Amino Acid and Protein Metabolism-I

Which of the following statements best describes the role of tryptophan in the body?

• Tryptophan is primarily used in the muscles to synthesize branched-chain amino acids (BCAAs).

• Tryptophan is required for the synthesis of hepatic tyrosine.

• Tryptophan is necessary for the production of serotonin and melatonin.

• Tryptophan is converted directly into glucose in the liver during starvation

• Tryptophan is a non-essential amino acid that does not need to be provided in the diet.

Answer explanation

Tryptophan is an essential amino acid that serves several critical functions in the body. One of its primary roles is as a precursor for the neurotransmitter serotonin, which regulates mood, appetite, and sleep. Tryptophan is also crucial in the synthesis of melatonin, a hormone that controls the sleep-wake cycle. This role is vital for maintaining healthy sleep patterns and overall mental health.

What is the primary function of methionine in the body?

• Methionine is necessary for the synthesis of cysteine.

• Methionine is essential for the synthesis of nucleic acids.

• Methionine is used primarily for energy production in the muscle tissue.

• Methionine is converted directly into urea in the liver.

• Methionine is not essential for adults.

Answer explanation

Methionine is an essential sulfur-containing amino acid. It plays a critical role as a methyl donor in many biochemical processes and is necessary for the synthesis of cysteine, another sulfur-containing amino acid. This function is crucial for protein synthesis and many metabolic processes.

Where does protein digestion begin in the dog?

• In the pancreas

• In the stomach

• In the colon

• In the mouth

• In the small intestine

Answer explanation

Protein digestion begins in the stomach. The enzyme pepsin plays a significant role in breaking down proteins into smaller peptides. This process is facilitated by the acidic environment of the stomach, which is essential for the activation of pepsin from its inactive form, pepsinogen.

Which of the following enzymes is not involved in the final steps of protein digestion in the intestines?

• Aminopeptidase

• Trypsin

• Chymotrypsin

• Pepsin

• Carboxypeptidase

Answer explanation

Pepsin is an enzyme that functions in the stomach, not the intestines. In the intestines, enzymes like trypsin, chymotrypsin, carboxypeptidase, and aminopeptidase continue the process of breaking down polypeptides into smaller peptides and free amino acids.

Which amino acids are considered branched-chain amino acids (BCAAs) and are routinely oxidized in muscle tissue?

• Phenylalanine, Methionine, Tryptophan

• Lysine, Histidine, Arginine

• Cysteine, Tyrosine, Threonine

• Alanine, Glycine, Serine

• Leucine, Isoleucine, Valine

Answer explanation

Leucine, Isoleucine, and Valine are known as branched-chain amino acids (BCAAs). They are unique among the essential amino acids for their role in muscle metabolism and are primarily oxidized in the muscle rather than in the liver, which is typical for most other amino acids. They play critical roles in protein synthesis and energy production during exercise.

What is the primary role of ubiquitin in protein metabolism?

• To signal the transport of proteins into the mitochondria.

• To act as a signaling molecule that triggers protein synthesis.

• To mark proteins for degradation by the proteasome.

• To enhance the activity of digestive enzymes in the stomach.

• To function as a structural component of muscle fibers.

Answer explanation

Ubiquitin is a small regulatory protein that is found in almost all tissues of eukaryotic organisms. It attaches to proteins and labels them for degradation by the proteasome, a large protein complex in the cell that breaks down and recycles unwanted or damaged proteins. This process is crucial for maintaining cellular health and function.

What triggers the release of pancreatic proteases that are involved in protein digestion?

• The hormone insulin

• The enzyme pepsin

• The hormone glucagon

• The hormone cholecystokinin

• The hormone gastrin

Answer explanation

Cholecystokinin (CCK) is a hormone that is released by the small intestine in response to the presence of fats and proteins in the stomach content. CCK stimulates the pancreas to secrete a juice rich in pancreatic enzymes, including proteases such as trypsin and chymotrypsin, which are crucial for the digestion of proteins in the small intestine.

What is the main function of carboxypeptidase in protein digestion?

• To break down carbohydrates

• To remove one amino acid at a time from the carboxyl end of a peptide

• To synthesize new proteins

• To transfer amino groups between amino acids

• To initiate protein digestion in the stomach

Answer explanation

Carboxypeptidase is an exopeptidase enzyme that works in the small intestine, where it continues the process of protein digestion by removing one amino acid at a time from the carboxyl (C-terminal) end of peptides. This sequential cleavage helps in breaking down peptides into free amino acids that can be absorbed into the bloodstream.

What is the primary outcome of amino acid deamination in the liver?

• Synthesis of more complex proteins

• Conversion into fatty acids

• Release of ammonia and production of urea

• Immediate use as energy

• Storage as body fat

Answer explanation

Deamination is the process by which amino acids are stripped of their amino groups. This reaction, which primarily occurs in the liver, releases ammonia. The liver then quickly converts the toxic ammonia into urea, a much less toxic compound, which is then transported to the kidneys for excretion in the urine. This process is vital for the detoxification of nitrogenous wastes from the body.

Which of the following is a ketogenic amino acid?

• Alanine

• Leucine

• Aspartate

• Valine

• Methionine