Question

Muscle proteolysis releases branched-chain amino acids which A. can also be synthesized from alanine. B. can be catabolized by muscle but not liver. C. are the main amino acids metabolized by intestine. D. are the amino acids released in largest amounts by muscle. E. are a major source of nitrogen for alanine and glutamine produced in muscle.

Short answer

a. BCAAs can also be synthesized from alanine. b. BCAAs can be catabolized by muscle but not liver. c. BCAAs are the main amino acids metabolized by the intestine. d. BCAAs are the amino acids released in the largest amounts by muscle. e. BCAAs are a major source of nitrogen for alanine and glutamine produced in muscle. Answer: d. Branched-chain amino acids are the amino acids released in the largest amounts by muscle during muscle proteolysis.

Step-by-step solution

Understanding Muscle Proteolysis

Muscle proteolysis is the process of breaking down muscle protein into its constituent amino acids. This process occurs in the body during periods of stress, starvation, or intense exercise. The breakdown of muscle protein releases branched-chain amino acids, which can be utilized for various functions in the body.

Defining Branched-Chain Amino Acids

Branched-chain amino acids (BCAAs) are a specific group of three essential amino acids: leucine, isoleucine, and valine. These amino acids possess a unique branched structure and are essential because they cannot be synthesized by the body and must be obtained through one's diet.

Analyzing Option A

Option A states that BCAAs can also be synthesized from alanine. This statement is incorrect, as BCAAs cannot be synthesized by the body and must be obtained through dietary sources.

Analyzing Option B

Option B states that BCAAs can be catabolized by muscle but not liver. This statement is partially correct. BCAAs are primarily metabolized in the skeletal muscle, while the liver has a limited ability to metabolize them. However, the liver is not entirely incapable of metabolizing BCAAs, so this statement can't be considered fully correct.

Analyzing Option C

Option C states that BCAAs are the main amino acids metabolized by the intestine. This statement is incorrect; while BCAAs can be metabolized by the intestine, they are not the primary amino acids undergoing metabolism in this tissue.

Analyzing Option D

Option D states that BCAAs are the amino acids released in the largest amounts by muscle. This statement is correct. During muscle proteolysis, BCAAs are released in large amounts, particularly leucine. Thus, Option D is the correct choice.

Analyzing Option E

Option E states that BCAAs are a major source of nitrogen for alanine and glutamine produced in muscle. This statement is also correct. BCAAs can serve as a nitrogen donor for the synthesis of alanine and glutamine in muscle. However, Option D is a more characteristic feature of BCAAs as they are indeed released in the largest amounts by muscle during proteolysis. In conclusion, the correct answer is: D. Branched-chain amino acids are the amino acids released in the largest amounts by muscle during muscle proteolysis.

Key concepts

Branched-Chain Amino Acids

Branched-chain amino acids (BCAAs) comprise a group of three essential amino acids: leucine, isoleucine, and valine. Essential amino acids are named so because our bodies cannot manufacture them, and therefore, they must be obtained through our diet. BCAAs are found in high-protein foods such as meat, dairy products, and legumes.

These amino acids are labeled 'branched-chain' due to their chemical structure which includes a side chain with one carbon atom bonded to three or more other carbon atoms. Among their many roles in the body, BCAAs are integral for building and repairing muscles, serving as a nitrogen source for the synthesis of other amino acids, and may help regulate blood sugar levels by stimulating the secretion of insulin.

BCAA Metabolism

The metabolism of branched-chain amino acids is a specialized process that primarily occurs in muscle tissue rather than the liver, which is the site for metabolism of most other amino acids. During periods of muscle proteolysis, which can occur during exercise, stress, or fasting, muscles break down proteins and release BCAAs into the bloodstream.

Once in the bloodstream, these amino acids can be used by different tissues for protein synthesis or can be further broken down to be used as energy. Interestingly, unlike other amino acids, BCAAs are oxidized in the muscle, rather than the liver. This unique aspect of BCAA metabolism plays a critical role in energy production during exercise and could also influence muscle recovery and growth.

Amino Acid Catabolism

The catabolism of amino acids, including BCAAs, is the process by which proteins are broken down into their constituent amino acids. Generally, amino acid catabolism occurs when there is a deficiency of dietary protein or carbohydrates, during which the body resorts to using its protein reserves for energy.

BCAAs, in particular, are significant in this process as they provide an important source of energy for muscle protein synthesis and can be converted into other important molecules. For instance, during this catabolic process, BCAAs donate their nitrogen to the formation of alanine and glutamine in muscles. Alanine can transport nitrogen from peripheral tissues to the liver, where it is converted into urea for excretion. Meanwhile, glutamine serves several purposes, including being an energy source for cells of the immune system and increasing the production of muscle protein.