PHR202: Biochemistry and Molecular Biology Mir Ishruna Muniyat

Amino Acid Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen. An alpha-amino acid has the generic formula H 2 NCHRCOOH, where R is an organic substituent, the amino group is attached to the carbon atom immediately adjacent to the carboxylate group (the α–carbon).

Essential Amino Acids An essential amino acid is an amino acid that cannot be synthesized itself by the organism (usually referring to humans), and therefore must be supplied in the diet. 10 amino acids are essential amino acid They are - arg, his, ile, leu, lys, met, phe, thr, trp, val Must be obtained from the diet An adequate diet must contain these essential amino acids. Typically, they are supplied by meat and dairy products Essential amino acids help the body function and regulate neurotransmitters, chemicals in the brain that control mood and behavior. A lack of essential amino acids can cause emotional or physical difficulties and lead to health disorder

List of Essential & Nonessential Amino Acid EssentialNonessential HistidineAlanine IsoleucineAspartate LeucineCysteine LysineGlutamate MethionineGlutamine PhenylalanineGlycine ThreonineProline TryptophanSerine ValineTyrosine Arginine Asparagine 20

Although  -amino acids are commonly written in the unionized form, they are more properly written in the zwitterion (internal salt) form (Germ. Zwitter means hybrid) Both the –NH 2 and the –COOH groups in an amino acid undergo ionization in water. At physiological pH (7.4), a zwitterion forms – Both + and – charges – Overall neutral – Amphoteric- contain both positive and negative charges Amino group is protonated Carboxyl group is deprotonated Soluble in polar solvents due to ionic character Zwitterions

Acid-Base Properties of Amino Acids Figure : The ionic forms of the amino acids. 22

Isoelectric Point  The isoelectric point is the pH at which there is zero net charge Using Glycine again: Charges in the first ionization: +1  0 Charges in the second ionization: 0  -1 So, in the case of glycine, the pH at which there is most of the zero net charge form occurs half way between the first and second ionizations. 23

Peptides and Proteins 20 amino acids are commonly found in protein. These 20 amino acids are linked together through “peptide bond” The chains containing less than 50 amino acids are called “peptides”, while those containing greater than 50 amino acids are called “proteins”. Peptide bond is the Peptide bond formation: Peptide bond is the amide bond between the  -carboxyl group of one amino acid and the  -amino group of another 24

Peptide bond The trans conformation of the peptide bond.

Peptides and Proteins Peptide: a short polymer of amino acids joined by peptide bonds; they are classified by the number of amino acids in the chain dipeptide: a molecule containing two amino acids joined by a peptide bond tripeptide tripeptide: a molecule containing three amino acids joined by peptide bonds Polypeptide: a macromolecule containing many amino acids Linear polymers (no branches) Protein: a biological macromolecule of molecular weight 5000 g/mol or greater, consisting of one or more polypeptide chains Oligopeptide: An oligopeptide, often just called peptide (oligo-, "a few"), consists of two to twenty amino acids and can include dipeptides, tripeptides, tetrapeptides, and pentapeptides. 26

Examples on Peptides: 1- Dipeptide ( two amino acids joined by one peptide bond): Example: Aspartame which acts as sweetening agent being used in replacement of cane sugar. It is composed of aspartic acid and phenyl alanine. 2- Tripeptides ( 3 amino acids linked by two peptide bonds). Example: GSH ( Glutathione) which is formed from 3 amino acids: glutamic acid, cysteine and glycine. It helps in absorption of amino acids, protects against hemolysis of RBC by breaking H 2 O 2 which causes cell damage. 3- Octapeptides: (8 amino acids) Examples: Two hormones; oxytocin and vasopressin (Antidiuretic Hormone - ADH). 4- Polypeptides: More than 10 amino acids: e.g. Insulin hormone 27

 Biologically active peptides: oxytocin, bradykinin, vasopressin Vasopressin  Peptides (and proteins) have their unique pI values depending on the AA compositions. Insulin has two peptide chains (A chain has 21 AAs & B chain has 30 AAs) held together by two disulfide linkages ArgArg - Pro - Pro - Gly - Phe – Ser - Pro - Phe - ArgPro GlyPheSerProPheArg Bradykinin causes blood vessels to dilate & causes blood pressure to lower 28