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Proteins, Amino Acids, Peptides, and Polypeptides

Proteins consist of amino acids which are characterized 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). Amino acids are the basic building blocks of enzymes, hormones, proteins, and body tissues. 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.

Eggs
Egg whites are mostly protein

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. Individual amino acids serve as neurotransmitters and modulators of various physiological processes, while proteins catalyze most chemical reactions in the body, regulate gene expression, regulate the immune system, form the major constituents of muscle, and are the main structural elements of cells. 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. By weight, bone tissue is 70% mineral, 8% water and 22% protein. Muscle tissue consists of approximately 65% actin and myosin, which are the contractile proteins that enable muscle movement. Casein is a nutritive phosphorus-containing protein present in milk. It makes up approximately 80% of the protein in milk and contains all the common amino acids.

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 *
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 *
phenylalanine
Pro = proline
proline
Ser = serine
HOCH2CH(NH2)COOH
Thr = threonine *
CH3CH(OH)CH(NH2)COOH
Trp = tryptophan *
tryptophan
Tyr = tyrosine
tyrosine
Val = valine *
CH3CH(CH3)CH(NH2)COOH

The term "essential amino acid" refers to an amino acid that is required to meet physiological needs and must be supplied in the diet. 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" (BCAA) because their carbon chains are branched.

Stereochemistry

In all twenty amino acids, except glycine, the carbon atom with the amino group is attached to four different substituents. The tetrahedral bond angles of carbon and the asymmetry of the attachments make it possible for amino acids to have two non-superimposable structures, the L and R forms, which are mirror images of each other. Only L-amino acids are found in proteins. L-amino acids have the amino group to the left when the carboxyl group is the top, as illustrated here. The wedge bonds are above the display plane and the dotted bonds are below the display plane. The rotating molecular model represents oxygen atoms as red, nitrogen as blue, carbon as black, and hydrogen as white for the dipolar ion form of L-alanine (CH3CH(NH3+)COO-).

L-Alanine      L-Alanine 3D
L-Alanine

Formation of a peptide from two amino acids

Formation of a peptide

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-).

The reverse reaction, i.e., the breakdown of peptide bonds into the component amino acids, is achieved by hydrolysis. Many commercial food products use hydrolyzed vegetable proteins as flavoring agents. Soy sauce is produced by hydrolyzing soybean and wheat protein by fungal fermentation or by boiling with acid solutions. Monosodium glutamate (MSG), a flavor enhancer, is a sodium salt of glutamic acid that is found naturally in seaweed and fermented soy products.


CONTINUED: Peptides and Proteins
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