Protein Chemistry PDF

Proteins chemistry Proteins are organic compounds of high molecular weight formed by a number of amino acids united by a peptide linkage. Proteins contain C. H. O. S. N. and some of them contain P. The nitrogen content of protein represents about 16% i.e. every 100gm protein contain 16 gm nitrogen....

Proteins chemistry Proteins are organic compounds of high molecular weight formed by a number of amino acids united by a peptide linkage. Proteins contain C. H. O. S. N. and some of them contain P. The nitrogen content of protein represents about 16% i.e. every 100gm protein contain 16 gm nitrogen. Biochemical importance: 1. proteins serve as structural component of the cell. i.e. All enzymes, antibodies and some hormones. 2. Certain amino acids are involved in transmission of nerve impulses like, glycine and glutamic acid. 3. Essential amino acids are necessary to support growth in infants and maintain health in adults. 4. Many compounds of biochemical importance are produced from protein metabolism e.g. histamine and γ-aminobutyrate. Amino acids Amino acids are structural units of proteins and obtained from them by hydrolysis. Naturally occurring amino acids are amino acids containing amino group and carboxyl group on the same carbon atoms and represented by the general formula:  About 300 amino acids are present in nature , from which 20 only inter in protein of living organisms.  All amino acids found in living systems, plant and animal proteins are L-amino acids (amino group found on the left side). D- amino acids are found in the cell walls of bacteria. H R Importance of amino acids:  they serves as building block of proteins.  They act as precursors of:  Hormones: like thyroid hormones, adrenal cortical hormones and Melatonin.  Vitamins: like niacin.  Purines and pyrimidine.  Porphyrins. Classification of amino acids  Amino acids can be classified into: I. Chemical classification. According to the number of amino groups and carboxyl groups in the amino acids. II. Biological classification. According to whether amino acids are essential or non-essential. III. Metabolic classification. According to whether amino acids are glucogenic or ketogenic. I. Chemical classification: According to the number of amino groups and carboxyl groups in the amino acids into; 1.Neutral amino acids: They contain one amino group and one carboxyl group. 2. Acidic amino acids: They contain 2 carboxyl groups and one amino group. 3. Basic amino acids: They contain 2 amino groups and one carboxyl group. 1-Neutral amino acids:  The neutral amino acids are further classified into: A- Aliphatic amino acids: B-Hydroxy amino acids: C-Sulfur containing amino acids: D- Aromatic amino acids: E- Heterocyclic amino acids: A- Aliphatic amino acids: Which include :  The neutral amino acids are further classified into: 1.Glycine: (α-amino acetic acid). 2. Alanine: (α-amino propionic acid). 3*. Valine: (α-amino iso-valeric acid). 4*- Leucine: (α-amino isocaproic acid). 5*.Isoleucine: (α-amino β -methyl valeric acid). B-Hydroxy amino acids: Which include: 1.Serine: (α-amino β –hydroxyl propionic acid). 2*.Threonine: (α-amino β –hydroxyl butyric acid). C-Sulfur containing amino acids: Which include: 1- Cysteine: (α-amino β –thiol propionic acid). 2- Cystine: Which formed from oxidation of two molecules of cysteine. 3*- Methionine: (α-amino β –thiol butyric acid). D- Aromatic amino acids :Which include 1*. Phenyl alanine: (α-amino β –phenyl propionic acid) 2. Tyrosine : (α-amino β –parahydroxy phenyl propionic acid). 3*. Tryptophan: (α-amino β –indol propionic acid). E- Heterocyclic amino acids: Which include: 1*- Histidine: (α-amino β –imidazole propionic acid). 2- Proline: (α-pyrrolidine carboxylic acid). 3- Hydroxy proline. N.B. Proline and hydroxyl proline are known as imino acids characterized by the presence of imino group (NH) since the amino nitrogen is a part of the ring structure. 2- Acidic amino acids 1. Glutamic acid: (α-amino glutaric acid). 2- Aspartic acid: (α-amino succinic) N.B. These acidic amino acids can occur in tissues in the form of amides e.g. glutamines and asparagines in which the last COOH group is converted into CO-NH2. 3- Basic amino acids 1*- Arginine: (α-amino σ-guanido valeric acid). 2*- Lysine: (α -ε-diamino caproic acid). 3- Hydroxy lysine: (α -ε-diamino σ-hydroxy caproic acid). 4-Ornithine:This amino acid does not enter in the synthesis of proteins and is usually present in the free form as intermediate product in the synthesis of urea. (α -σ-diamino valeric acid). 5-Citrulline: is produced also during urea synthesis.  II- Biological (Nutritional) classification According to whether amino acids are essential or non essential into: 1- Essential (Indispensable) amino acids: Those amino acids which are not synthesized in the body and so they must be taken in diet. They are ten amino acids: Valine Leucine, Isoleucine, Threonine, Methionine, Phenyl alanine,Tryptophan, Histidine, Lysine and arginine. 2- Non-essential (Dispensable) amino acids: Those amino acids which are synthesized in the body. They are ten amino acids:. These amino acids are derived from carbon skeletons of lipids and carbohydrates during their metabolism or from the transformation of essential amino acids. They includes the remaining ten of amino acids. 3-Semi-essentia) amino acids: Those amino which can be formed in the body but in small quantities which are not sufficient for the physiological requirements of the body. They include histidine and argininie.  III- Metabolic classification According to the fate of amino acids inside the body.  1-Glucogenic amino acids: These are amino acids which can  give glucose or carbohydrate intermediate product inside  the body.These include: Glycine, alanine, serine, proline,  hydroxyl proline, tyrosine, cysteine, cystine, glutamic and  aspartic acids.  2-Ketogenic amino acids: These are amino acids which can  give ketone bodies inside the body.They include Leucine.  3-Mixed amino acids :Glucogenic and Ketogenic. They include  phenyl alanine, tyrosine ,isoleucine and lysine  Properties of amino acids A-Physical properties 1- Solubility:  - Soluble in polar solvents as water and ethanol due to the presence of charged groups except  cysteine and tyrosine.  - Soluble in strong acids and bases.  - Insoluble in alcohols except proline and hydroxyl proline.  2-Optical activity:  - All amino acids are optically active except glycine due to absence of asymmetric carbon atoms.  3-Optical isomer:  - The natural abundant are L-amino acids. D and L-forms are mirror image to each others.  4-Amphoterism and Zwitter ion  -Substances that can behave chemically both as acids and bases are said to be amphoteric. -So amphoteric substance can react as an acid when a base is added to it, or as a base when an  acid is added.  Zwitter ion or Dipolar ion: is form of amino acid carrying equal number of positive and  negative charges Isoelectric point: The pH point at which the amino acids are isotonic i.e. carry equal number  of (+) and (-) charges. At this point the amino acid dose not migrate toward anode or cathode and will be precipitated B-Chemical properties  1-Reactions due to carboxyl group.  2-Reactions due to amino group.  3-Reactions due to radical group.