Amino acids: are molecules containing an amine group (that contain a basic nitrogen atom with a lone pair), a carboxyl acid group (the presence of at least one carboxyl group), and a side-chain that is specific to each amino acid (called "main chain" or backbone). The key elements of an amino acid are carbon, hydrogen, oxygen and nitrogen. • has the generic formula H2NCHRCOOH • in which side-chain (R-group) is attached to their alpha carbon, and can vary in size from just one hydrogen atom in glycine to a large hetercyclic group in tryptophan • serve as building blocks of proteins • can be linked together in varying sequences to form a vast variety of proteins • Twenty amino acids are naturally incorporated into polypeptides and are called proteinogenic or standard amino acids • Nine standard amino acids are called "essential" for humans because they cannot be created from other compounds by the human body, and so must be taken in as food
|Essential |Nonessential |
|Histidine |Alanine |
|Isoleucine |Arginine* |
|Leucine |Asparagine |
|Lysine |Aspartic acid |
|Methionine |Cysteine* |
|Phenylalanine |Glutamic acid |
|Threonine |Glutamine* |
|Tryptophan |Glycine |
|Valine |Ornithine* |
| |Proline* |
| |Selenocysteine* |
| |Serine* |
| |Taurine* |
| |Tyrosine* |

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Essential Amino Acids:

Histidine: (abbreviated as His or H) has a positively charged imidazole functional group (with the formula C3H4N2 ). • used to develop and maintain healthy tissues in all parts of the body, particularly the myelin sheaths that coat nerve cells and ensure the transmission of messages from the brain to various parts of the body • High levels of this amino acid have been linked to the presence of psychological disorders such as anxiety and schizophrenia, while low levels of histidine are thought contribute to the development of rheumatoid arthritis and the type of deafness that results from nerve damage • important to normal sexual functioning, because it gets converted into histamine, a chemical needed to stimulate sexual arousal • needed to help the immune system know when the body is experiencing an allergic reaction, and for the production of gastric juices needed for normal digestion • also acts as a natural detoxifier, protecting against radiation damage, and removing heavy metals from the system • it may even help prevent the onset of AIDS—histidine is crucial to the production of both red and white blood cells • found in many high-protein foods, such as meat and dairy products, as well as grains such as rice, wheat, and rye • Because it has a proven effect on the central nervous system and histamine production, people with manic (bipolar) depression should not take supplemental histidine without first consulting their physician. Anyone with liver or kidney disorders should not take histidine without first consulting with a licensed health care provider. Taking any one amino acid supplement may cause levels of nitrogen in the body to become imbalanced, as well as disrupt the Krebs cycle by which toxins are eliminated from the liver and kidneys.

• Chemical Properties: o has a pKa of approximately 6.0, and, overall, the amino acid has a pKa of 6.5 o This means that, at physiologically relevant pH values, relatively small shifts in pH will change its average charge • Metabolism: o a precursor for