Amino Acids and Proteins PDF

Dr. mohammed Lecture 1 of protein tawfiq by Pro. Dr. Sura A. Abdulsattar ( 2019-2020) Amino acids and proteins AMINO ACID: STRUCTURE AND CLASSIFICATION. Amino Acids are the building units of proteins. There are about 300 amino acids occur in nature....

Dr. mohammed Lecture 1 of protein tawfiq by Pro. Dr. Sura A. Abdulsattar ( 2019-2020) Amino acids and proteins AMINO ACID: STRUCTURE AND CLASSIFICATION. Amino Acids are the building units of proteins. There are about 300 amino acids occur in nature. Only 20 of them enter in proteins synthesis. Structure of amino acids: Each amino acid has 4 different groups attached to α- carbon (which is C-atom next to COOH). These 4 groups are: amino group, COOH group, Hydrogen atom and side Chain (R). At physiological pH (7.4), -COOH group is dissociated forming a negatively charged carboxylate ion (COO- ) and amino group is protonated forming positively charged ion (NH3 + ) forming Zwitter ion. Proline is an imino acid not amino acid. 1 Lecture 1 of protein by Pro. Dr. Sura A. Abdulsattar ( 2019-2020) Classification of Amino Acids: I. Classification by R group II. Chemical Classification III. Nutritional Classification IV. Metabolic Classification Classification according to polarity of side chain (R): A- Polar amino acids: in which R contains polar hydrophilic group so can forms hydrogen bond with H2O. In those amino acids, R may contain: 1- OH group : as in serine, threonine and tyrosine 2- - SH group : as in cysteine 3- amide group: as in glutamine and aspargine 4- NH2 group or nitrogen act as a base (basic amino acids ): as lysine, arginine and histidine 5- COOH group (acidic amino acids): as aspartic and glutamic. 2 Lecture 1 of protein by Pro. Dr. Sura A. Abdulsattar ( 2019-2020) Figure: Classification by R group B- Non polar amino acids: R is alkyl hydrophobic group which can’t enter in hydrogen bond formation. 9 amino acids are non-polar (glycine, alanine, valine, leucine, isoleucine, phenyl alanine, tryptophan, proline and methionine). The twenty common amino acids are often referred to using three-letter abbreviations. The structures, names, and abbreviations for the twenty common amino acids are shown below. Note that they are all α-amino acids. 3 Lecture 1 of protein by Pro. Dr. Sura A. Abdulsattar ( 2019-2020) 4 Lecture 1 of protein by Pro. Dr. Sura A. Abdulsattar ( 2019-2020) Each amino acid, aside from its name, has a three letter abbreviation and a one letter code. Nutritional Classification 1- Essential Amino Acids 10 in number can’t be synthesized in the body, essential to be taken in diet. Their deficiency affects growth, health and protein synthesis. 5 Lecture 1 of protein by Pro. Dr. Sura A. Abdulsattar ( 2019-2020) 2- Semi-essential formed in the body but not in sufficient amount for body requirements especially in children. Arginine and histidine are semi-essential 3- Non-essential can be synthesized in the body. Non Standard Amino Acids A nonstandard amino acid is an amino acid that occurs naturally in cells but do not participate in peptide synthesis. Many are metabolic intermediates eg. ornithine and citrulline are intermediates in urea biosynthesis Amino Acid Derivatives Chemical derivatives of amino acids also have important biological functions, eg. Catecholamines (below) lack the carboxylate of amino acids GABA & Dopamine are neurotransmitters. Histamine mediates parts of the immune response. Functions of Amino Acids Apart from being the monomeric constituents of proteins and peptides, amino acids serve variety of functions. (a) Some amino acids are converted to carbohydrates and are called as glucogenic amino acids. 6 Lecture 1 of protein by Pro. Dr. Sura A. Abdulsattar ( 2019-2020) (b) Specific amino acids give rise to specialized products, e.g. Tyrosine forms hormones such as thyroid hormones, (T3, T4), epinephrine and norepinephrine and a pigment called melanin. Tryptophan can synthesize a vitamin called niacin. Glycine, arginine and methionine synthesis creatine. Glycine and cysteine help in synthesize of Bile salts. Glutamate, cysteine and glycine synthesis glutathione. Histidine changes to histamine on decarboxylation. Serotonin is formed from tryptophan. Glycine is used for the synthesis of haem. Pyrimidines and purines use several amino acids for their synthesis such as aspartate and glutamine for pyrimidines and glycine, aspartic acid, Glutamine and serine for purine synthesis. (c) Some amino acids such as glycine and cysteine are used as detoxicants of specific substances. (d) Methionine acts as “active” methionine (S-adenosylmethionine) and transfers methyl group to various substances by transmethylation. (e) Cystine and methionine are sources of sulphur 7