Metabolism of Proteins

Metabolism of Proteins

Proteins must also be broken down and replaced before they deteriorate. During their catabolism, con-tained amino acids are recycled for use in building new proteins. They may also be modified into different nitrogen-containing compounds. Active transport processes allow the cells to absorb newly ingested amino acids from the blood. The cells can then use the amino acids to replace tissue proteins. This occurs at a rate of approximately 100 grams in 24 hours. Excess protein cannot be stored by the body. When excess protein is present, amino acids are either oxidized for energy or converted to fat so they may be used for energy in the future.

Oxidation of Amino Acids

Amino acids must be deaminated before they can be oxidized for energy. To do this, their amine group (NH ₂) must be removed. The molecule that results is then converted to pyruvic acid or to a keto acid inter-mediate during the Krebs cycle. These intermediates include acetyl CoA, α-ketoglutaric acid, fumaric acid, oxaloacetic acid, or succinyl CoA. With amino acids that contain sulfur (cysteine, methionine, etc.), sulfur is released before deamination occurs. Glutamic acid is the key molecule used in the oxidation of amino acids. The oxidation of amino acids occurs in the fol-lowing steps:

■■ Transamination: The reversible exchange of amino groups between different amino acids. A variety of amino acids are able to transfer their amine group to the Krebs cycle keto acid known as α-­ketoglutaric acid. Then, this acid is transformed to glutamic acid. The original amino acid there fore becomes a keto acid, having an oxygen atom in the place where the amine group used to be.

■■ Oxidative deamination: The oxidative breakdown of amino acids. Specialized enzyme systems carry out the process such as d-amino oxidase. The amine group of glutamic acid is removed in the liver as ammonia (NH₃). Regeneration of α-ketoglutaric acid occurs. The freed ammonia molecules combine with carbon dioxide to yield urea and water. The urea enters the blood to be excreted from the body in urine. It is vital that glutamic acid can move the amine groups into the urea cycle because ammonia is toxic. Therefore, this cycle removes ammonia that was produced by oxidative deamination and ammonia in the blood that was produced by intestinal bacteria.

■■ Modification of keto acids: As amino acids are degraded, molecules are produced that may be oxidized during the Krebs cycle or converted into glucose. Transamination produces keto acids that may be altered to form metabolites, which can enter the Krebs cycle. Pyruvic acid, acetyl CoA,α-ketoglutaric acid, and oxaloacetic acid are the most important of these metabolites. Deaminated amino acids converted to pruvic acid can be reconverted to glucose, because glycolytic reactions are reversible. These converted amino acids can then act as part of gluconeogenesis.

Protein Synthesis

The most important anabolic nutrients are amino acids, which form all protein structures as well as most of the body’s functional molecules. Protein synthesis occurs on ribosomes, where enzymes control forma-tion of peptide bonds. These bonds link amino acids together, forming protein polymers. The amount and type of protein that is synthesized are regulated by growth hormone, sex hormones, thyroxine, and other hormones. The anabolism of protein is directly related to the individual’s hormonal balance, which is differ-ent through the various stages of life.

The human body easily forms the nonessen-tial amino acids by removing keto acids from the Krebs cycle and transferring amine groups to them. Therefore, during an average lifetime, the body synthesizes­ approximately 500–1,000 pounds of proteins (225–450 kg). This means that an indi-vidual is not required to consume, via the diet, the amount of protein required by the body. The liver is where most of these transformations occur. Nearly all nonessential amino acids are provided there, producing the small amount of protein synthesized by the body in a 24-hour period.

However, protein synthesis requires a complete set of amino acids to occur. The diet must provide all the essential amino acids. If it does not, the remain-der is oxidized for energy. This occurs even if they are needed for anabolism. Therefore, the protein in the body is “consumed” to supply the essential amino acids required and it experiences a negative nitrogen balance.