Biochemistry - MCQ Practice Questions

The Maillard reaction is a non-enzymatic browning reaction between carbonyl groups of reducing sugars and amino groups of proteins/amino acids, producing AGEs. This is significant in glycemic control and diabetes complications.

Aldose reductase catalyzes the reduction of glucose to sorbitol using NADPH. Sorbitol dehydrogenase catalyzes the oxidation of sorbitol to fructose in the second step of the polyol pathway.

Glycogen branching enzyme transfers segments of 6-7 glucose residues from the outer chains to create α(1→6) branch points, increasing solubility and accessibility for glycogen phosphorylase.

Pyruvate carboxylase catalyzes the first committed step of gluconeogenesis, converting pyruvate to oxaloacetate. Lactate is converted to pyruvate by LDH, then to glucose via gluconeogenesis.

The anomeric carbon (C1 in glucose) is the carbon that becomes chiral when the open-chain form cyclizes to form the hemiacetal. This is the site of α/β isomerism.

HbA1c is formed through non-enzymatic glycation of hemoglobin by glucose, reflecting average blood glucose over 2-3 months (RBC lifespan). Normal HbA1c <5.7%, diagnostic for diabetes ≥6.5%.

Cellulose contains β(1→4) glycosidic bonds linking glucose units, making it indigestible to humans (no cellulase enzyme). Starch contains α(1→4) bonds, which are digestible.

The oxidative phase of PPP generates 2 NADPH per glucose-6-phosphate, essential for fatty acid synthesis, cholesterol synthesis, and antioxidant defense (glutathione reduction). This is particularly important in adipose tissue, liver, and RBCs.

Type 2 diabetes is characterized by insulin resistance (defective glucose uptake and utilization) and progressive β-cell dysfunction. Impaired fasting glucose (100-125 mg/dL) is a prediabetic state. Fasting glucose ≥126 mg/dL on 2 occasions is diagnostic for diabetes.

The Cori cycle describes lactate produced in muscles during anaerobic glycolysis being transported to the liver where it is converted back to glucose via gluconeogenesis, which is then returned to muscles.

Ethanol metabolism consumes NAD+, reducing the NAD+/NADH ratio. This inhibits NAD+-dependent dehydrogenases including those in gluconeogenesis and lactate oxidation, leading to lactate accumulation.

Glucuronic acid, derived from glucose, is a key component of glycosaminoglycans like heparin, hyaluronic acid, and chondroitin sulfate, providing their acidic properties.

Through the non-oxidative phase of the pentose phosphate pathway, sugars are rearranged to produce fructose-6-phosphate and glyceraldehyde-3-phosphate, which directly enter glycolysis.

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.