DEFINITION: Amino acids are the basic organic substances that constitute building blocks of enzymes, hormones, peptides, and proteins, (i.e. body tissues.)  Amino acids are defined by the -CH(NH₂)COOH substructure. Nitrogen and two hydrogens comprise the amino group, -NH₂, and the acid entity is the carboxyl group, -COOH. Amino acids link to each when the carboxyl group of one molecule reacts with the amino group of another molecule, creating a peptide bond -C(=O)NH- and releasing a molecule of water (H₂O).   A peptide is a compound consisting of 2 or more amino acids. Oligopeptides have 10 or fewer amino acids. Polypeptides and proteins are chains of 10 or more amino acids, but peptides consisting of more than 50 amino acids are classified as proteins.

In the animal kingdom, peptides and proteins regulate metabolism and provide structural support. The cells and the organs of our body are controlled by peptide hormones (see table below). Insufficient protein in the diet may prevent the body from producing adequate levels of peptide hormones and structural proteins to sustain normal bodily functions. Deficiency of good quality protein in the diet may contribute to seemingly unrelated symptoms such as sexual dysfunction, blood pressure problems, fatigue, obesity, diabetes, frequent infections, digestive problems, and bone mass loss leading to osteoporosis. Severe restriction of dietary protein causes kwashiorkor which is a form of malnutrition characterized by loss of muscle mass, growth failure, and decreased immunity.

Allergies are generally caused by the effect of foreign proteins on our body. Proteins that are ingested are broken down into smaller peptides and amino acids by digestive enzymes called "proteases". Allergies to foods may be caused by the inability of the body to digest specific proteins. Cooking denatures (inactivates) dietary proteins and facilitates their digestion. Allergies or poisoning may also be caused by exposure to proteins that bypass the digestive system by inhalation, absorption through mucous tissues, or injection by bites or stings. Spider and snake venoms contain proteins that have a variety of neurotoxic, proteolytic, and hemolytic effects.

Many structures of the body are formed from protein. Hair and nails are made of keratins which are long protein chains containing a high percentage (15%-17%) of the amino acid cysteine. Keratins are also components of animal claws, horns, feathers, scales, and hooves. Collagen is the most common protein in the body and comprises approximately 20-30% of all body proteins. It is found in tendons, ligaments, and many tissues that serve structural or mechanical functions. Collagen consists of amino acid sequences that coil into a triple helical structure to form very strong fibers. Glycine and proline account for about 50% of the amino acids in collagen. Gelatin is produced by boiling collagen for a long time until it becomes water soluble and gummy. Tooth enamel and bones consist of a protein matrix (mostly collagen) with dispersed crystals of minerals such as apatite, which is a phosphate of calcium. Muscle tissue consists of approximately 65% actin and myosin, which are the contractile proteins that enable muscle movement.

Amino Acids

Naturally occurring amino acids, their abbreviations, and structural formulas
* Essential amino acids

Ala = alanine CH3CH(NH2)COOH	Arg = arginine H2N-C(=NH)NHCH2CH2CH2CH(NH2)COOH
Asn = asparagine H2N-C(=O)CH2CH(NH2)COOH	Asp = aspartic acid HOOC-CH2CH(NH2)COOH
Cys = cysteine HS-CH2CH(NH2)COOH	Gln = glutamine H2N-C(=O)CH2CH2CH(NH2)COOH
Glu = glutamic acid HOOC-CH2CH2CH(NH2)COOH	Gly = glycine H2N-CH2COOH
His = histidine *	Ile = isoleucine * CH3CH2CH(CH3)CH(NH2)COOH
Leu = leucine * CH3CH(CH3)CH2CH(NH2)COOH	Lys = lysine * H2N-CH2CH2CH2CH2CH(NH2)COOH
Met = methionine * CH3-S-CH2CH2CH(NH2)COOH	Phe = phenylalanine *
Pro = proline	Ser = serine HOCH2CH(NH2)COOH
Thr = threonine * CH3CH(OH)CH(NH2)COOH	Trp = tryptophan *
Tyr = tyrosine	Val = valine * CH3CH(CH3)CH(NH2)COOH

Arginine is synthesized by the body but at a rate that is insufficient to meet growth needs. Methionine is required in large amounts to produce cysteine if the latter amino acid is not adequately supplied in the diet. Similarly, phenylalanine can be converted to tyrosine, but is required in large quantities when the diet is deficient in tyrosine. Tyrosine is essential for people with the disease phenylketonuria (PKU) whose metabolism cannot convert phenylalanine to tyrosine. Isoleucine, leucine, and valine are sometimes called "branched-chain amino acids" because their carbon chains are branched.

Stereochemistry

L-Alanine

Formation of a peptide from two amino acids

This illustration shows the reaction of two amino acids, where R and R' are any functional groups from the table above. The blue circle shows the water (H₂O) that is released, and the red circle shows the resulting peptide bond (-C(=O)NH-).

Peptides and Proteins

Peptide hormones are produced by the endocrine glands (pituitary, thyroid, pineal, adrenal, pancreas) or by various organs such as the kidney, stomach, intestine, placenta, or liver. Peptide hormones can have complex, convoluted structures with hundreds of amino acids. The following graphics illustrate the chemical structure of human insulin and its three-dimensional shape. Insulin is made of two amino acid sequences. The A-Chain has 21-amino acids, and the B-Chain has 30-amino acids. The chains are linked together through the sulfur atoms of cysteine (Cys). Peptide hormones are generally different for every species, but they may have similarities. Human insulin is identical to pig insulin, except that the last amino acid of the B-Chain for the pig is alanine (Ala) instead of threonine (Thr).

Chemical Structure of Human Insulin

Ribbon representation shows shape of peptide links	Stick representation shows all the atoms	Space-filling representation shows external shape

How are proteins created?

Nucleotide Bases
Adenine (A)	Cytosine (C)	Guanine (G)	Thymine (T)

Chemical Structure of DNA

The nucleotide bases in the center of the DNA helix are flanked by deoxyribose units linked by phosphate groups. The figure on the right represents oxygen as red, nitrogen as blue, and phosphorus as olive green.

Transcription of DNA to mRNA

Uracil (U)

Amino acid profiles of some proteins

Egg protein is considered to have one of the best amino acids profiles for human nutrition. Plant proteins generally have lower content of some essential amino acids such as lysine and methionine. Soy protein is one of the best plant proteins, nevertheless, the most prominent difference in this chart is the proportion of the sulfur-containing amino acids cysteine and methionine. Egg protein has approximately twice as much cysteine and three times as much methionine than is found in soy protein. Also, whey protein has half the amount of arginine and phenylalanine than egg and soy proteins.