Biomolecules (Carbohydrates & Proteins)

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Biomolecules — Key Facts for SLMC Medical (Sri Lanka)

• Carbohydrates: polyhydroxyaldehydes or ketones; classified as monosaccharides, disaccharides, polysaccharides
• Monosaccharides: glucose, fructose (C₆ = hexose); ribose, deoxyribose (C₅ = pentose)
• Proteins: polymers of α-amino acids linked by peptide bonds
• Amino acids have both acidic (–COOH) and basic (–NH₂) groups — they are amphoteric
• ⚡ Exam tip: Glucose structure, glycosidic bonds, protein denaturation, and zwitterion forms are high-yield for SLMC

---

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Biomolecules — SLMC Medical (Sri Lanka) Study Guide

Carbohydrates

Classification

Type	Example	Composition	Bond
Monosaccharide	Glucose, Fructose, Ribose	Single sugar unit	Cannot hydrolyze further
Disaccharide	Sucrose, Lactose, Maltose	2 monosaccharides	Glycosidic bond
Polysaccharide	Starch, Glycogen, Cellulose	Many monosaccharides	Glycosidic bonds

Monosaccharides

Glucose (C₆H₁₂O₆) — the most important hexose:

• Also called dextrose (due to its ability to rotate plane-polarized light clockwise)
• Open-chain form has an aldehyde group (C-1) and 5 hydroxyl groups
• Cyclizes to form a 6-membered pyranose ring (5 carbons + 1 oxygen) via hemiacetal formation
• In the ring form, the anomeric carbon (C-1) can be α-D-glucose or β-D-glucose

Fructose (C₆H₁₂O₆) — ketose hexose:

• Open-chain form has a ketone group at C-2
• Can form a 5-membered furanose ring
• Present in fruits and honey; a component of sucrose

Ribose (C₅H₁₀O₅) and 2-deoxyribose — pentoses in nucleic acids:

• DNA contains 2-deoxyribose (H at C-2 instead of –OH)
• RNA contains ribose (–OH at C-2)

Epimers

Glucose has multiple stereoisomers called epimers (differ at only one specific carbon):

• Mannose: epimer at C-2 (differs from glucose only at position 2)
• Galactose: epimer at C-4 (differs from glucose only at position 4)

These are clinically relevant in galactosemia (galactose-1-phosphate uridyltransferase deficiency) and mannose metabolism.

Reducing and Non-reducing Sugars

Reducing sugars have a free or potentially free aldehyde/keto group (can reduce Tollen’s/Fehling’s):

• All monosaccharides are reducing sugars
• Maltose, lactose (has free C-1 in one glucose unit)
• Sucrose is NON-reducing — glycosidic bond connects both reducing ends

Disaccharides

Disaccharide	Units	Bond Type	Key Fact
Sucrose	Glucose + Fructose	α-1,β2 glycosidic	Non-reducing; table sugar
Lactose	Glucose + Galactose	β-1,4 glycosidic	Reducing; milk sugar
Maltose	Glucose + Glucose	α-1,4 glycosidic	Reducing; from starch hydrolysis

Glycosidic bond: formed between the anomeric carbon of one sugar and any –OH group of another, with elimination of water.

Polysaccharides

Polysaccharide	Function	Structure	Branching
Starch	Energy storage in plants	Amylose (α-1,4 linear) + Amylopectin (α-1,4 + α-1,6 branches)	Branching at α-1,6
Glycogen	Energy storage in animals	Like amylopectin but more extensively branched	Highly branched; α-1,6
Cellulose	Structural support in plants	β-1,4 glucose polymer	No branching; forms fibers

Cellulose cannot be digested by humans (we lack β-1,4 cellulase) — dietary fiber. Starch (amylose + amylopectin) IS digestible — broken down by salivary and pancreatic amylase.

Carbohydrate Metabolism (Key Points)

• Glycolysis: glucose → 2 pyruvate + 2 ATP + 2 NADH (occurs in cytoplasm)
• Krebs cycle: acetyl-CoA → 2 CO₂ + 3 NADH + 1 FADH₂ + 1 GTP (mitochondria)
• Gluconeogenesis: formation of glucose from non-carbohydrate sources (lactate, amino acids, glycerol)
• Glycogenolysis: breakdown of glycogen to glucose (liver and muscle)

Proteins

Amino Acids — The Building Blocks

α-Amino acids have the general formula H₂N–CH(R)–COOH:

• The –NH₂ is on the α-carbon (adjacent to –COOH)
• 20 standard amino acids; categorized as essential, non-essential, conditionally essential

Category	Examples	Key Points
Essential	Valine, Leucine, Isoleucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Histidine	Must be obtained from diet
Non-essential	Glycine, Alanine, Serine, Glutamate, Aspartate	Synthesized by body
Conditionally essential	Glutamine, Arginine, Tyrosine, Cysteine	Required under specific conditions

Zwitterion: at physiological pH (~7.4), amino acids exist as H₃N⁺–CH(R)–COO⁻ (zwitterion — both positive and negative charges):

• pH < pI: predominantly positively charged (cationic)
• pH > pI: predominantly negatively charged (anionic)
• pI = isoelectric point = pH at which net charge = 0

Peptide Bond

Amino acids link via peptide bonds (–CO–NH–):

• Formed by condensation reaction between the –COOH of one amino acid and –NH₂ of another
• The peptide bond has partial double-bond character due to resonance → planar, rigid structure
• The N–C bond of the peptide backbone has restricted rotation (φ rotation ~ –180° to +180°)

Dipeptide: 2 amino acids linked Oligopeptide: 3–10 amino acids Polypeptide: >10 amino acids Protein: functional polypeptide (typically >50 amino acids, or specifically folded)

Protein Structure — Four Levels

Level	Description	Stabilizing Forces
Primary	Linear amino acid sequence (covalent peptide bonds)	Peptide bonds (covalent)
Secondary	Local folding patterns: α-helix (right-handed) and β-sheet (pleated)	H-bonds between backbone C=O and N–H groups
Tertiary	3D shape of entire polypeptide; domains, active sites	Hydrophobic interactions, disulfide bonds (–S–S–), H-bonds, ionic bonds
Quaternary	Assembly of multiple polypeptide subunits	Same forces as tertiary; also interface interactions

α-helix: 3.6 amino acids per turn; H-bond between C=O(i) and N–H(i+4) — intramolecular H-bonding β-sheet: H-bond between C=O and N–H of adjacent strands — can be parallel or antiparallel

Denaturation

Denaturation = disruption of tertiary and secondary structure WITHOUT breaking peptide bonds:

• Physical agents: heat, UV radiation, mechanical agitation
• Chemical agents: acids, bases, organic solvents (ethanol), heavy metal salts, urea, SDS (detergents)

Renaturation is possible if the denaturing agent is removed and the primary structure remains intact (e.g., ribonuclease can be renatured after urea/β-mercaptoethanol treatment).

Permanent denaturation: involves cleavage of peptide bonds (e.g., by proteases) or breaking of disulfide bonds without proper reformation.

Protein Classifications

• Fibrous proteins: keratin (hair, nails), collagen (connective tissue), myosin/actin (muscle) — structural, insoluble
• Globular proteins: enzymes, hemoglobin, antibodies — soluble, compactly folded

Clinical and Medical Relevance

• Lactose intolerance: deficiency of lactase enzyme → inability to hydrolyze lactose (β-1,4 bond) → osmotic diarrhea
• Galactosemia: deficiency of galactose-1-phosphate uridyltransferase → accumulation of galactose-1-phosphate → hepatotoxicity, cataracts
• Diabetes mellitus: impaired glucose metabolism; fasting blood glucose, HbA1c as diagnostic markers
• Phenylketonuria (PKU): deficiency of phenylalanine hydroxylase → phenylalanine accumulation → intellectual disability if untreated
• Cystic fibrosis: mutation in CFTR chloride channel; affects respiratory and GI systems
• Sickle cell anemia: Glu→Val substitution at position 6 of β-globin chain (HbS); deoxygenated HbS polymerizes → sickling
• Hemoglobin: tetramer of 2 α-globin + 2 β-globin chains; each subunit carries one heme (Fe²⁺) binding one O₂

Common SLMC Exam Traps

• Sucrose is NON-reducing — students often think all disaccharides are reducing
• Glycogen and amylopectin are branched via α-1,6 glycosidic bonds — amylose is linear (α-1,4 only)
• The α-helix is RIGHT-HANDED (most common in proteins); left-handed α-helices are rare
• Disulfide bonds (–S–S–) stabilize tertiary structure, not primary (they are covalent but between distant cysteines)
• Denaturation does NOT break peptide bonds — it only disrupts secondary/tertiary/quaternary structure
• Zwitterion has both + and – charges but is electrically neutral overall — net charge = 0 at pI

---

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.