Proteins in the body are constantly synthesized and degraded, partially draining and refilling the cellular amino acid pools. The metabolism of amino acids involve a wide range of synthetic and degradative reactions by which amino acids are gathered and broken down to recover metabolic energy. Since amino acids contain element nitrogen, their chemical transformation is unique as it is different from metabolism of carbohydrates and lipids. There is a dynamic pool of amino acids in the blood. Most of them are incorporated in proteins. Non-essential amino acids are synthesized in the body while essential amino acids are required in the diet. Tissues have continuous access to individual amino acids for the synthesis of proteins and essential amino acid derivatives, such as neurotransmitters. The amino acid pool also provides the liver with substrates for gluconeogenesis and ketogenesis. The free amino acid pool is derived from dietary amino acids and the proteolysis of body proteins.

The liver is the major site of amino acid metabolism in the body and the major site of urea synthesis. The liver is also the major site of amino acid degradation, and partially oxidizes most amino acids, converting the carbon skeleton to glucose, ketone bodies, or C0₂. In liver, the urea cycle converts ammonia and the amino groups from amino acids to urea (see “Nitrogen > Urea Cycle” in the top menu), which is non-toxic, water- soluble, and easily excreted in the urine.

All nitrogen-containing compounds of the body are synthesized from amino acids - cellular proteins, hormones (e.g., thyroxine, epinephrine, insulin), neurotransmitters, creatine phosphate, heme in hemoglobin and cytochromes, melanin, purine and pyrimidine bases.

Nucleic acid metabolism is the process by which nucleic acids (DNA and RNA) are synthesized and degraded. Nearly all the cells can synthesize nucleotides de novo and from degradation products of nucleic acids. Nucleotides are also components of some of the central cofactors of metabolism.

After studying this course, you should be able to:

• explain the importance of amino acid and nucleotide metabolism;

• describe the synthesis of non-essential amino acids and their catabolism;

• enlist de novo and salvage biosynthetic pathways of purine and pyrimidine nucleotides ; and

• describe regulation and degradation of purine and pyrimidine nucleotides.