MBBS Biochemistry 1 — Questions and Answers
Question 1: Which enzyme is deficient in Phenylketonuria (PKU)?
- Phenylalanine transaminase
- Phenylalanine hydroxylase (Correct answer)
- Tyrosine aminotransferase
- Homogentisate oxidase
Correct answer: Phenylalanine hydroxylase
PKU is caused by deficiency of phenylalanine hydroxylase, which converts phenylalanine to tyrosine, leading to phenylalanine accumulation and intellectual disability.
Question 2: In which organelle does the citric acid (Krebs) cycle occur?
- Cytoplasm
- Smooth endoplasmic reticulum
- Mitochondrial matrix (Correct answer)
- Nucleus
Correct answer: Mitochondrial matrix
The citric acid cycle occurs in the mitochondrial matrix, where acetyl-CoA is oxidized to CO2 with generation of NADH, FADH2, and GTP.
Question 3: Hemoglobin S (sickle cell) differs from hemoglobin A by a substitution of which amino acid?
- Valine replaces glutamate at position 6 of the beta chain (Correct answer)
- Glutamate replaces valine at position 6 of the beta chain
- Lysine replaces glutamate at position 6 of the alpha chain
- Serine replaces threonine at position 6 of the beta chain
Correct answer: Valine replaces glutamate at position 6 of the beta chain
Sickle cell disease results from a single point mutation causing valine to replace glutamic acid at position 6 of the beta-globin chain.
Question 4: Which vitamin serves as a coenzyme for pyruvate dehydrogenase?
- Riboflavin (B2)
- Thiamine (B1) (Correct answer)
- Niacin (B3)
- Biotin (B7)
Correct answer: Thiamine (B1)
Thiamine pyrophosphate (TPP) is a required cofactor for pyruvate dehydrogenase; its deficiency impairs aerobic metabolism, causing Wernicke's encephalopathy.
Question 5: DNA replication proceeds in which direction?
- 3' to 5' only
- 5' to 3' only
- Both 3' to 5' and 5' to 3' simultaneously
- Bidirectional from multiple origins but always 5' to 3' (Correct answer)
Correct answer: Bidirectional from multiple origins but always 5' to 3'
DNA replication is bidirectional from origins of replication, but each new strand is synthesized only in the 5' to 3' direction by DNA polymerase.
Question 6: Which enzyme catalyzes the rate-limiting step of glycolysis?
- Hexokinase
- Phosphofructokinase-1 (PFK-1) (Correct answer)
- Pyruvate kinase
- Phosphoglucose isomerase
Correct answer: Phosphofructokinase-1 (PFK-1)
PFK-1 is the rate-limiting enzyme of glycolysis, catalyzing the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, and is regulated by ATP, AMP, and citrate.
MBBS Biochemistry 1
Which enzyme is deficient in Phenylketonuria (PKU)?