MDS PREP
The lipids associated with very low density lipoprotein
1. are usually linked covalently to apoproteins.
2. belong primarily to the phosphatidylcholine class.
3. incorporate fatty acids that are synthesized in the liver.
4. reflect, in general, the types of lipids found in the diet.
Biochemistry
Answer: 3
VLDL is a lipoprotein particle synthesized in the liver and secreted into the bloodstream to transport endogenously synthesized triglycerides and cholesterol to peripheral tissues. The triglycerides in VLDL are derived from fatty acids synthesized in the liver, and the cholesterol is derived from both dietary sources and de novo synthesis in the liver. They are not usually linked covalently to apoproteins (Answer 1), nor do they primarily consist of phosphatidylcholine (Answer 2). While the types of lipids found in the diet (Answer 4) can influence the liver's synthesis of VLDL, the statement does not accurately describe the direct composition of VLDL.
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.
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.
Urea formation take place in liver by:
1. Salvage pathway
2. Krebs cycle
3. Krebs Henseleit cycle
4. none
Biochemistry
Answer: 3
The synthesis of urea takes place in the liver by the process called ''Ornithine
cycle'' or "Urea cycle"
This cycle was discovered by Hans Krebs and Kurt Henseleit in the year 1932.
The Ornithine cycle is completed in 5 steps:
1. Conversion of ammonia into carbamoyl phosphate
2.The transfer of carbamoyl group from carbamoyl phosphate to ornithine
3. Releasing of the formed citrulline into the cytosol
4. Cleavage of argininosuccinate
5. Hydrolysis of Arginine: The final step involves the hydrolysis of Arginine
into urea and ornithine
Arginase is the sixth and final enzyme of this cycle.
If a biochemical test gives the same reading for a sample on repeated testing, it is inferred that the measurement is:
1. Precise.
2. Accurate.
3. Specific.
4. Sensitive.
repeatablity of test is precision and getting results within reference range is accuracy
Site of â-oxidation of fatty acid Is:
1. Cytoplasm
2. Mitochondria
3. Both cytoplasm and mitochondria
4. Lysosomes
Biochemistry
Answer: 2
â-oxidation of fatty acid occursin Mitochondria
Hyperuricemia in Lesch-Nyhan syndrome is due to a defect in which of the following pathways?
1) Purine biosynthesis
2) Pyrimidine biosynthesis
3) Purine salvage
4) Pyrimidine salvage
Biochemistry Answer: 3
Uric acid is a purine derivative, increased by purine salvage reactions that convert purines, purine ribonucleosides, and purine deoxyribonucleoside to mononucleotides (incorrect answer 4).
Such salvage reactions require much less energy than de novo synthesis (incorrect answers 1, 2). The liver is the major site of purine nucleotide biosynthesis and provides excess purines for other tissues that cannot synthesize purines.
A defect in hypoxanthine-guanine phosphoribosyl transferase, one of the enzymes of purine salvage, is responsible for purine overproduction and subsequent hyperuricemia observed in Lesch-Nyhan syndrome.
Symptoms from a riboflavin deficiency can be:
1. Cracks in the corner of the mouth (angular stomatitis)
2. Inflammation of the tongue (glossitis)
3. Eye and skin changes
4. All of the above
Biochemistry
Answer: 4
The eye change include an increase in blood vessels and inflammation of the conjunctivae, cornea is invaded by capillaries, producing opaque areas and even ulceration. Dermatitis characterized by a greasy and scaly reddened lesion develops on the skin around the nasolabial folds and may extend to a butterfly shape on the cheeks. There many also be lesions at the corners (canthi) of the eyes and lobes of the mouth.