Biochemistry - MCQ Practice Questions

Essential fructosuria results from fructokinase deficiency. It is benign because fructose is simply excreted in urine without accumulating in tissues or causing metabolic harm.

Phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme of glycolysis. It is inhibited by ATP and citrate (signals of sufficient energy) and activated by AMP and ADP (signals of energy depletion).

GLUT4 is the insulin-responsive glucose transporter present in skeletal muscle and adipose tissue. Insulin signaling causes GLUT4 translocation to the cell membrane, increasing glucose uptake.

After liver glycogen is depleted (8-12 hours), gluconeogenesis becomes the primary source of glucose. The substrates are amino acids (from muscle proteolysis) and glycerol (from lipolysis).

Transketolase requires thiamine pyrophosphate (TPP/vitamin B1) as a cofactor to transfer 2-carbon ketol groups between sugar phosphates in the non-oxidative phase of the pentose phosphate pathway.

PFK-1 catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate and is the rate-limiting enzyme of glycolysis. It is negatively regulated by ATP and citrate, making it a key control point.

The Cori cycle involves lactate produced in muscles being transported to the liver where it undergoes gluconeogenesis to form glucose, which is sent back to muscles. This is crucial during anaerobic exercise.

During the fight-or-flight response, epinephrine activates adenylyl cyclase leading to increased cAMP and PKA activation. PKA phosphorylates and activates glycogen phosphorylase, promoting glycogen breakdown for rapid glucose availability.

Both phosphoglycerate kinase (1,3-bisphosphoglycerate → 3-phosphoglycerate) and pyruvate kinase (phosphoenolpyruvate → pyruvate) catalyze substrate-level phosphorylation in glycolysis, directly generating ATP.

GLUT4 is the insulin-dependent glucose transporter found primarily in skeletal muscle and adipose tissue. Impaired GLUT4 translocation or function is a hallmark of type 2 diabetes, leading to reduced muscle glucose uptake.

The branching enzyme (amylo-1,6-transglucosidase) transfers segments of α-1,4-linked glucose to create α-1,6-branch points, making glycogen more soluble and accessible. Its deficiency causes Andersen disease with abnormal glycogen accumulation.

2,3-BPG is synthesized from 1,3-bisphosphoglycerate by bisphosphoglycerate mutase. 2,3-BPG binds to hemoglobin, decreasing its oxygen affinity, facilitating oxygen release to tissues. This shunt allows RBCs to regulate oxygen delivery without producing ATP.

During prolonged fasting, ketone bodies produced from hepatic fatty acid oxidation become the preferred fuel for the brain (up to 60% of energy needs), reducing the requirement for gluconeogenesis and preserving muscle protein.

Hexokinase is inhibited by its product glucose-6-phosphate, making it rate-limiting. Glucokinase, found in liver, is NOT inhibited by G6P, allowing continuous glucose phosphorylation in fed state. This differential regulation is crucial for glucose sensing.

McArdle disease involves glycogen phosphorylase deficiency in muscle, preventing glycogen breakdown during exercise. This causes exercise intolerance with muscle pain, cramps, fatigue, and myoglobinuria. The 'second wind' phenomenon (improved symptoms after 10 minutes as blood glucose increases) is characteristic.

NADPH from the pentose phosphate pathway serves as the primary reducing agent for biosynthetic reactions, especially fatty acid and cholesterol synthesis. It is also crucial for antioxidant defense (glutathione reduction) and biosynthesis of nucleotides.

Glycogen's highly branched structure with α-1,6-branch points every 8-12 glucose residues increases its solubility, provides multiple substrate sites for glycogen phosphorylase simultaneously, and enables rapid glucose mobilization during energy demands. This is metabolically superior to linear polymers.

Acetyl-CoA condenses with oxaloacetate to form citrate, which enters the citric acid cycle. Citrate synthase catalyzes this reaction.

During exercise, epinephrine increases and ATP/AMP ratio decreases, both of which activate phosphorylase kinase and promote glycogenolysis.

Under anaerobic conditions, pyruvate is reduced to lactate by lactate dehydrogenase (LDH) to regenerate NAD+ for continued glycolysis.