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Biochemistry Questions: Detailed Answers

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5. Role of Biochemistry in the Pharmacy Field

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6. Disaccharides - Definition and Structures of Sucrose and Maltose

Definition

Structure of Sucrose

• Glucose (pyranose form, α configuration at C-1)
• Fructose (furanose form, β configuration at C-2)

α-D-Glucopyranose ——(1↔2)——β-D-Fructofuranose
         |__________________________|
              α,β-1,2-glycosidic bond

• Non-reducing sugar
• Hydrolyzed by the enzyme sucrase (invertase) → glucose + fructose
• The hydrolysis product is called "invert sugar" because the optical rotation changes from (+) to (-)

Structure of Maltose

• Two D-glucose units (both in pyranose form)
• Joined by an α-1,4 glycosidic bond (C-1 of one glucose to C-4 of the second glucose, α configuration)

α-D-Glucopyranose ——(1→4)——D-Glucopyranose (free anomeric -OH)
         |__________________________|
              α-1,4-glycosidic bond

• Reducing sugar (free anomeric OH at C-1 of second glucose)
• Produced during starch digestion by the enzyme amylase
• Hydrolyzed by maltase → 2 glucose molecules
• Important intermediate in brewing and malting processes

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7. Functions of Essential and Non-Essential Amino Acids

Essential Amino Acids (EAA)

Evidence is accumulating that histidine is essential even for adults. - Park's Textbook of Preventive and Social Medicine, p. 1749

Non-Essential Amino Acids

Combined Functions

• Structural proteins (collagen, keratin, actin, myosin)
• Enzymatic proteins
• Antibodies and plasma proteins (albumin, globulins)
• Hemoglobin
• Hormones and neurotransmitters
• Coagulation factors
• Maintenance of osmotic pressure
• Energy supply (4 kcal/g) when caloric intake is inadequate

"New tissues cannot be formed unless all the essential amino acids are present in the diet." - Park's Textbook of Preventive and Social Medicine, p. 1764

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8. Structure and Functions of Cholesterol

Structure of Cholesterol

1. Cyclopentanoperhydrophenanthrene nucleus - the steroid core made of:
   • Three fused 6-membered rings (rings A, B, C - cyclohexane)
   • One 5-membered ring (ring D - cyclopentane)
   • All four rings are fused together
2. Hydroxyl group (-OH) at C-3 of ring A (makes it amphipathic)
3. Hydrocarbon tail - a branched 8-carbon side chain at C-17 of ring D
4. Double bond between C-5 and C-6 in ring B (making it a Δ⁵-sterol)
5. Two angular methyl groups at C-10 and C-13

         CH₃  CH₃
          |    |
   HO--[Ring A]-[Ring B]-[Ring C]-[Ring D]--C₈H₁₇ side chain
     (C-3-OH)         (Δ⁵ double bond)

Functions of Cholesterol

"Appreciate the importance of cholesterol as the precursor of many biologically important steroids, including steroid hormones, bile acids, and vitamin D." - Harper's Illustrated Biochemistry, 32nd Ed., p. 217

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9. Nucleotides vs. Nucleosides - Differences with Examples

Definitions

"The difference between a nucleotide and a nucleoside is that the latter must be phosphorylated to become active." - Comprehensive Clinical Nephrology, 7th Edition, p. 823

Components of Each

1. Nitrogenous base - either a purine (adenine, guanine) or pyrimidine (cytosine, uracil, thymine)
2. Pentose sugar - ribose (in RNA) or deoxyribose (in DNA)
3. Phosphate group (one or more: mono-, di-, triphosphate)

1. Nitrogenous base
2. Pentose sugar (no phosphate)

Examples

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10. Factors Affecting Enzyme Activity

1. Temperature

• Enzyme activity increases with temperature up to an optimal point (typically 37°C for human enzymes)
• Above the optimum temperature, heat causes denaturation - unfolding of the enzyme's 3D structure, destroying the active site
• Below the optimum, molecular motion is too slow for effective substrate-enzyme collisions
• The Q₁₀ rule: for every 10°C rise, reaction rate approximately doubles (within physiological range)

2. pH

• Each enzyme has an optimal pH at which it is most active
• Examples:
  • Pepsin (stomach): optimal pH ~2
  • Trypsin (intestine): optimal pH ~8
  • Most intracellular enzymes: optimal pH ~7.4
• Deviations from optimal pH alter the ionization state of amino acid residues in the active site, reducing substrate binding and catalysis
• Extreme pH causes denaturation

3. Substrate Concentration [S]

• At low [S]: reaction rate is proportional to [S] (first-order kinetics)
• As [S] increases: rate increases but begins to plateau
• At very high [S]: rate reaches Vmax (maximum velocity) - all enzyme active sites are saturated
• Described by the Michaelis-Menten equation: v = Vmax[S] / (Km + [S])
• Km (Michaelis constant) = [S] at which v = Vmax/2; reflects enzyme-substrate affinity

4. Enzyme Concentration

• At constant [S], reaction rate increases proportionally with enzyme concentration (as long as substrate is not limiting)
• Enzyme levels depend on the rate of synthesis (transcriptional/translational control) and degradation
• Inducers (e.g., substrate molecules) can rapidly increase enzyme levels

5. Inhibitors

• Inhibitor resembles the substrate; binds reversibly to the active site
• Competes with the substrate - increases apparent Km, but Vmax is unchanged
• Effect overcome by increasing [S]
• Example: statins competitively inhibit HMG-CoA reductase

• Inhibitor binds to a site other than the active site (allosteric site)
• Does not affect substrate binding (Km unchanged)
• Reduces Vmax - cannot be overcome by increasing [S]
• Example: heavy metal ions (Pb²⁺, Hg²⁺)

• Inhibitor binds only to the enzyme-substrate (ES) complex
• Both Km and Vmax decrease

• Covalently modify the active site
• Example: organophosphates irreversibly inhibit acetylcholinesterase

6. Cofactors and Coenzymes

• Many enzymes require cofactors (inorganic ions: Mg²⁺, Zn²⁺, Fe²⁺) or coenzymes (organic molecules derived from vitamins: NAD⁺, FAD, CoA) for activity
• Absence of cofactors greatly reduces or abolishes enzyme activity
• Apoenzyme (enzyme without cofactor) + cofactor = holoenzyme (active enzyme)

7. Allosteric Regulation

• Binding of small molecules (allosteric effectors) at sites distinct from the active site causes conformational changes
• Allosteric activators increase activity; allosteric inhibitors decrease it
• Common in enzymes that control rate-limiting steps in metabolic pathways
• This is a key regulatory mechanism for feedback control of metabolic pathways

8. Product Concentration

• Accumulation of reaction products can slow enzyme activity by inhibiting the forward reaction (product inhibition) or by mass action effects
• In reversible reactions, enzymes accelerate both forward and reverse reactions equally; equilibrium is not changed, only reached faster

9. Presence of Activators

• Some enzymes require prior activation (e.g., zymogens/proenzymes must be cleaved: pepsinogen → pepsin; trypsinogen → trypsin)
• Phosphorylation/dephosphorylation switches many enzymes on or off

"Enzymatic activity is also subject to control through the binding of small molecules that produce conformational changes in the structure of the enzyme... Generally, enzymes that catalyze rate-determining steps in metabolic pathways are subject to this type of regulation." - Henry's Clinical Diagnosis and Management by Laboratory Methods

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