Q-111. The activity of following enzyme is affected by biotin deficiency
a) Trans-ketolase
b) Oxidase
c) Carboxylase
d) Dehydrogenase
Answer: Carboxylase
Explanation:
Biotin is a water-soluble B-vitamin (vitamin B7). Biotin is important in fatty acid synthesis, branched-chain amino acid catabolism, and gluconeogenesis.
Biotin deficiency is rare because, in general, intestinal bacteria produce biotin in excess of the body’s daily requirements.
Biotin is a cofactor responsible for carbon dioxide transfer in several carboxylase enzymes:
Acetyl-Co-A carboxylase
Propionyl-CoA carboxylase
Pyruvate carboxylase
Q-112. Acetyl Co-A acts as a substrate for all the enzymes except:
a) HMG-Co A synthetase
b) Malic enzyme
c) Malonyl Co A synthetase
d) Fatty acid synthetase
Answer: Malic enzyme
Explanation:
Coenzyme-A is a coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle.
Acetyl Co-A acts as a substrate for the enzymes:
HMG-Co A synthetase
Malonyl Co A synthetase
Fatty acid synthetase
Q-113. At the physiological pH the DNA molecules are:
a) Positively charged
b) Negatively charged
c) Neutral
d) Amphipathic
Answer: Negatively charged
Explanation:
For all forms of DNA the global structure has an overall negative charge and the overall electrostatic potential is negative.
This is largely due to the phosphate groups of DNA which bear an overall negative charge due to the negatively charged oxygen atoms they have.
However, DNA has both positive and negative charges covering its outer structure. The charge dispersion over the global structure of DNA varies between A-DNA, B-DNA, and Z-DNA.
B-DNA has mostly negative electrostatic potential spread throughout its global exterior and concentrated mostly in major and minor grooves.
Q-114. Cholesterol present in LDL:
a) Represents primarily cholesterol that is being removed from peripheral cells.
b) Binds to a receptor and diffuses across the cell membrane.
c) On accumulation in the cell inhibits replenishment of LDL receptors.
d) When enters a cell, suppresses activity of acyl-CoA: cholesterol acyl-transferase (ACAT).
Answer: On accumulation in the cell inhibits replenishment of LDL receptors.
Explanation:
The mechanism of suppression of cholesterol biosynthesis by LDL-bound cholesterol involves specific LDL receptors that project from the surface of human cells.
The first step of the regulatory mechanism involves the binding of the lipoprotein LDL to these LDL receptors, thereby extracting the LDL particles from the blood.
Accumulation of intracellular cholesterol eventually inhibits the replenishment of LDL receptors on the cell surface, a phenomenon called down regulation, thereby blocking further uptake and accumulation of cholesterol.
Q-115. A newborn infant refuses breast milk since the 2nd day of birth, vomits on force-feeding but accepts glucose-water, develops diarrhea on the third day, by 5th day she is jaundiced with liver enlargement and eyes show signs of cataract. Urinary reducing sugar was positive but blood glucose estimated by glucose oxidation method was found low. The most likely cause is deficiency of:
a) Galactose-1-phosphate uridyl transferase
b) Beta galactosidase
c) Glucose-6-phosphatase
d) Galactokinase
Answer: Galactose-1-phosphate uridyl transferase
Explanation:
Galactose 1 phosphate uridyl-transferase is missing in individuals with classical Galactosemia.
Galactose 1 phosphate and therefore Galactose accumulate in cells.
Accumulated Galactose is shunted into side-pathways such as that of Galactitol production.
This reaction is catalyzed by aldose reductase, same enzyme that converts glucose to sorbitol.
More benign form of Galactosemia is caused by a deficiency of galactokinase.
Classical Galactosemia:
Uridyl-transferase deficiency
Autosomal recessive disorder
It causes Galactosemia and galactosuria, vomiting, diarrhoea and jaundice.
Accumulation of Galactose 1 phosphate and Galactitol in nerve, lens, liver and kidney tissue causes liver damage, severe mental retardation and cataracts.
Antenatal diagnosis is possible by chorionic villus sampling.
Therapy: Rapid removal Galactose i.e. lactose from the diet.
Q-116. Replication and transcription are similar processes in mechanistic terms because both
a) Use RNA primers for initiation
b) Use deoxy-ribo-nucleotides as precursors
c) Are semi conserved events
d) Involve phosphodiester bond formation with elongation occurring in the 5′ – 3′ direction
Answer: Involve phosphodiester bond formation with elongation occurring in the 5′ – 3′ direction
Explanation:
Q-117. A buffer that is most effective at a pH of about 4.5 is
a) Acetate buffer
b) Bicarbonate buffer
c) Phosphate buffer
d) Tris buffer
Answer: Acetate buffer
Explanation:
The solution resists changes in pH most effectively at pH values close to the pKa.
A solution of a weak acid and its conjugate base, buffers most effectively in the pH range pKa ± 1.0 pH unit.
Acid———–pK
Acetic———pK = 4.76
Carbonic—–pK1=6.37and pK2=10.25
Phosphoric–pK1= 2.15, pK2=6.82 and pK3=12.38
Q-118. NADPH is produced by:
a) Glycolysis
b) Citric acid cycle
c) HMP Shunt
d) Glycogenesis
Answer: HMP Shunt
Explanation:
HMP Shunt provides a major portion of the body’s NADPH which functions as a biochemical reductant.
Uses of NADPH:
Reductive biosynthesis- Synthesis of steroids and fatty acids
Reduction of hydrogen per-oxide
Cytochrome P450 mono-oxygenase system
Phagocytosis by WBCs
Synthesis of nitric oxide
Q-119. Mitochondria are involved in all of the following except:
a) ATP production
b) Apoptosis
c) Tri-carboxylic acid cycle
d) Fatty acid biosynthesis
Answer: Fatty acid biosynthesis
Explanation:
The main pathway for de novo synthesis of fatty acid occurs in the cytosol.
The biochemical processes taking place in mitochondria:
TCA or Kreb’s cycle and fatty acid oxidation for ATP production
Formation of acetyl Co-A
Part of urea cycle
Part of gluconeogenesis
Q-120. The conjugation of bilirubin to glucuronic acid in the liver:
a) Converts a hydrophilic compound to a hydrophobic one
b) Converts a hydrophobic compound to a hydrophilic one
c) Enables the bilirubin molecule to cross the cell membrane
d) Is increased during neonatal jaundice
Answer: Converts a hydrophobic compound to a hydrophilic one
Explanation:
Bilirubin is a tetra-pyrrole created by the normal breakdown of heme.
Most bilirubin is produced during the breakdown of hemoglobin and other hemo-proteins.
Because bilirubin is highly insoluble in water, it must be converted into a soluble conjugate before elimination from the body.
In the liver, uridine di-phosphate (UDP)-glucuronyl transferase converts bilirubin to a mixture of mono-glucuronides and di-glucuronides, referred to as conjugated bilirubin, which is then secreted into the bile by an ATP-dependent transporter.
The kidneys do not filter un-conjugated bilirubin because of its avid binding to albumin. For this reason, the presence of bilirubin in the urine indicates the presence of conjugated hyper-bilirubinemia.