucose + Glucose
• Linkage: α-1,4
• Reducing sugar
• Product of starch digestion (amylase activity)
• Storage carbs = glycogen, starch; structural = cellulose, chitin.
• Glycoproteins = short, branched sugars; GAGs = long, unbranched, acidic.
• Amino sugars = glucosamine, galactosamine; deoxy sugars = deoxyribose.
• Sucrose = nonreducing; lactose & maltose = reducing.
• Lactose → β-1,4; Maltose → α-1,4; Sucrose → α-1,β-2.
Large polymers of monosaccharides linked by glycosidic bonds.
• Storage polysaccharides – starch, glycogen
• Structural polysaccharides – cellulose, chitin
• Mucopolysaccharides (GAGs) – hyaluronic acid, chondroitin sulfate, etc.
• Plant storage carbohydrate
• Mixture of:
– Amylose → linear α-1,4 link
– Amylopectin → branched α-1,4 and α-1,6 links
• Digested by salivary and pancreatic amylase
• Major dietary carbohydrate
• Lack of amylase → poor starch digestion
• Resistant starch → improves gut flora (prebiotic)
• Major animal storage polysaccharide
• Highly branched polymer of glucose
• Linkages: α-1,4 (chains) and α-1,6 (branches)
• Liver → maintains blood glucose
• Muscle → for contraction energy
• Glycogen storage diseases (GSD)
– Type I: von Gierke (G6Pase deficiency)
– Type V: McArdle (muscle phosphorylase deficiency)
– Type II: Pompe (lysosomal acid maltase deficiency)
• Linear chains of glucose linked by β-1,4 bonds.
• Humans cannot digest (lack cellulase)
• Provides dietary fiber → gut motility, reduced cholesterol
Long, negatively charged polysaccharides composed of repeating disaccharide units containing an amino sugar + uronic acid.
• Lubrication (synovial fluid)
• Shock absorption
• Structural ECM support
• Hydration due to negative charge
• Hyaluronic acid – synovial fluid, ECM
• Chondroitin sulfate – cartilage
• Dermatan sulfate – skin, vessels
• Heparin – anticoagulant
• Heparan sulfate – basement membranes
• Keratan sulfate – cornea, cartilage
• Lysosomal storage disorders
• Due to failure to degrade GAGs
• Accumulation → coarse facies, hepatosplenomegaly, joint stiffness, developmental delay
• Enzyme: α-L-iduronidase deficiency
• Accumulated GAGs: dermatan sulfate, heparan sulfate
• Features:
– Coarse facies
– Corneal clouding
– Hepatosplenomegaly
– Developmental delay
• Enzyme: iduronate sulfatase deficiency
• Accumulated GAGs: dermatan sulfate, heparan sulfate
• Features:
– Coarse facies
– No corneal clouding
– Behavioral issues
• Defects in heparan sulfate degradation
• Predominantly neurological deterioration
• Enzyme: galactose-6-sulfatase deficiency
• Accumulated GAG: keratan sulfate
• Features:
– Skeletal abnormalities
– Short stature
– Normal intelligence
• Enzyme: arylsulfatase B deficiency
• Accumulated GAG: dermatan sulfate
• Features:
– Similar to Hurler
– Normal intelligence
• Excess glucose → sorbitol (aldose reductase) → osmotic swelling → lens opacity.
• Lactase deficiency → bloating, diarrhea.
• GALT deficiency → galactose ↑ → galactitol in lens → cataracts.
• Aldolase B deficiency → vomiting, hypoglycemia, jaundice.
• Accumulation of GAGs → coarse facies, organ enlargement, joint deformity.
• Impaired glycogen metabolism → hypoglycemia, muscle weakness.
• Prevents constipation, lowers cholesterol.
• Starch → amylose (α-1,4) + amylopectin (α-1,4 & α-1,6).
• Glycogen → highly branched (α-1,4 & α-1,6).
• Cellulose → β-1,4; non-digestible.
• GAGs → long, acidic, structural polysaccharides.
• MPS disorders → failure to degrade GAGs.
• Hurler = corneal clouding; Hunter = no clouding.
• Morquio = skeletal deformity, keratan sulfate accumulation.
Pentoses are 5-carbon monosaccharides important in nucleic acid structure and metabolic pathways.
• Formula: C₅H₁₀O₅ (ribose)
• Exist mainly in furanose (5-membered) ring form
• All pentoses are reducing sugars
• Aldopentose
• Component of RNA, ATP, NAD⁺, FAD, CoA
• Part of ribose-5-phosphate in the HMP shunt
• Required for synthesis of:
– RNA nucleotides
– ATP, GTP
– Coenzymes (NAD⁺, NADP⁺, FAD, CoA)
• Generated from glucose-6-phosphate via:
– HMP shunt (oxidative)
– Transketolase reactions (non-oxidative)
• Aldopentose
• Differs from ribose by absence of OH at C-2 → replaced by H
• Component of DNA
• Gives DNA chemical stability (less reactive than RNA).
• Essential for genome replication and repair.
• Produced from ribose via ribonucleotide reductase (requires NADPH).
• Defects in enzymes like transketolase decrease ribose-5-phosphate production.
• Thiamine deficiency reduces transketolase function → neurological symptoms.
• Deoxyribose deficiency affects DNA stability and repair mechanisms.
• Seen in some inherited nucleotide metabolism disorders.
• Ribose is needed for NADPH production (via pentose phosphate pathway).
• NADPH deficiency → increased oxidative damage (RBC hemolysis in G6PD deficiency).
• Cancer cells require high ribose for accelerated nucleotide synthesis.
• Basis for targeting nucleotide metabolism in chemotherapy.
• Overproduction or underutilization of ribose-based nucleotides causes:
– Gout (excess purines)
– SCID (adenosine deaminase deficiency)
– Orotic aciduria (pyrimidine synthesis defect)
• Ribose → RNA, ATP, NAD⁺, FAD, CoA
• Deoxyribose → DNA
• Ribose made from HMP shunt; deoxyribose from ribose via ribonucleotide reductase
• Pentoses are reducing sugars
• Clinically relevant in G6PD deficiency, cancer metabolism, nucleotide disorders
Pentoses are 5-carbon monosaccharides important for nucleic acid structure and cellular metabolism.
Deoxyribose lacks the OH group at carbon-2, making DNA more stable than RNA.
In RNA, ATP, GTP, NAD⁺, FAD, and CoA.
In DNA only.
From glucose-6-phosphate via the HMP shunt.
By reduction of ribonucleotides via ribonucleotide reductase, using NADPH.
Yes, all pentoses are reducing sugars.
It generates NADPH, protecting RBCs from oxidative damage.
Defect → G6PD deficiency → hemolysis.
Deoxyribose is less reactive and provides long-term chemical stability to DNA.
Cancer cells require large amounts of ribose for rapid nucleotide synthesis.
Nucleotide synthesis decreases → impaired cell proliferation and DNA repair.
Transketolase deficiency (exacerbated by thiamine deficiency).
Defects in nucleotide metabolism (e.g., adenosine deaminase deficiency) impair DNA synthesis.
Yes. Ribose → NADPH → antioxidant protection via glutathione.
Because fructose bypasses the PFK-1 step and enters at the triose phosphate level (side concept linking monosaccharides).
• Ribose = RNA sugar, Deoxyribose = DNA sugar
• Deoxyribose has no OH at C-2 → increases DNA stability
• Ribose is essential for ATP, NAD⁺, FAD, CoA, RNA
• Ribose-5-phosphate is formed via the HMP shunt
• Deoxyribose is formed by ribonucleotide reductase (needs NADPH)
• All pentoses are reducing sugars
• NADPH generated by ribose pathway protects RBCs (G6PD deficiency → hemolysis)
• Deoxyribose defects affect DNA replication and repair
• Pentose metabolism is tightly linked to cell division, cancer metabolism, oxidative balance
• Ribose and deoxyribose form the backbone of nucleic acids
Carbohydrates are essential for energy metabolism, structural integrity, cellular communication, and detoxification. Their clinical importance becomes clear when examining genetic diseases, acquired metabolic disorders, nutritional issues, and diagnostic applications.
• Glucose is the universal fuel for most tissues.
• Brain, RBCs, renal medulla, retina → highly glucose-dependent.
• Hypoglycemia → confusion, seizures, coma.
• Any condition causing low blood glucose (insulin overdose, sepsis, adrenal insufficiency) becomes a medical emergency.
• Glucose-6-phosphatase deficiency
• Severe fasting hypoglycemia, lactic acidosis, hepatomegaly.
• Lysosomal acid maltase deficiency
• Cardiomegaly, hypotonia.
• Muscle phosphorylase deficiency
• Exercise intolerance, muscle cramps, myoglobinuria.
• GALT deficiency
• Jaundice, vomiting, cataracts, hepatomegaly.
• Galactitol accumulation → lens opacity.
• Aldolase B deficiency
• Hypoglycemia, liver failure after consuming fruits/sucrose.
• Benign fructokinase deficiency
• Fructose spills into urine.
• Low NADPH → oxidative stress → hemolysis.
• Triggers: fava beans, infections, sulfa drugs, antimalarials.
• Lactase deficiency (genetic or post-infectious).
• Bloating, diarrhea, gas.
• Intolerance to sucrose and starch digestion.
• Watery diarrhea in infants.
• Anaerobic glycolysis → lactate production.
• Excess lactate → lactic acidosis in:
– Shock
– Sepsis
– Liver failure
– Thiamine deficiency
– Metformin toxicity
• 50–60% of daily calories.
• Complex carbs preferred over simple sugars.
• Hyperinsulinemia → weight gain
• Increased triglycerides
• Increased risk of fatty liver
• Used for weight loss and type 2 diabetes control.
• Risk: ketoacidosis in type 1 diabetics if insulin is absent.
• Prevents constipation
• Reduces postprandial glucose spikes
• Lowers LDL cholesterol
• Improves gut microbiota
• Reduces colon cancer risk
• Recommended in diabetes, obesity, hyperlipidemia.
• Blood group antigens
• Immunoglobulins
• Cell adhesion molecules
• Receptor recognition (viral attachment)
• Carbohydrate defects contribute to congenital disorders of glycosylation (CDG).
• Important in joints, cartilage, ECM.
• Hurler (cloudy cornea)
• Hunter (no corneal clouding)
• Morquio (skeletal deformity)
• Sanfilippo (CNS degenerative)
• Chronic hyperglycemia → nonenzymatic glycation of proteins.
• Complications:
– Retinopathy
– Nephropathy
– Neuropathy
– Atherosclerosis
• In lens, nerves, kidney → osmotic damage.
• Seen in uncontrolled diabetes.
• Cancer cells rely on aerobic glycolysis, converting glucose to lactate even with oxygen.
• High glucose uptake → basis of FDG-PET scans.
• PET scan detects tumors based on glucose uptake.
• Liver stores glycogen → fasting glucose maintenance.
• Liver failure → hypoglycemia + lactic acidosis.
• High carbohydrate diet can worsen fatty liver disease.
• Diagnosis of diabetes mellitus and reactive hypoglycemia.
• Measures long-term glycemic control (AGE formation).
• Screens for galactosemia, fructosuria, glucosuria.
• Glucose is the primary metabolic fuel; brain & RBCs depend on it.
• GSDs, galactosemia, and fructose intolerance are important genetic carbohydrate disorders.
• G6PD deficiency → oxidative hemolysis; linked to pentose pathway.
• Lactase deficiency → common cause of carbohydrate malabsorption.
• Sorbitol accumulation → diabetic cataract and neuropathy.
• Chronic hyperglycemia leads to glycation, causing microvascular complications.
• Fiber improves digestion, lowers cholesterol, and helps control diabetes.
• GAG defects → mucopolysaccharidoses (Hurler, Hunter, Morquio).
• Carbohydrates are essential for nucleic acids (ribose, deoxyribose), immunity, and ECM.
• Excess carbohydrates → fatty liver, obesity, hypertriglyceridemia.
• PET scans use glucose analogs to detect tumors.
• Hypoglycemia is more dangerous acutely; hyperglycemia dangerous chronically.
A. Liver
B. Kidney cortex
C. Brain
D. Skeletal muscle
Answer: C
A. Retinopathy
B. Cataract
C. Hypoglycemia
D. Ketoacidosis
Answer: B
A. Fructokinase
B. Aldolase B
C. Galactokinase
D. Pyruvate kinase
Answer: B
A. Galactokinase
B. Aldose reductase
C. GALT
D. Hexokinase
Answer: C
A. Glycolysis
B. TCA cycle
C. Pentose phosphate pathway
D. Beta-oxidation
Answer: C
A. Galactokinase
B. Aldolase B
C. G6PD
D. Transketolase
Answer: A
A. McArdle disease
B. Hurler syndrome
C. Hunter syndrome
D. Pompe disease
Answer: B
A. Hurler syndrome
B. Maroteaux–Lamy syndrome
C. Hunter syndrome
D. Morquio syndrome
Answer: C
A. Liver phosphorylase
B. Muscle phosphorylase
C. G6Pase
D. Acid maltase
Answer: B
A. Pompe disease
B. von Gierke disease
C. McArdle disease
D. Cori disease
Answer: B
A. NADH
B. NADPH
C. ATP
D. FADH₂
Answer: B
A. Glutathione
B. NAD⁺
C. Peroxide
D. Sorbitol
Answer: C
A. Constipation
B. Colon cancer
C. LDL elevation
D. Hypoglycemia
Answer: D
A. Benedict’s test
B. Seliwanoff test
C. Barfoed test
D. Rapid urease test
Answer: A
A. Glucose
B. Fructose
C. Galactose
D. Ribose
Answer: B
A. Hemolytic anemia
B. Liver disease
C. Myocardial infarction
D. Muscle injury
Answer: C
A. Glycogen
B. Amylose
C. Amylopectin
D. Cellulose
Answer: D
A. Glycogen breakdown in liver
B. Aerobic glycolysis in cancer cells
C. Lactose intolerance
D. Ketone body formation in diabetes
Answer: B
A. Maltose
B. Lactose
C. Sucrose
D. Cellobiose
Answer: C
A. Hurler
B. Hunter
C. Sanfilippo
D. Morquio
Answer: C
To provide immediate energy, mainly in the form of glucose.
Brain, RBCs, retina, renal medulla.
Approximately 120 grams of glucose.
RBCs lack mitochondria, so they depend entirely on glycolysis.
Hypoglycemia, because it affects the brain quickly.
Deficiency of glucose-6-phosphatase prevents glucose release from liver.
Galactokinase deficiency.
Aldolase B deficiency → hereditary fructose intolerance.
Low NADPH → inability to neutralize oxidants → oxidative damage to RBCs.
Fava beans, infections, antimalarials, sulfa drugs.
To generate NADPH for glutathione regeneration.
Sorbitol, due to aldose reductase activity.
Lens, retina, kidney, peripheral nerves.
Tissue hypoxia → anaerobic glycolysis → lactate accumulation.
Bloating and diarrhea due to undigested lactose fermenting in colon.
Hurler syndrome (α-L-iduronidase deficiency).
Hunter syndrome.
Defective degradation of keratan sulfate.
Lack of β-1,4 glucanase (cellulase).
Acts as fiber, improving bowel movement and lowering cholesterol.
Cancer cells preferentially perform aerobic glycolysis, producing lactate even with oxygen.
FDG-PET scan for tumor detection.
LDH-1 exceeds LDH-2 due to cardiac muscle damage.
Both anomeric carbons are involved in the α-1,β-2 glycosidic bond.
Rapid digestion and absorption of simple carbohydrates → glucose spike → insulin release.
Slows glucose absorption → reduces postprandial spikes.
Helps in detoxification (conjugation reactions).
Ribonucleotide reductase.
Component of ATP, NAD⁺, RNA.
Benedict’s test.
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