Properties and structure:  Are carboxylic acids with α - amino group.  Are the basic building blocks of proteins.  Nearly all have an asymmetric carbon atom (chiral center).  Possible stereoisomers is 2 n., where: n= No of chiral centers.

H 2 N COOH CH R Amino Acid

 So, Glycine have no chiral center and therefore cannot exist in stereoisomeric forms., Alanine has 2 isomers., and some amino acids have more than 2.  Threonine and isoleucine which have 4 stereoisomers, but only one of the four possible isomers occurs in proteins.  The amino acids present in protein molecules are the L stereoisomers.

 Amino acids are ionized in water solutions.  The ionized form (dipolar ion) with a net charge of zero is called "zwitterion".  The dipolar ion can act as acid or base (proton donor or acceptor).

 Substances having this property are amphoteric and often called (ampholytes).  Amino acids are also said to be polyprotic, since they have at least two dissociable protons.  The proton from the COOH transfer to the NH 2 end of the same molecule, since the COO- is stronger acid than NH 2.

 Amino acids have characteristic titration curves.  The COOH end of the amino acid tend to lose its proton, since it is stronger as acid (e.g. the pKa for Glycine is 2.34 much lower than acetic acid 4.75).  The NH 2 end of the molecule tend to accept the proton, since it is stronger as base (e.g. the pKa for Glycine is 9.6).  The R groups which may be acidic, basic, aliphatic or aromatic are also have different pKa values.  The titration curve around the pKa values is flat indicating buffering power of amino acids at these pH values.

 The net electric charge on the amino acid depends on the pH of the solution.  At low pH, amino acid has a net positive charge and move towards the negative electrode (the cathode).  At high pH, amino acid has a net negative charge and move towards the positive electrode (the anode).  At some pH value, where the ion has no net charge, it will not move in the electric field.

 This characteristic pH is called the (isoelectric pH or isoelectric point pI).  The pI of Glycine = ( )/2 =  pI is of practical importance, since separation of a mixture of amino acids can be done in an electric field at known pH.

 Amino acids can be classified on the bases of their R groups.  The R groups of the amino acids vary widely with respect to their polarity.  Amino acids are classified nonpolar (hydrophobic –water hating-)when the R groups are H, alkyl or aromatic. E.g Glycine, Alanine, Phenylalanine.

 The amino acids with polar (hydrophilic –water loving-) have R groups of alcohols, thiols or amides.  Essential amino acids (8), cannot be synthesized in the body, and so, should be taken in the diet.  Non essential amino acids can be synthesized in the body from other precursors such as (Alanine).

A number of hormones are known to be peptide hormones. Hormones are chemical messengers secreted by specific cells of endocrine glands, such as the pancreas, pituitary, or adrenal cortex, and sent via the blood to stimulate specific functions of other tissues or organs. Insulin, secreted by the β -cells of the pancreas is carried in blood to the cells of liver and muscles where exerts it is action and stimulates the capacity of these cells to use glucose as a metabolic fuel. Insulin contains two polypeptide chains: one having 30 amino acid residues and the other 21. Glucagon, secreted by the A-cells of the pancreas and oppose the action of insulin is also a polypeptide.

Corticotrophin, a hormone of the anterior pituitary gland, stimulate the adrenal cortex is a polypeptide with 39 amino acid residues. Oxytocin, secreted by the posterior pituitary, and stimulates uterine contraction during delivery is a polypeptide of only 9 amino acids. Bradykinin, a hormone that inhibit the inflammation of tissues, also have 9 amino acids. Thyrotropin-releasing factor (TRF), formed in the hypothalamus and stimulate the secretion of the TSH of the anterior pituitary, have 3 amino acid residues (Glu-His-Pro). Enkephalins, formed in the central nervous system, and when bind to specific receptors in certain cells of the brain, they induce analgesia,(deadening of pain sensations), they have opiate-like activity. ( they bound to the sites in brain which also bind morphine, heroin, and other addicting opiate drugs. Tyr-Gly-Gly-Phe-Met. Tyr-Gly-Gly-Phe-Leu.

***It is important to notice that these peptides have such potent biological effects, despite the fact that amino acid forming them are harmless, nontoxic substances. So, it is the sequence of amino acids that gives the polypeptides their biological effects and specificity.