MLT GlobeChemistry: Carbohydrates & diabetes – page 4
121 Chemistry MCQs on Carbohydrates & diabetes with answers and explanations.
Fluoride tubes preserve blood glucose by inhibiting which glycolytic enzyme that converts 2-phosphoglycerate to phosphoenolpyruvate?
Fluoride with Mg2+ and phosphate forms a complex that blocks enolase, stopping glycolysis in collected blood. Hexokinase acts at the first step and is not the fluoride target.
Salivary and pancreatic α-amylase hydrolyse which bonds in dietary starch?
α-Amylase cleaves internal α-1,4 links, producing maltose, maltotriose and α-limit dextrins. The α-1,6 branch bonds are removed by brush-border isomaltase; β-1,4 bonds (cellulose) are not digested.
Which sugar is the C-2 epimer of glucose?
Mannose differs from glucose only at C-2; galactose is the C-4 epimer. Fructose is a ketose isomer, not an epimer, and ribose is a pentose.
Which statement about glycolysis is correct?
Hexokinase, PFK-1 and pyruvate kinase are both irreversible and regulated. Fructose-2,6-bisphosphate activates PFK-1, and NADH made is reoxidised by lactate dehydrogenase, so no net NADH remains.
Which liver enzyme's synthesis is increased by insulin in the fed state?
Insulin induces hepatic glucokinase to trap incoming glucose. It represses the gluconeogenic enzymes glucose-6-phosphatase, PEPCK and fructose-1,6-bisphosphatase. Hexokinase in other tissues is not insulin-induced.
A young adult has exercise-induced cramps and myoglobinuria, and blood lactate does not rise during forearm exercise. Which enzyme is deficient?
McArdle disease (GSD V) affects skeletal muscle only: muscle cannot break down glycogen, so no lactate is produced. Glucose-6-phosphatase deficiency is von Gierke disease; acid α-glucosidase deficiency is Pompe disease.
Making one glucose from two lactate molecules in the liver consumes how many high-energy phosphate bonds?
Pyruvate carboxylase uses 2 ATP, PEP carboxykinase 2 GTP, and phosphoglycerate kinase 2 ATP, totalling 6. The Cori cycle therefore costs the liver more than the 2 ATP muscle gains.
Using current P/O ratios, complete aerobic oxidation of one glucose molecule yields approximately:
Current estimates (2.5 ATP per NADH, 1.5 per FADH2) give about 30–32 ATP. 38 is the older textbook figure; 2 ATP is anaerobic glycolysis.
Whether pyruvate is converted to lactate or enters the mitochondria depends mainly on which factor?
Glycolysis needs NAD+. With oxygen, NADH is reoxidised by the electron transport chain. Without it, lactate dehydrogenase regenerates NAD+ and forms lactate.
Which compound cannot give a net synthesis of glucose in humans?
Pyruvate dehydrogenase is irreversible, and the two carbons of acetyl-CoA are lost as CO2 in the TCA cycle. Lactate, glycerol and alanine are gluconeogenic substrates.
Which enzyme works without a phosphorylated vitamin coenzyme?
Glucose-6-phosphatase is a simple hydrolase. Pyruvate dehydrogenase and transketolase need thiamine pyrophosphate, and aminotransferases need pyridoxal phosphate.
In the liver, which regulator activates PFK-1 and inhibits fructose-1,6-bisphosphatase, so glycolysis and gluconeogenesis do not run at the same time?
Insulin raises and glucagon lowers fructose-2,6-bisphosphate, switching the liver between glycolysis and gluconeogenesis. Acetyl-CoA activates pyruvate carboxylase instead.
Which molecule acts as a coenzyme, carrying acyl groups in the pyruvate dehydrogenase complex?
Lipoic acid, bound to E2 as lipoamide, carries the acetyl group and electrons between subunits. Glucose-6-phosphate and glucose-1-phosphate are metabolic intermediates, not coenzymes.
Which class of antidiabetic drug lowers circulating free fatty acids by activating PPAR-gamma in adipose tissue?
Glitazones increase fatty acid uptake and storage in adipocytes and improve insulin sensitivity. Glucocorticoids and beta-agonists increase lipolysis and raise FFA.
Sodium fluoride in the grey-top tube preserves glucose by:
Cells keep using glucose after collection, lowering it by about 5-7% per hour at room temperature. Fluoride inhibits enolase and slows this; potassium oxalate in the same tube is the anticoagulant.
A glucose oxidase–peroxidase (Trinder) method gives falsely low results on a sample from a patient receiving high-dose vitamin C. Why?
Reducing substances such as ascorbic acid and uric acid consume hydrogen peroxide, so less coloured dye forms. The glucose oxidase step itself is not inhibited.
A 48-year-old has a fasting plasma glucose of 112 mg/dL (6.2 mmol/L) and no symptoms. By ADA criteria this is:
Fasting glucose of 100–125 mg/dL (5.6–6.9 mmol/L) is impaired fasting glucose. Impaired glucose tolerance is defined by the 2-hour OGTT value, not the fasting value.
An adult has an HbA1c of 6.1% (43 mmol/mol). According to ADA, this result indicates:
HbA1c of 5.7–6.4% (39–47 mmol/mol) is the ADA prediabetes range. Diabetes needs 6.5% (48 mmol/mol) or higher.
In the two-step approach to gestational diabetes screening, the first step is:
The first step is a 50 g glucose challenge without fasting, with plasma glucose at 1 hour. Women above the cut-off then have the 100 g 3-hour OGTT.
A diabetic patient with autoimmune hemolytic anemia has an HbA1c of 5.2% while home glucose readings are often above 200 mg/dL. Best explanation?
Hemolysis leaves a younger red cell population with less time to be glycated, so HbA1c is falsely low. Iron deficiency tends to raise HbA1c, not lower it.