for BDS First Year Biochemistry and are written in a format suitable for 5–6 mark university examinations.

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BDS First Year Biochemistry - Section "B" Model Answers

Q1. Classify proteins on the basis of functions with suitable examples. Write down the structural organization of secondary proteins. [3+3=6]

Part A: Classification of Proteins Based on Functions (3 marks)

Proteins perform diverse biological roles. Based on their functions, they are classified as:

Function Class	Description	Examples
Structural Proteins	Provide mechanical support and shape to cells and tissues	Collagen (bones, tendons, skin), Keratin (hair, nails), Elastin (blood vessels)
Enzymatic Proteins (Enzymes)	Catalyze biochemical reactions	Hexokinase, Pepsin, DNA polymerase, Amylase
Transport Proteins	Carry molecules through blood or across membranes	Hemoglobin (O2), Myoglobin (O2 in muscle), Albumin (fatty acids, hormones), Transferrin (iron)
Hormonal Proteins	Chemical messengers that regulate physiological processes	Insulin (blood glucose), Growth hormone, Glucagon
Receptor Proteins	Bind specific ligands to trigger intracellular responses	Insulin receptor, Acetylcholine receptor
Contractile/Motor Proteins	Responsible for movement	Actin, Myosin (muscle contraction)
Immunological Proteins	Defense against pathogens	Immunoglobulins (IgG, IgA, IgM), Complement proteins
Storage Proteins	Store amino acids or other molecules	Ferritin (iron storage), Casein (in milk), Ovalbumin (egg white)
Regulatory Proteins	Control gene expression or cell processes	Histones, Transcription factors, p53

Part B: Structural Organization of Secondary Proteins (3 marks)

Secondary structure refers to the regular, repeating local folding of the polypeptide backbone stabilized by hydrogen bonds between the peptide NH and C=O groups.

1. Alpha-Helix (alpha-helix)

The most common secondary structure
The polypeptide backbone coils into a right-handed spiral (helix)
Each turn of the helix contains approximately 3.6 amino acid residues
Pitch (rise per turn) = 5.4 Angstroms
Stabilized by hydrogen bonds between the C=O of residue n and the N-H of residue n+4
The R-groups (side chains) project outward from the helical axis
Proline cannot fit in an alpha-helix due to its ring structure - it acts as a "helix breaker"
Examples: alpha-keratin (hair), myoglobin

2. Beta-Pleated Sheet (beta-sheet)

The polypeptide backbone is almost fully extended
Stabilized by hydrogen bonds between adjacent strands (not within the same strand)
Two types:
- Parallel beta-sheet: adjacent strands run in the same N-to-C direction
- Antiparallel beta-sheet: adjacent strands run in opposite directions (more stable, straighter H-bonds)
R-groups alternate above and below the plane of the sheet
Examples: beta-keratin (silk fibroin), beta-amyloid (in Alzheimer's disease)

3. Beta-Turn (Reverse Turn)

A sharp 180-degree reversal in the direction of the polypeptide chain
Involves 4 amino acid residues
Stabilized by a hydrogen bond between residue 1 (C=O) and residue 4 (N-H)
Commonly found at the surface of globular proteins

4. Random Coil

Regions with no regular repeating structure
Not truly "random" - conformation is specific but not periodic

Diagram tip: Draw a helical ribbon for alpha-helix showing H-bonds every 4th residue; draw flat arrows for beta-strands showing parallel/antiparallel arrangement.

Q2. Define enzyme, coenzyme, and isoenzyme. Explain factors that affect enzyme activity. [2+3=5]

Part A: Definitions (2 marks)

Enzyme: An enzyme is a biological catalyst - almost always a protein - that speeds up biochemical reactions without being consumed in the process. Enzymes are highly specific (both substrate specificity and reaction specificity) and work by lowering the activation energy of a reaction. They are characterized by an active site where the substrate binds and the reaction occurs.

Coenzyme: A coenzyme is a small, non-protein organic molecule that is loosely or tightly associated with an enzyme and is essential for its catalytic activity. Coenzymes carry chemical groups (e.g., electrons, acyl groups) between reactions. Most coenzymes are derived from vitamins.

Examples: NAD+ (from niacin/Vitamin B3), FAD (from riboflavin/Vitamin B2), Coenzyme A (from pantothenic acid/Vitamin B5), Pyridoxal phosphate (from Vitamin B6), Thiamine pyrophosphate (from Vitamin B1)

Isoenzyme (Isozyme): Isoenzymes are multiple molecular forms of the same enzyme that catalyze the same chemical reaction but differ in their physical, chemical, and kinetic properties (amino acid sequence, electrophoretic mobility, substrate affinity, inhibitor sensitivity). They are encoded by different genes.

Classic Example: Lactate dehydrogenase (LDH) - has 5 isoforms (LDH-1 to LDH-5). LDH-1 (H4) predominates in heart; LDH-5 (M4) in liver. This has diagnostic significance - LDH-1 rises in myocardial infarction.
Another example: Creatine kinase (CK) - CK-MB (heart), CK-MM (skeletal muscle), CK-BB (brain)

Part B: Factors That Affect Enzyme Activity (3 marks)

1. Temperature

Enzyme activity increases with temperature (up to an optimum, usually 37°C in humans)
A rise of 10°C doubles the reaction rate (Q10 rule)
Beyond the optimal temperature, the enzyme undergoes denaturation - the 3D structure is disrupted, active site loses shape, and activity drops sharply
Most human enzymes are denatured above 55-60°C

2. pH

Each enzyme has an optimal pH at which it functions maximally
Examples: Pepsin (gastric enzyme) works best at pH 1.5-2.0; Trypsin (pancreatic) at pH 7-8; Alkaline phosphatase at pH 9-10
Changes in pH alter the ionization state of amino acid residues at the active site (especially histidine, lysine, aspartate, glutamate), disrupting substrate binding and catalysis

3. Substrate Concentration ([S])

At low [S], reaction rate (V) increases proportionally with [S]
At high [S], all enzyme active sites are occupied (enzyme is saturated) and the rate reaches a maximum - Vmax
This relationship is described by the Michaelis-Menten equation: V = Vmax[S] / (Km + [S])
Km (Michaelis constant) = [S] at which V = Vmax/2; reflects the affinity of enzyme for substrate (low Km = high affinity)

4. Enzyme Concentration

At constant [S] (saturating), the reaction rate is directly proportional to enzyme concentration
Doubling enzyme doubles the reaction rate

5. Inhibitors

Competitive inhibitors: structurally similar to substrate, compete for the active site; increase apparent Km but Vmax unchanged; can be overcome by excess substrate. Example: Methotrexate inhibits dihydrofolate reductase
Non-competitive inhibitors: bind to a site other than the active site (allosteric site); reduce Vmax but Km unchanged. Example: Heavy metals (Pb2+, Hg2+)
Irreversible inhibitors: permanently inactivate the enzyme by covalent bonding. Example: Organophosphates inhibiting acetylcholinesterase

6. Cofactors and Coenzymes

Many enzymes require metal ions (Mg2+, Zn2+, Fe2+) or coenzymes for full activity
Absence of these reduces or abolishes enzyme function

7. Allosteric Effectors

Allosteric enzymes have regulatory sites separate from the active site
Allosteric activators increase activity; allosteric inhibitors decrease it
These are important for metabolic regulation (e.g., PFK-1 in glycolysis is inhibited by ATP and citrate, activated by AMP)

Q3. Define neurotransmitter, write down the criteria to be considered a neurotransmitter, and classify them. [1+2+2=5]

Part A: Definition (1 mark)

A neurotransmitter is a chemical substance synthesized and stored in presynaptic neurons that is released into the synaptic cleft upon neuronal stimulation (nerve impulse/action potential), diffuses across the synapse, binds to specific receptors on the postsynaptic membrane, and produces a physiological effect (excitation or inhibition).

Part B: Criteria to Be Considered a Neurotransmitter (2 marks)

For a substance to qualify as a neurotransmitter, it must meet the following criteria:

Synthesis and presence: The substance must be synthesized within the neuron and present in the presynaptic terminal
Release: It must be released from the presynaptic terminal in response to nerve stimulation (in a Ca2+-dependent manner)
Receptor binding: The released substance must bind to specific receptors on the postsynaptic cell (or presynaptic autoreceptors) and produce a measurable response
Mimicry: Exogenous application of the substance must replicate the same effect as nerve stimulation
Removal mechanism: There must be a mechanism for terminating its action - by enzymatic degradation (e.g., ACh by acetylcholinesterase), reuptake into the presynaptic neuron (e.g., dopamine), or diffusion
Blocked by specific antagonists: The effect of the NT must be blocked by specific pharmacological antagonists

Part C: Classification of Neurotransmitters (2 marks)

I. Based on Chemical Nature:

A. Amino Acids (small molecule, fast-acting)

Excitatory: Glutamate (major excitatory NT of CNS), Aspartate
Inhibitory: GABA (gamma-aminobutyric acid - major inhibitory NT), Glycine

B. Biogenic Amines (Monoamines)

Catecholamines: Dopamine, Norepinephrine (Noradrenaline), Epinephrine (Adrenaline)
Indoleamines: Serotonin (5-HT)
Others: Histamine

C. Acetylcholine (ACh)

At neuromuscular junctions and in the parasympathetic nervous system
Not an amino acid but a choline ester

D. Neuropeptides (large molecule, slow-acting)

Endogenous opioids: Endorphins, Enkephalins, Dynorphins
Others: Substance P (pain), Somatostatin, Vasoactive intestinal peptide (VIP), Neuropeptide Y

E. Purinergic Neurotransmitters

ATP, Adenosine

F. Gaseous Neurotransmitters

Nitric oxide (NO), Carbon monoxide (CO)

II. Based on Function:

Type	Examples
Excitatory	Glutamate, Aspartate, Acetylcholine, Norepinephrine
Inhibitory	GABA, Glycine, Endorphins

Q4. Write Short Notes on: [3×3=9]

a. Reactions of Glycolysis with Rate-Limiting Enzymes

Glycolysis is the anaerobic pathway that converts one molecule of glucose (6C) into two molecules of pyruvate (3C), yielding a net gain of 2 ATP and 2 NADH.

The 10 Steps of Glycolysis:

Preparatory Phase (Energy Investment - consumes 2 ATP):

Step	Reaction	Enzyme	Notes
1	Glucose → Glucose-6-phosphate	Hexokinase (or Glucokinase in liver)	Irreversible; consumes 1 ATP; RATE-LIMITING in liver (Glucokinase)
2	Glucose-6-phosphate → Fructose-6-phosphate	Phosphoglucose isomerase	Reversible
3	Fructose-6-phosphate → Fructose-1,6-bisphosphate	Phosphofructokinase-1 (PFK-1)	Irreversible; consumes 1 ATP; MAIN RATE-LIMITING STEP of glycolysis; Activated by AMP, ADP, fructose-2,6-bisphosphate; Inhibited by ATP, citrate
4	Fructose-1,6-bisphosphate → DHAP + Glyceraldehyde-3-phosphate	Aldolase	Splits 6C into two 3C molecules
5	DHAP ⇌ Glyceraldehyde-3-phosphate	Triose phosphate isomerase	Reversible; DHAP converted to G3P

ATP-Generating Phase (Energy Payoff - yields 4 ATP, 2 NADH):

Step	Reaction	Enzyme	Notes
6	Glyceraldehyde-3-P → 1,3-Bisphosphoglycerate	Glyceraldehyde-3-phosphate dehydrogenase	Generates NADH
7	1,3-Bisphosphoglycerate → 3-Phosphoglycerate	Phosphoglycerate kinase	Substrate-level phosphorylation; generates ATP
8	3-Phosphoglycerate → 2-Phosphoglycerate	Phosphoglycerate mutase
9	2-Phosphoglycerate → Phosphoenolpyruvate (PEP)	Enolase
10	PEP → Pyruvate	Pyruvate kinase	Irreversible; RATE-LIMITING; generates ATP; Inhibited by ATP, alanine; Activated by fructose-1,6-bisphosphate

Three Rate-Limiting (Irreversible) Enzymes in Glycolysis:

Hexokinase/Glucokinase - Step 1
Phosphofructokinase-1 (PFK-1) - Step 3 (most important regulatory step)
Pyruvate kinase - Step 10

Net yield of glycolysis: 2 ATP + 2 NADH + 2 Pyruvate per glucose molecule

b. Thalassemia

Definition: Thalassemias are a group of inherited autosomal recessive hemolytic anemias caused by reduced or absent synthesis of one or more globin chains of hemoglobin.

Biochemical Basis: Normal adult hemoglobin (HbA) = alpha2-beta2 (two alpha and two beta globin chains + heme). In thalassemia, mutations in the globin genes reduce chain synthesis, leading to:

Imbalanced globin chain production
Accumulation of excess unpaired chains which precipitate inside RBCs
Hemolysis, ineffective erythropoiesis, and anemia

Types:

1. Alpha-Thalassemia (defect in alpha-globin genes on chromosome 16)

4 alpha-globin genes; severity depends on how many are deleted
- 1 gene deleted: Silent carrier (asymptomatic)
- 2 genes deleted: Alpha-thalassemia trait (mild microcytic anemia)
- 3 genes deleted: HbH disease - unstable HbH (beta4 tetramers), moderate-severe hemolytic anemia
- 4 genes deleted: Hydrops fetalis (Hb Barts - gamma4) - incompatible with life

2. Beta-Thalassemia (defect in beta-globin gene on chromosome 11)

Thalassemia minor (trait, heterozygous beta+): mild microcytic hypochromic anemia, usually asymptomatic; elevated HbA2
Thalassemia intermedia: moderately severe anemia, splenomegaly, may need occasional transfusions
Thalassemia major (Cooley's anemia) (homozygous, beta0/beta0): severe hemolytic anemia from 6 months of age (when fetal Hb switches to adult Hb), severe splenomegaly, hepatomegaly, "hair-on-end" appearance of skull X-ray (marrow expansion), growth retardation, iron overload; requires regular blood transfusions and chelation therapy

Lab Findings in Beta-Thalassemia Major:

Very low Hb (2-8 g/dL)
Microcytic, hypochromic RBCs; target cells; nucleated RBCs on blood film
Elevated HbF, elevated HbA2
Elevated serum ferritin and iron (due to transfusions and increased absorption)

Treatment:

Regular blood transfusions
Iron chelation therapy (Deferoxamine or Deferasirox) to prevent iron overload
Folic acid supplementation
Splenectomy in selected cases
Bone marrow/stem cell transplantation (curative)

c. Myasthenia Gravis

Definition: Myasthenia gravis (MG) is an autoimmune neuromuscular disorder characterized by weakness and fatigability of skeletal muscles due to the production of autoantibodies (primarily against nicotinic acetylcholine receptors [nAChR] at the neuromuscular junction).

Biochemical/Immunological Basis:

In normal neuromuscular transmission: ACh is released from the presynaptic terminal, diffuses across the synaptic cleft, binds to nAChR on the motor end plate, and causes muscle contraction
In MG: IgG antibodies are produced against nAChR
- These antibodies block ACh binding to the receptor
- Cause receptor internalization and degradation (reducing the number of functional receptors)
- Activate complement and damage the postsynaptic membrane
Net result: Reduced functional receptors → impaired neuromuscular transmission → muscle weakness

Autoantibodies in MG:

Anti-nAChR antibodies (80-85% of patients) - most common
Anti-MuSK antibodies (muscle-specific tyrosine kinase) - ~10% of seronegative MG
Anti-LRP4 antibodies - less common

Clinical Features:

Characteristic fatigable weakness - weakness that worsens with activity and improves with rest
Ptosis (drooping eyelid) and diplopia (double vision) - often first symptoms (ocular MG)
Dysphagia, dysarthria, facial weakness
Proximal limb weakness
Myasthenic crisis: respiratory muscle paralysis requiring ventilatory support (medical emergency)

Association with Thymus:

~75% of MG patients have thymic abnormalities
~15% have thymoma (thymic tumor); ~85% have thymic hyperplasia
Thymectomy improves symptoms in many patients

Diagnosis:

Tensilon (edrophonium) test: short-acting AChE inhibitor; transient improvement in strength confirms diagnosis
Anti-nAChR antibody titers
Repetitive nerve stimulation: shows decremental response
Single-fiber EMG (most sensitive)
CT scan of chest to detect thymoma

Treatment:

Acetylcholinesterase inhibitors (Pyridostigmine/Neostigmine): inhibit ACh breakdown, increasing ACh at the synapse - symptomatic treatment
Immunosuppressants: Corticosteroids (Prednisolone), Azathioprine, Mycophenolate
Thymectomy: improves long-term outcome
Plasmapheresis and IV Immunoglobulins (IVIG): short-term use in myasthenic crisis

Exam Tips:

For Q1, always draw labeled diagrams of alpha-helix and beta-sheet

For Q2, the Michaelis-Menten curve (V vs [S]) and Lineweaver-Burk plot earn extra marks

For Q4a, a flowchart of the 10 glycolytic steps with the 3 irreversible enzymes highlighted in red earns full marks

For Q4c, a labeled diagram of the normal vs. MG neuromuscular junction is expected

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