MDS PREP
Which of the following is a common dietary saturated fatty acid?
1. Oleic acid
2. Stearic acid
3. Linolenic acid
4. Arachidonic acid
Biochemistry
Answer: 4
Saturated fatty acids are a type of fat that has a chemical structure with a
single bond between each carbon atom in the fatty acid chain and no double
bonds. These types of fats are typically solid at room temperature. Out of the
given options, stearic acid (C18:0) is the only saturated fatty acid. The others
are:
1. Oleic acid (C18:1) is a monounsaturated fatty acid, which means it has one
double bond. It is commonly found in olive oil, canola oil, and avocados.
2. Linolenic acid (C18:3) is a polyunsaturated fatty acid, specifically an
omega-3 fatty acid, with three double bonds. It is found in plant sources like
flaxseeds, chia seeds, and walnuts.
3. Arachidonic acid (C20:4) is also a polyunsaturated fatty acid, an omega-6
fatty acid with four double bonds. It is found in animal fats, such as meat, and
in smaller amounts in some plant oils like sunflower oil and peanut oil.
Stearic acid, on the other hand, is a common saturated fatty acid found in
various animal fats and certain vegetable oils like coconut oil, palm kernel
oil, and cocoa butter. It is known for its role in the structure of cell
membranes and as a precursor for the biosynthesis of cholesterol in the body.
While it is important to consume fatty acids for good health, excessive
consumption of saturated fatty acids has been associated with higher levels of
LDL (low-density lipoprotein) cholesterol, which is often referred to as "bad"
cholesterol, and an increased risk of heart disease. However, it is important to
maintain a balanced diet that includes both saturated and unsaturated fatty
acids in moderation.
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 component of the coenzyme required in a transamination process is
1. Thiamine
2 Folic acid
3 Pyridoxine
4 Riboflavin
Biochemistry
Answer: 3
A component of the coenzyme required in a transamination process is Pyridoxine
Which of the following dental tissues is most likely to be effected by Vit-A
deficiency
1. enamel
2. dentin
3. cementum
4. pulp
Biochemistry Answer: 1
Enamel most likely effected by deficiency of vitamin A
Glycine is a:
1. Glycogenic amino acid, only
2. Ketogenic only
3. Glucogenic and ketogenic
4. Since It Is optically inactive therefore none of the above
Biochemistry
Answer: 1
Glycine is a Glycogenic amino acid, only
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.
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.
Approximately 3 hours following a well-balanced meal, blood levels of which of the following are elevated?
1) Fatty acids
2) Glucagon
3) Glycerol
4) Chylomicrons
Biochemistry Answer: 4
Following digestion, the products of digestion enter the bloodstream.
These include glucose, amino acids, triacylglycerides packaged into chylomicrons from the intestine, and very low density lipoproteins from the liver.
The hormone of anabolism, insulin, is also elevated because of the signaling of the glucose and amino acids in the blood, which allows release of insulin from the β-cells of the pancreas. Insulin aids the movement of glucose and amino acids into cells. In contrast, all the hormones and energy sources associated with catabolism are decreased in the blood during this time. Long-chain fatty acids and glycerol released by lipolysis from adipocytes are not elevated. Glucagon and epinephrine are not released. The only time glucose levels rise significantly above approximately 80 mM is following a well-balanced meal when glucose is obtained from the diet. The concentration of glucose reaches a peak 30 to 45 minutes after a meal and returns to normal within 2 hours after eating. This response of blood glucose after eating (mimicked by giving 50 g of oral glucose) is the basis for the glucose tolerance test. In the event of insulin deficiency (diabetes mellitus), the peak glucose concentration is abnormally high and its return to normal is delayed.