Because proteins are a highly evolved and diverse class of molecules, they perform endless tasks and functions within both plants and animals. They are important in the biosyntheses of hormones, enzymes, and membrane channels and pumps. In animals, proteins also function in the immune system and can be used in the production of energy. In essence, proteins are the currency of life.
Since proteins constitute the majority of tissues in the body and since these tissues are constantly in protein flux, proteins are degraded and synthesized within all tissues on a regular basis. Some of the amino acids that are degraded can be recycled by the liver and used again for other biosyntheses, but a significant portion of this protein cannot be replaced.
Through a process known as transamination, the liver synthesizes amino acids. During this reaction, an amino group from glutamic acid is transferred to an alpha keto acid, which is a precursor for amino acid sythesis. Aminotransferases, which are derived from vitamin B6, are the enzyme responsible for the reaction. The amino acids that can be produced through transanimation include alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine. These are obviously the non-essential amino acids, since they can be synthesized in the body.
When the body's energy sources are low, it begins to degrade proteins for use as an alternative energy source. Amino acids can be classified as glucogenic or ketogenic.
Glucogenic amino acids can be degraded to pyruvate or an intermediate in the Krebs Cycle. They are named glucogenic because they can produce glucose under conditions of low glucose. This process is also known as gluconeogenesis, or the production of "new glucose." Amino acids form glucose through degradation to pyruvate or an intermediate in the Krebs Cycle. The intermediates can then be converted to oxaloacetate, the main precursor for gluconeogenesis. The following amino acids are glucogenic: alanine, cysteine, glycine, serine, threonine, tryptophan, asparagine, aspartate, phenylalanine, tyrosine, isoleucine, methionine, threonine, valine, arginine, glutamate, glutamine, histidine, and proline.
In contrast, ketogenic amino acids can produce ketones when energy sources are low. Some of these amino acids are degraded directly to ketone bodies such as acetoacetate (see ). They include leucine, lysine, phenylalanine, tryptophan, and tyrosine. The other ketogenic amino acids can be converted to acetyl CoA. Acetyl CoA has several different fates, one of which is the conversion to acetoacetate. Although not a preferential energy source, acetoacetate can be metabolized by the brain and muscle for energy when blood glucose is low. Acetoacetate cannot be used in gluconeogenesis, since acetyl CoA cannot be converted directly to oxaloacetate.