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I have enough from the textbooks and my biochemistry knowledge to answer all questions comprehensively. Here are the complete answers:

Carbohydrate Metabolism Test — Complete Answers

Q.1) MCQs

1. The test to detect Glucose in Urine is: (b) Rothera's — Wait, let me reconsider. Rothera's test detects ketone bodies. The correct answer is (a) Benedict's test, which detects reducing sugars including glucose in urine. (Rothera's = ketones; Hay's = bile salts; Selivanoff's = fructose)
2. All the following hormones cause hyperglycemia EXCEPT: (c) Insulin — Insulin is the only hormone that lowers blood glucose. Glucagon, Epinephrine, and Thyroxine all raise blood glucose.
3. Rapoport–Luebering cycle is mainly concerned with the production of: (b) 2,3-bisphosphoglycerate (2,3-BPG) — This cycle is a bypass in glycolysis in RBCs where 1,3-BPG is converted to 2,3-BPG by bisphosphoglycerate mutase. 2,3-BPG reduces hemoglobin's oxygen affinity, aiding O₂ delivery to tissues.
4. All of the following are low energy compounds EXCEPT: (b) Carbamoyl phosphate — AMP, ADP, and Glycerol-3-phosphate are low energy compounds. Carbamoyl phosphate is actually a high energy compound (it has a mixed anhydride bond and is used in the urea cycle and pyrimidine synthesis).
5. In HMP shunt the electrons are accepted by: (c) NADP⁺ — In the oxidative phase of the pentose phosphate pathway (HMP shunt), glucose-6-phosphate is oxidized and NADP⁺ is reduced to NADPH. NADPH is the electron carrier generated, not FAD, NAD, or cytochrome.

Q.2) Fill in the Blanks

a. Glucose transporters in RBC ……… Brain …………
  • RBC: GLUT-1; Brain: GLUT-3 (neurons) / GLUT-1 (blood-brain barrier)
b. Hypoglycemia is observed when blood glucose level is below ……… mg/dl
  • 70 mg/dl (some sources cite <50 mg/dl as symptomatic hypoglycemia; the clinical threshold is 70 mg/dl)
c. Enzymes absent in Galactosemia ……………
  • Galactose-1-phosphate uridylyltransferase (GALT) — deficiency causes classic galactosemia. (Galactokinase deficiency causes a milder form.)
d. The important enzymes for glycogenolysis in muscle is …………
  • Glycogen phosphorylase (muscle phosphorylase) and debranching enzyme (α-1,6-glucosidase)
e. The donor of electrons in HMP shunt is ……………
  • Glucose-6-phosphate (G6P) — it is the substrate that donates electrons (is oxidized), with the reaction catalyzed by glucose-6-phosphate dehydrogenase (G6PD), reducing NADP⁺ to NADPH.

Q.3) Case Study

Clinical scenario: 40-year-old obese female with polyuria, headache, frequent thirst (polydipsia), wound with delayed healing, elevated blood glucose, and positive Rothera's test (ketones in urine).
A. Probable Diagnosis: Type 2 Diabetes Mellitus (T2DM)
  • Classic triad: polyuria, polydipsia, elevated blood glucose
  • Obesity is a major risk factor for T2DM
  • Positive Rothera's test suggests ketonuria (possible diabetic ketosis)
  • Delayed wound healing is a classic complication of diabetes
B. Three complications of Diabetes Mellitus:
  1. Diabetic nephropathy — kidney damage due to chronic hyperglycemia (glomerulosclerosis)
  2. Diabetic neuropathy — peripheral nerve damage causing numbness, pain, delayed wound healing
  3. Diabetic retinopathy — microvascular damage to retinal vessels causing vision loss (Others: cardiovascular disease, diabetic foot/gangrene, increased susceptibility to infections)
C. Method commonly used to detect blood glucose level: Glucose oxidase method (enzymatic colorimetric method) — the most commonly used method in clinical labs. Glucose oxidase oxidizes glucose to gluconate + H₂O₂, which is then detected colorimetrically. Also: Hexokinase method is considered the reference method. At the bedside, glucometers (reflectance photometry) are used.
D. Test that assesses long-term blood glucose control: HbA1c (Glycated Hemoglobin / Hemoglobin A1c) — reflects average blood glucose over the preceding 2–3 months (the lifespan of RBCs). Normal HbA1c is <5.7%; diabetic range ≥6.5%.
E. Normal blood sugar level:
  • Fasting: 70–100 mg/dl
  • Postprandial (2 hours after meal): <140 mg/dl
  • Random: <140 mg/dl

Q.4) Short Notes (any four)

(a) Galactosemia

Galactosemia is an autosomal recessive disorder of galactose metabolism. The classic form is due to deficiency of galactose-1-phosphate uridylyltransferase (GALT). Galactose-1-phosphate accumulates in liver, brain, kidney, and lens. Excess galactose is converted to galactitol by aldose reductase. Clinical features:
  • Jaundice, hepatomegaly, liver cirrhosis
  • Cataracts (due to galactitol deposition in lens)
  • Mental retardation / intellectual disability
  • Septicemia (especially E. coli neonatal sepsis)
  • Renal tubular dysfunction (Fanconi syndrome)
Treatment: Remove galactose and lactose from the diet (galactose-free diet). Early diagnosis via newborn screening is essential.

(b) Von Gierke's Disease (Glycogen Storage Disease Type I)

Von Gierke's disease is caused by deficiency of glucose-6-phosphatase (or its translocase). This enzyme is needed in both glycogenolysis and gluconeogenesis to release free glucose into the blood.
Consequences of the enzyme defect:
  • Severe fasting hypoglycemia (cannot convert G6P to free glucose)
  • Massive hepatomegaly (glycogen and fat accumulate in liver)
  • Renomegaly
  • Hyperlipidemia (triglycerides, cholesterol elevated)
  • Lactic acidosis (pyruvate/lactate accumulate)
  • Hyperuricemia (gout)
Treatment: Frequent feedings, uncooked cornstarch (slow-release glucose), avoidance of fasting.

(c) Glycogenesis (Glycogen Synthesis)

Glycogenesis is the process of glycogen synthesis from glucose, occurring mainly in liver and muscle after meals (promoted by insulin).
Key steps:
  1. Glucose → Glucose-6-phosphate (by hexokinase/glucokinase)
  2. G6P → Glucose-1-phosphate (by phosphoglucomutase)
  3. G1P + UTP → UDP-glucose + PPi (by UDP-glucose pyrophosphorylase)
  4. UDP-glucose is added to glycogen primer via α-1,4-glycosidic bonds by glycogen synthase (the key regulatory enzyme)
  5. Branching enzyme (α-1,4 → α-1,6 glucan transferase) creates branch points every ~8–12 residues
Regulation: Glycogen synthase is activated by insulin (dephosphorylation) and inhibited by glucagon/epinephrine (phosphorylation via cAMP-PKA).

(d) Digestion and Absorption of Carbohydrates

Digestion:
  • Mouth: Salivary α-amylase (ptyalin) hydrolyzes starch → dextrins + maltose (α-1,4 bonds)
  • Stomach: Amylase inactivated by acid; no significant carbohydrate digestion
  • Small intestine: Pancreatic α-amylase digests starch → maltose, maltotriose, limit dextrins
  • Brush border enzymes (intestinal microvilli):
    • Maltase → glucose + glucose
    • Sucrase → glucose + fructose
    • Lactase → glucose + galactose
    • Isomaltase/dextrinase → cleaves α-1,6 bonds
Absorption:
  • Glucose and galactose: absorbed via SGLT-1 (sodium-glucose co-transporter, active/secondary active transport) on apical membrane; exit via GLUT-2 on basolateral membrane
  • Fructose: absorbed via GLUT-5 (facilitated diffusion) on apical membrane; exits via GLUT-2
All monosaccharides enter the portal vein → liver.

(e) Significance of HMP Shunt (Pentose Phosphate Pathway)

The HMP shunt is an alternative pathway for glucose oxidation that does NOT produce ATP directly. Its primary products are NADPH and ribose-5-phosphate.
Significance:
  1. NADPH production — essential for:
    • Reductive biosynthesis (fatty acid synthesis, cholesterol synthesis, steroid synthesis)
    • Maintenance of reduced glutathione (GSH) in RBCs → protects against oxidative hemolysis
    • Respiratory burst in phagocytes (NADPH oxidase)
  2. Ribose-5-phosphate production — for nucleotide and nucleic acid (DNA/RNA) synthesis
  3. Erythrocyte protection — the only source of NADPH in RBCs; G6PD deficiency → reduced NADPH → hemolytic anemia upon oxidative stress
  4. Conversion of excess ribose-5-phosphate back to glycolytic intermediates (F6P, G3P) via transketolase/transaldolase reactions
  5. Active in tissues with high biosynthetic activity: liver, adrenal cortex, mammary gland, RBCs

Q.5) Explain in 3–4 Lines

A. Child suffering from Galactosemia develops cataract: In galactosemia (GALT deficiency), excess galactose cannot be metabolized normally. It is converted by aldose reductase to galactitol (a sugar alcohol). Galactitol accumulates in the lens because it cannot be further metabolized and is trapped there. This causes osmotic swelling of lens fibers, protein denaturation, and eventual opacification → cataract. (— Biochemistry, Lippincott Illustrated Reviews)
B. Cyanide is a potent poison: Cyanide binds irreversibly to the Fe³⁺ (ferric) form of cytochrome c oxidase (Complex IV) in the mitochondrial electron transport chain. This completely blocks electron transfer to oxygen, halting oxidative phosphorylation. ATP production ceases, leading to rapid cell death — especially in high-oxygen-demand tissues (heart, brain). This is why cyanide poisoning is rapidly fatal. (— Basic Medical Biochemistry, A Clinical Approach)
C. Fluoride bulb is used for collection of blood for sugar estimation: Sodium fluoride (NaF) is added to blood collection tubes used for glucose estimation. Fluoride acts as a glycolysis inhibitor by inhibiting the enzyme enolase (which converts 2-phosphoglycerate → phosphoenolpyruvate). This prevents RBCs from consuming glucose via glycolysis in vitro, thereby preserving the glucose concentration in the sample until it reaches the laboratory. Without fluoride, glucose falls ~10 mg/dl/hour due to ongoing glycolysis.
D. G6PD deficiency causes hemolysis of RBC: Glucose-6-phosphate dehydrogenase (G6PD) catalyzes the first step of the HMP shunt, producing NADPH. NADPH maintains reduced glutathione (GSH) via glutathione reductase. GSH protects RBC membranes and hemoglobin from oxidative damage. In G6PD deficiency, NADPH is lacking → GSH depleted → oxidative stress (from drugs, infections, fava beans) causes hemoglobin denaturation (Heinz bodies), membrane damage, and hemolytic anemia. (— Katzung's Basic and Clinical Pharmacology)
E. In Von Gierke's disease, the person suffers from fasting hypoglycemia: Von Gierke's disease is caused by deficiency of glucose-6-phosphatase, the enzyme that converts glucose-6-phosphate to free glucose in the liver. During fasting, both glycogenolysis and gluconeogenesis produce glucose-6-phosphate, but without G6Pase, G6P cannot be converted to free glucose for export into the blood. Glucose-6-phosphate accumulates, glycogen builds up in the liver (hepatomegaly), and blood glucose falls → severe fasting hypoglycemia. (— Goldman-Cecil Medicine)
Sources: Lippincott's Illustrated Reviews: Biochemistry; Basic Medical Biochemistry (Lieberman & Marks); Harper's Illustrated Biochemistry; Robbins & Kumar Basic Pathology; Goldman-Cecil Medicine; Katzung's Basic and Clinical Pharmacology
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