Biochemistry 1. 3 rd year College of Pharmacy University of Kerbala

Transcription

1 Biochemistry 1 3 rd year College of Pharmacy University of Kerbala

2 AMINO ACIDS Amino acids are compounds having an amino (- NH 2 ) and a carboxyl (-COOH) groups. α- amino acids have the general formula NH 2 - CHR-COOH. Although there are more than 300 naturally occurring amino acids, the human genome allows for only 20 L-α-amino acids listed in the table below.

3 Standard amino acids: there are 20 standard amino acids that are commonly found in proteins.

4 H H2N C α COOH R Where "R" represents a side chain specific to each amino acid. Amino acids are usually classified by properties of the side chain into four groups: acidic, basic, hydrophilic (polar), and hydrophobic (nonpolar).

5 α-amino acids α-amino acids are amino acids in which the amino and carboxylate functionalities are attached to the same carbon, the so-called α carbon. They are the building blacks of proteins. Both one and three letter abbreviations for each amino acid can be used. Some proteins contain additional amino acids that arise by modification of an amino acid already present in a peptide. For example the conversion of proline and lysine into 4-hydroxyproline and 5-hydroxylysine respectively. This will exttend the biologic diversity of proteins by altering the solubility, stability and interaction with other proteins. Except glycine all AAS

6 ISOELECTRIC POINT A zwitterion is an amino acid having positively and negatively charged groups, a dipolar molecule, + NH 3 CHRCOO -. Isoelectric species is the form of a molecule that has an equal number of negative and positive charges and thus electrically neutral. The isolecltric ph, Ip is the ph midway between pka values on either side of the isoelecric species, e.g. for Ala pi = Q: Work out the Ip for.,..and. Ip guides the selection of conditions for separation of Aas.

7 Acidity of AAs Both HA and HB+ are weak acids. However, HA is more acic than HB+. At ph 7.4, carboxyl groups exist almost entirely as RCCO- and amine groups predominately as RNH3+. The effect of ph on the charge state of aspartic acid can be illustrated as; see text. Q: Illustrate the charge state of, and from phs 1-13.

8 Charge on AAs The net charge on an AA depends on the pka values of its functional groups and on the ph of the surounding medium. Hence the net the charge on Aas can be changed by altering the ph pf the surrounding medium. The forms the basis of analysis involving AAs. Ex. Use pka values, show the net charge on glutamic acid in the ph range 1-13.

9 Properties of AAS The functional groups on an AA dictate the chemical reactions of AAs. Examples are,..and. The solubility and melting points of AAs reflect its ionic characters. Charged functional groups ensures they are soluble in aqueous media and alcohols. Aas are solids with m.p. of about 200oC reflecting the presence of inter and intra interactions in these molecules. Aas do not absorb visible light, however those containing a

10 ph effect on the charge of amino acids The proton dissociation constants are K1 and K2. K [ HA][ H [ HAH 1 ] ]..3 K 2 [ A ][ H [ HA] ].4 Taking the logs of equations 3 and 4, yields, ph pk [ HA] log [ HAH 1 ] ph pk [ ] represents concentration in dilute solution 2 [ A ] log [ HA]

11 ESSENTIAL amino acids An essential amino acid for an organism is an amino acid that cannot be synthesized by the organism from other available resources, and therefore must be supplied as part of its diet. Most of the plants and microorganism cells are able to use inorganic compounds to make amino acids necessary for the normal growth.

12 ESSENTIAL amino acids Eight amino acids are generally regarded as essential for humans: tryptophan, lysine, methionine, phenylalanine, threonine, valine, leucine, isoleucine. Two others, histidine and arginine are essential only in children. A good memonic device for remembering these is "Private Tim Hall", abbreviated as: PVT TIM HALL: Phenylalanine, Valine, Tryptophan Threonine, Isoleucine, Methionine Histidine, Arginine, Lysine, Leucine

13 ESSENTIAL amino acids limiting amino acid content: The essential amino acid found in the smallest quantity in the foodstuff. Protein source Limiting amino acid Wheat lysine Rice lysine and threonine Maize lysine and tryptophan Pulses methionine Beef methionine and cysteine Whey none Milk none

14 Use of Amino Acids Aspartame (aspartyl-phenylalanine-1-methyl ester) is an artificial sweetener. 5-HTP (5-hydroxytryptophan) has been used to treat neurological problems associated with PKU (phenylketonuria), as well as depression. L-DOPA (L-dihydroxyphenylalanine) is a drug used to treat Parkinsonism. Monosodium glutamate is a food additive to enhance flavor.

15 The Peptide bond Peptide bond: It is a chemical bond formed between two amino acids. The.. of one amino acid reacts with the..of the other amino acid releasing a molecule of... This is a condensation reaction (also called dehydration synthesis) reaction.