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Model Answers
B.Sc. (Hons.) (Second Semester)
Biotechnology (Biomolecules II)
Subject Code: AR-7858
Objective type questions (all compulsory)
i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
ix.
x.
Formation of glucose from non-carbohydrate source called
(b) Gluconeogenesis
Sucrose is a
(b) Disaccharide
How many molecules of ATP are hydrolysed to form two molecule of ammonia?
(d) 16
The nitrogen atoms of urea produced in the urea cycle are derived from
(a) Ammonia and aspartic acid
Which of the following supplies the 2 corbon units that are added to the elongation of fatty acid
chain?
(b) Malonyl co A
Conversion of Acetyl co A to Malonyl co A requires which of the following
(a) Biotin
FAD is reduced to FADH2 during
(c) Krebs cycle
A biological redox reaction always involves
(a) An oxidized agent, a gain of electron, a reducing agent
Which statement best describes Xanthine
(b) It is oxidized to form Uric acid
Methylated hetrocyclic bases of plants includes all except
(b) Thymine
Answer-2, TCA Cycle
Ans: 3
Nitrogen metabolism in brief:
It is the polymeric nitrogen containing compounds proteins and nucleic acids that define the
major attributes of organism such as function and structure. Operation and mechanism of
metabolic pathways is provided by proteins. Genetic information is stored in nucleic acid
polymers. Each of the monomer of these macromolecules has an individual metabolic pathway.
In addition, the monomeric nucleotides are essential for energy turnover as key intermediates in
all metabolic pathways and also as second messenger molecules, often in form of cyclic
nucleotides.
Amino acids contribute to carbohydrate synthesis via gluconeogenesis, to fat synthesis or energy
production via acetyl-CoA, and special nitrogen compounds such as catecholamines
(neurotransmitters), thyroid hormones, creatine(-phosphate), the protoporphyrin ring (heme), and
contribute to nucleic acid and phospholipid synthesis as nitrogen group donor.
The majority of useful nitrogen for animal metabolism comes from proteins in the form of
reusable ammonia (NH3). Nitrogen is fixated in form of ammonia by microorganisms and all
'higher' forms of life (eukaryotes) depend on this primordial source of nitrogen extracted from
the air. The 'usefulness' of proteins depends on four distinct properties:
1. total amount of protein ingested
2. digestibility of proteins
3. amino acid composition of proteins
4. total caloric intake
Removal of Nitrogen from Amino Acids
The dominant reactions involved in removing amino acid nitrogen from the body are known as
transaminations. This class of reactions funnels nitrogen from all free amino acids into a small
number of compounds; then, either they are oxidatively deaminated, producing ammonia, or
their amine groups are converted to urea by the urea cycle. Transaminations involve moving an
α-amino group from a donor α-amino acid to the keto carbon of an acceptor α-keto acid. These
reversible reactions are catalyzed by a group of intracellular enzymes known as
aminotransferases, which generally employ covalently bound pyridoxal phosphate as a cofactor.
However, some aminotransferases employ pyruvate as a cofactor.
Aminotransferases exist for all amino acids except threonine and lysine. The most common
compounds involved as a donor/acceptor pair in transamination reactions are glutamate and αKG, which participate in reactions with many different aminotransferases. Serum
aminotransferases such as aspartate aminotransferase, AST (also called serum glutamateoxaloacetate-aminotransferase, SGOT) and alanine transaminase, ALT (also called serum
glutamate-pyruvate aminotransferase (SGPT) have been used as clinical markers of tissue
damage, with increasing serum levels indicating an increased extent of damage (see the Enzyme
Kinetics page for description of the use of enzyme levels in diagnosis). Alanine transaminase has
an important function in the delivery of skeletal muscle carbon and nitrogen (in the form of
alanine) to the liver. In skeletal muscle, pyruvate is transaminated to alanine, thus affording an
additional route of nitrogen transport from muscle to liver. In the liver, alanine transaminase
transfers the ammonia to α-KG and regenerates pyruvate. The pyruvate can then be diverted into
gluconeogenesis. This process is referred to as the glucose-alanine cycle (see above).
The ammonia produced in the latter two reactions is excreted as NH4+ in the urine, where it helps
maintain urine pH in the normal range of pH4 to pH8. The extensive production of ammonia by
peripheral tissue or hepatic glutamate dehydrogenase is not feasible because of the highly toxic
effects of circulating ammonia. Normal serum ammonium concentrations are in the range of 20–
40μM, and an increase in circulating ammonia to about 400μM causes alkalosis and
neurotoxicity. A final, therapeutically useful amino acid-related reaction is the amidation of
aspartic acid to produce asparagine. The enzyme asparagine synthase catalyzes the ATPrequiring transamidation reaction shown below:
Most cells perform this reaction well enough to produce all the asparagine they need. However,
some leukemia cells require exogenous asparagine, which they obtain from the plasma.
Chemotherapy using the enzyme asparaginase takes advantage of this property of leukemic cells
by hydrolyzing serum asparagine to ammonia and aspartic acid, thus depriving the neoplastic
cells of the asparagine that is essential for their characteristic rapid growth.
In the peroxisomes of mammalian tissues, especially liver, there exists a minor enzymatic
pathway for the removal of amino groups from amino acids. L-amino acid oxidase is FMNlinked and has broad specificity for the L-amino acids.
A number of substances, including oxygen, can act as electron acceptors from the flavoproteins.
If oxygen is the acceptor the product is hydrogen peroxide, which is then rapidly degraded by the
catalases found in liver and other tissues.
Missing or defective biogenesis of peroxisomes or L-amino acid oxidase causes generalized
hyperaminoacidemia and hyperaminoaciduria, generally leading to neurotoxicity and early death.
Answer: 4
The Urea Cycle
Earlier it was noted that kidney glutaminase was responsible for converting excess glutamine
from the liver to urine ammonium. However, about 80% of the excreted nitrogen is in the form
of urea which is produced exclusively in the liver, in a series of reactions that are distributed
between the mitochondrial matrix and the cytosol. The series of reactions that form urea is
known as the Urea Cycle or the Krebs-Henseleit Cycle.
Diagram of the urea cycle. The reactions of the urea cycle which occur in the mitochondrion are
contained in the red rectangle. All enzymes are in red, CPS-I is carbamoyl phosphate synthetaseI, OTC is ornithine transcarbamoylase. Click on the enzyme names to go to a descriptive page of
the urea cycle disorder caused by deficiency in the particular enzyme.
The essential features of the urea cycle reactions and their metabolic regulation are as follows:
Arginine from the diet or from protein breakdown is cleaved by the cytosolic enzyme arginase,
generating urea and ornithine. In subsequent reactions of the urea cycle a new urea residue is
built on the ornithine, regenerating arginine and perpetuating the cycle.
Ornithine, arising in the cytosol, is transported to the mitochondrial matrix via the action of
ornithine translocase encoded by the ORNT1 gene. The ORNT1 transporter is a member of the
solute carrier family of transporters and as such is also identified as SLC25A15. In the
mitochondria ornithine transcabamoylase catalyzes the condensation of ornithine with carbamoyl
phosphate, producing citrulline. The energy for the reaction is provided by the high-energy
anhydride of carbamoyl phosphate. Concomitant with ornithine transport into the mitochondria is
the export of citrulline, by SLC25A15, to the cytosol where the remaining reactions of the cycle
take place. Also important in the function of the urea cycle is the mitochodrial transporter called
citrin. Citrin is involved in the mitochondrial uptake of glutamate and export of aspartate and as
such functions in the malate-aspartate shuttle. Citrin is a Ca2+-dependent mitochondrial solute
transporter that is also a member of the solute carrier family of transporters identified as
SLC25A13.
The synthesis of citrulline requires a prior activation of carbon and nitrogen as carbamoyl
phosphate (CP). The activation step requires 2 equivalents of ATP and the mitochondrial matrix
enzyme carbamoyl phosphate synthetase-I (CPS-I). There are two CP synthetases: a
mitochondrial enzyme, CPS-I, which forms CP destined for inclusion in the urea cycle, and a
cytosolic CP synthetase (CPS-II), which is involved in pyrimidine nucleotide biosynthesis. CPSI is positively regulated by the allosteric effector N-acetylglutamate, while the cytosolic enzyme
is acetylglutamate independent.
In a 2-step reaction, catalyzed by cytosolic argininosuccinate synthetase, citrulline and aspartate
are condensed to form argininosuccinate. The reaction involves the addition of AMP (from ATP)
to the amido carbonyl of citrulline, forming an activated intermediate on the enzyme surface
(AMP-citrulline), and the subsequent addition of aspartate to form argininosuccinate.
Arginine and fumarate are produced from argininosuccinate by the cytosolic enzyme
argininosuccinate lyase (also called argininosuccinase). In the final step of the cycle arginase
cleaves urea from arginine, regenerating cytosolic ornithine, which can be transported to the
mitochondrial matrix for another round of urea synthesis. The fumarate, generated via the action
of arginiosuccinate lyase, is reconverted to aspartate for use in the argininosuccinate synthetase
reaction. This occurs through the actions of cytosolic versions of the TCA cycle enzymes,
fumarase (which yields malate) and malate dehydrogenase (which yields oxaloacetate). The
oxaloacetate is then transaminated to aspartate by AST.
Beginning and ending with ornithine, the reactions of the cycle consumes 3 equivalents of ATP
and a total of 4 high-energy nucleotide phosphates. Urea is the only new compound generated by
the cycle; all other intermediates and reactants are recycled. The energy consumed in the
production of urea is more than recovered by the release of energy formed during the synthesis
of the urea cycle intermediates. Ammonia released during the glutamate dehydrogenase reaction
is coupled to the formation of NADH. In addition, when fumarate is converted back to aspartate,
the malate dehydrogenase reaction used to convert malate to oxaloacetate generates a mole of
NADH. These two moles of NADH, thus, are oxidized in the mitochondria yielding 6 moles of
ATP.
Answer 5
Essential fatty acids, or EFAs, are fatty acids that humans and other animals must ingest because the
body requires them for good health but cannot synthesize them.[1] The term "essential fatty acid" refers
to fatty acids required for biological processes but does not include the fats that only act as fuel.
Only two fatty acids are known to be essential for humans: alpha-linolenic acid (an omega-3 fatty acid)
and linoleic acid (an omega-6 fatty acid).[2][3] Some other fatty acids are sometimes classified as
"conditionally essential," meaning that they can become essential under some developmental or disease
conditions; examples include docosahexaenoic acid (an omega-3 fatty acid) and gamma-linolenic acid
(an omega-6 fatty acid).
Answer: 6
A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane
rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex
hormones estradiol and testosterone and the anti-inflammatory drug dexamethasone.
The core of steroids is composed of twenty carbon atoms bonded together that take the form of four
fused rings: three cyclohexane rings (designated as rings A, B and C in the figure to the right) and one
cyclopentane ring (the D ring). The steroids vary by the functional groups attached to this four-ring core
and by the oxidation state of the rings. Sterols are special forms of steroids, with a hydroxyl group at
position-3 and a skeleton derived from cholestane.
Hundreds of distinct steroids are found in plants, animals and fungi. All steroids are made in cells either
from the sterols lanosterol (animals and fungi) or from cycloartenol (plants). Both lanosterol and
cycloartenol are derived from the cyclization of the triterpene squalene
Isoprene, or 2-methyl-1,3-butadiene, is a common organic compound with the formula
CH2=C(CH3)CH=CH2. It is a colorless volatile liquid. Isoprene is produced by many plants.
Answer: 7
Substrate-level phosphorylation is a type of metabolic reaction that results in the formation of
adenosine triphosphate (ATP) or guanosine triphosphate (GTP) by the direct transfer and donation of a
phosphoryl (PO3) group to adenosine diphosphate (ADP) or guanosine diphosphate (GDP) from a
phosphorylated reactive intermediate. Note that the phosphate group does not have to come directly
from the substrate. By convention, the phosphoryl group that is transferred is referred to as a
phosphate group.
The main part of substrate-level phosphorylation occurs in the cytoplasm of cells as part of glycolysis
and in mitochondria as part of the Krebs Cycle under both aerobic and anaerobic conditions. In the payoff phase of glycolysis, two ATP are produced by substrate-level phosphorylation: two and only two 1,3bisphosphoglycerate are converted to 3-phosphoglycerate by transferring a phosphate group to ADP by
a kinase; two phosphoenolpyruvate are converted to pyruvate by the transfer of their phosphate groups
to ADP by another kinase.
2 molecules of 1,3-bisphoshoglycerate (C3H4O4P2) + phosphoglycerokinase + 2 ADP → 2 molecules of 3phosphoglycerate (C3H5O4P1) + 2 ATP
Answer: 8
Pyrimidine Nucleotide Biosynthesis
Synthesis of the pyrimidines is less complex than that of the purines, since the base is much simpler. The
first completed base is derived from 1 mole of glutamine, one mole of ATP and one mole of CO 2 (which
form carbamoyl phosphate) and one mole of aspartate. An additional mole of glutamine and ATP are
required in the conversion of UTP to CTP. The pathway of pyrimidine biosynthesis is diagrammed below.
The carbamoyl phosphate used for pyrimidine nucleotide synthesis is derived from glutamine and
bicarbonate, within the cytosol, as opposed to the urea cycle carbamoyl phosphate derived from
ammonia and bicarbonate in the mitochondrion. The urea cycle reaction is catalyzed by carbamoyl
phosphate synthetase I (CPS-I) whereas the pyrimidine nucleotide precursor is synthesized by CPS-II.
Carbamoyl phosphate is then condensed with aspartate in a reaction catalyzed by the rate limiting
enzyme of pyrimidine nucleotide biosynthesis, aspartate transcarbamoylase (ATCase).
Enzyme names:
1. aspartate transcarbamoylase, ATCase
2. carbamoyl aspartate dehydratase
3. dihydroorotate dehydrogenase
4. orotate phosphoribosyltransferase
5. orotidine-5'-phosphate carboxylase