Amino Acids Lecture Notes PDF

 BY Leila Mustafa Taha Department of Biochemistry Faculty of Veterinary Medicine South valley university Definitions: Proteins are organic compounds with a high molecular weight formed of carbon ,oxygen, hydrogen and nitrogen and may also contain sulfur, phosphorus and metal ions. T...

 BY Leila Mustafa Taha Department of Biochemistry Faculty of Veterinary Medicine South valley university Definitions: Proteins are organic compounds with a high molecular weight formed of carbon ,oxygen, hydrogen and nitrogen and may also contain sulfur, phosphorus and metal ions. They are polymers formed of subunits called amino acids linked together by peptide linkage. N.B.The term protein is applied to describe molecules greater than 50 amino acids. Polypeptides are formed from less than 50 amino acids.  Biological importance of proteins: 1. Nutritional role : provide the body with essential amino acids, sulfur and nitrogen 2. Catalytic role: All enzymes are proteins in nature. 3. Defensive role: Antibodies (immunoglobulins). 4. Plasma proteins are responsible for most effective osmotic pressure of the blood. 5. Hormonal role: Most of hormones are protein in nature. 6. Transport role: proteins carry lipid in blood forming lipoprotein complexes. Protein also carry hormones e.g., (thyroid hormones) and minerals e.g.,( calcium, iron and Copper). Hemoglobin carry oxygen from lung to tissues is a protein. 7) Structural role: proteins are the main structural component in bone, muscle and cell membrane. 8)Blood clotting: coagulation factors are proteins. 9)Control of gene expression: most factors required for DNA replication, transcription and m RNA translation are protein in nature.  Amino acids are organic acids that contain NH2 group. They are the structural units of proteins and are obtained from them by hydrolysis. The general formula of any amino acid is as follows: Although about 300 amino acids exist in nature, only 20 of them can polymerize in protein structure. For each a specific codon exists in the genetic code. The metabolizible form of them are L-amino acids (amino group is on the left side conf iguration). All amino acids present in mammals are L-amino acids. D-amino acids are found in the cell wall of bacteria. Each amino acid has a carboxylic group (COOH) and amino group (NH2) except Proline and Hydroxyproline. In α-amino acids, both (COOH )and (NH2) groups are attached to the same (α-) carbon atom. this why they are amphoteric, react as acid by (COOH) and as alkali by (NH2). At physiological PH (approximately PH7.4) the carboxyl group is dissociated forming negatively charged carboxylate i o n ( CO O - ) a n d t h e a m i n o g r o u p i s protonated forming positively charged ion (-NH3). All amino acids (except glycine) are optically active, i.e., can rotate plane polarized light. This is because the 4 groups attached to α-carbon are different. In glycine, the α-carbon is attached to 2 hydrogen atoms, therefore, is optically inactive. Ø Classification of amino acids (A.A.) Amino acids can be classified according to one of the following methods: I. Chemical classification II. Metabolic classification III. Biological classification IV. C l a s s i f i c a t i o n a cco r d i n g t o w a t e r solubility 1. Chemical classification According to the number of (COOH) and (NH2) groups amino acids can be classified into: 1. N e u t r a l a m i n o a c i d s ( m o n o a m i n o , monocarboxylic). 2. Acidic amino acids (monoamino, dicarboxylic). 3. Basic amino acids (diamino, monocarboxylic). A. Neutral amino acids Contain one amino group and one carboxyl group. They can be further classified into: 1) Aliphatic amino acids e.g. N.B. Valine, Leucine and Isoleucine also called Branched chain amino acids. 2) Hydroxy amino acids: contain (-OH) group in t h e i r s i d e c h a i n e. g. , serine,threonine,tyrosine ,Hydroxyproline and hydroxylysine. 3) Sulfur-containing amino acids: e.g. 4) Aromatic amino acids: e.g., tyrosine, phenyl alanine and tryptophan. 5) Heterocyclic amino acids: contain ring other than benzene ring e.g., Histidine (Imidazole ring ), tryptophan (Indole ring), Proline and Hydroxyproline (pyrrolidone ring). 6)Imino Acids: contain imino group (-NH).e.g. Proline and Hydroxyproline.  Acidic amino acids.B They contain 2 carboxyl groups and one amino group, e.g. They can occur in the tissue in the form of amides e.g. C. Basic amino acids They contain 2 amino groups and one carboxyl group e.g. Arginine, Citrulline, ornithine ,lysine and hydroxylysine. 1- Arginine II. Metabolic classification Based upon the fate of AAs inside the body: a) Glucogenic AAs that can be converted to glucose. b) Ketogenic AAs that can be converted to ketone bodies. c) Ketogenic & glucogenic AAs, i.e. can be converted to both glucose and ketone bodies. III. Biological classification Based upon whether the amino acids can be synthesized in body or not: a) Essential amino acids (Indispensable amino acids): they called essential because they are not synthesized in body, must be taken in diet for optimal growth and health, otherwise their deficiency will lead to nutritionally deficiency disease that affect both growth and health. They include: N.B. Arginine and Histidine are called semi-essential: they are synthesized in a mount sufficient for healthy adult but not for growing children, lactating and pregnant women and convalescent patient.Therfore ,they are essential for growing children, lactating and pregnant women and convalescent patient. b) N o n - e s s e n t i a l a m i n o a c i d s (Dispensable amino acids): They can be synthesized inside the body. Their deficiency in diet does not affect t h e g row t h a n d t h e h e a l t h s o n o t essential to be present in diet. They include the rest of amino acids. IV. Classification according to water solubility a) Hydrophobic amino acids: They tend to repel the aqueous environment. They does not ionize nor participate in the formation of H-bonds and, therefore, they often involved in the core part of the proteins. b) Hydrophilic amino acids: They tend to interact with the aqueous environment, and are often participating in the formation of H- bonds. They often found on the external surfaces of proteins or in the reactive centers of enzymes.