MDS PREP
Rate limiting step in cholesterol synthesis is
1. HMG CoA synthetase
2. HMG CoA lyase
3. HMG CoA reductase
4. Mevalonate synthetase
Biochemistry
Answer: 3
The rate limiting step in cholesterol synthesis is HMG CoA reductase. Here's
a detailed explanation:
Cholesterol synthesis is a complex process that involves multiple enzymatic
steps. This process begins with the condensation of acetyl-CoA molecules to form
acetoacetyl-CoA, which is then converted into HMG CoA
(3-hydroxy-3-methylglutaryl-CoA) by the enzyme HMG CoA synthetase. HMG CoA is
further converted to mevalonate by the action of HMG CoA reductase. This
reaction is the rate limiting step of the cholesterol synthesis pathway. The
rate limiting step is the slowest step in a metabolic pathway and is responsible
for controlling the overall rate of the process.
HMG CoA reductase is a critical regulatory enzyme that is tightly controlled
because it is the first committed step in the synthesis of cholesterol from
acetate. This enzyme is responsible for reducing HMG CoA to mevalonate, which is
the precursor of all isoprenoids, including cholesterol, steroids, and other
important biological molecules. The rate limiting nature of this step is due to
the fact that HMG CoA reductase is subject to both allosteric regulation and
feedback inhibition.
Allosteric regulation involves the binding of regulatory molecules, such as ATP,
citrate, and NADH, which can either activate or inhibit the enzyme. For example,
when cellular ATP levels are high, the enzyme is inhibited, which reduces
cholesterol synthesis. Conversely, when ATP levels are low, the enzyme is
activated, leading to increased cholesterol production. Citrate, a molecule
derived from the citric acid cycle, inhibits HMG CoA reductase when it builds up
in the cytosol, indicating that the cell has enough energy and does not need to
synthesize additional cholesterol.
Feedback inhibition occurs when the end product of the pathway, cholesterol,
binds to the enzyme and reduces its activity. This is a form of negative
feedback regulation that helps to maintain homeostasis of cholesterol levels
within the cell. When cellular cholesterol levels are high, the enzyme is
inhibited, which slows down the synthesis of new cholesterol molecules.
Conversely, when cholesterol levels are low, the enzyme is less inhibited, and
the synthesis rate increases.
The other enzymes listed, HMG CoA synthetase and mevalonate synthetase, are
involved in the synthesis of HMG CoA and the subsequent transformation of
mevalonate, but they are not the rate limiting steps. HMG CoA lyase, on the
other hand, is part of an alternative pathway that breaks down HMG CoA into
acetyl-CoA and acetoacetate. This enzyme is not directly involved in the rate
limiting step of cholesterol synthesis.
A vitamin B12 deficiency may affect heme synthesis by reducing the concentration of which of the following? Choose the one best answer.
1) Acetyl-CoA
2) Succinyl-CoA
3) Glycine
4) Alanine
Biochemistry Answer: 2
Vitamin B12 participates in two reactions in the body-conversion of homocysteine to methionine and conversion ofmethylmalonyl-CoA to succinyl-CoA.
Methylmalonyl-CoA is produced via various amino acid degradation pathways, and from oddcarbon chain fatty acid oxidation. In the absence of B12, succinyl-CoA would only be produced as an intermediate of the TCA cycle, and if it were removed from the cycle for heme synthesis, energy production may suffer.
Glycine is usually obtained from the diet, although in a B12 deficiency a functional folate deficiency may also develop, leading to an inhibition of serine hydroxymethyltransferase, the enzyme that converts serine to glycine, and requires free tetrahydrofolate.
Succinyl-CoA and glycine are the precursors for heme synthesis. A B12 deficiency would not inhibit the production of acetyl-CoA, succinate, or alanine.
The essential. sulphur containing amino acid is:
1. Methionine 2. Cystetne
3. Cystine 4. Valine
Biochemistry
Answer: 1
Methionine is sulphur containing AminoAcid
An alpha helix of a protein is most likely to be disrupted if a missense mutation introduces the following armino acid within the alpha helical structure:
1. Alanine.
2. Aspartic acid.
3. Tyrosine.
4. Glycine.
Glycine and proline introduce a bend in structure of protein, hence alpha helix disrupted
The rate limiting step in glycolysis is catalyzed by:
1. Hexokinase
2. Phosphofructokinase
3. Enolase
4. Pyruvate kinase
Biochemistry
Answer: 2
The rate limiting step in glycolysis is catalyzed by Phosphofructokinase
After fasting for 24 hours, the major source of glucose for the whole body is
1. amino acids from muscle protein degradation.
2. ketone bodies from muscle triglycerides.
3. glycogen stored in the muscle.
4. creatine phosphate
Biochemistry Answer: 4
Creatine Phosphate is the major source of glucose for the whole body during starvation
Which of the following vitamins is MOST likely to be involved with bone loss in the elderly?
1. Vitamin A
2. Niacin
3. Thiamine
4. Vitamin D
Biochemistry
Answer: 4
Vitamin D is crucial for the maintenance of bone health as it aids in the
absorption of calcium from the digestive tract and facilitates the incorporation
of calcium into bones. A deficiency in vitamin D can lead to osteoporosis, a
condition characterized by weak and porous bones that are more susceptible to
fractures, which is common in the elderly. While vitamin A (Answer 1) is
important for vision and skin health, and niacin (Answer 2) and thiamine (Answer
3) have roles in energy metabolism and nerve function, respectively, vitamin D's
primary role in calcium homeostasis makes it most relevant to bone loss in older
individuals.
In addition to the influence of the oxygen concentration, the formation of oxyhemoglobin is influenced by
1. pH
2. CO2 concentration
3. Temperature
4. All of the above
Biochemistry
Answer: 4
The formation of oxyhemoglobin is influenced by
1. pH
2. CO2 concentration
3. Temperature