Contenu connexe


Amino acids.pptx

  1. Chemistry of Amino acids 1
  2. Major concepts 1.Define amino acids &with basic structure. 2. Classify amino acids. 3. standard & the nonstandard amino acids 4. Learn three-letter abbreviations, and their single-letter code & the occurrence of amino acids 5. Verify the Specific Roles of Some Side Chains 6. List essential amino acids, semi-essential amino acids and non- essential amino acids 7. List general functions of amino acids. 8. Describe physical and chemical properties of amino acids 2
  3. Amino acids • The term protein is derived from a Greek word proteios, meaning first place. • Berzelius ( Swedish chemist ) suggested the name proteins to the group of organic compounds that are utmost important to life. • proteins are a high molecular weight . nitrogenous macromolecules composed of many amino acids. • Protein can be broken down into smaller monomers called as amino acids on hydrolysis. These • 20 amino acids are used for the synthesis of proteins by the mRNA-directed process that occurs on ribosomes 3
  4. Amino acids… • There are hundreds of amino acids in nature, but only the 20 standard amino acids in different sequences and numbers are used as building blocks of proteins in humans • So an indefinite number of proteins can be formed and occur in nature. • Thus proteins are the unbranched polymers of L- α- amino acids. • Amino acids are a group of organic compounds containing two functional groups – amino and carboxyl 4
  5. AMINO ACIDS General Structure • All amino acids have a central carbon atom attached to a carboxyl group, an amino group, and a hydrogen atom, as shown in Figure I. asymmetric carbon atom NB. the amino group is Basic and -COOH is acidic. • R is called a side chain and can be a hydrogen, aliphatic, aromatic or heterocyclic group • Each amino acid has an amino group –NH2, a carboxylic acid group– COOH and a hydrogen atom each attached to carbon located next to the – COOH group. • The amino acids differ from one another only in the chemical nature of the side chain (R). 5
  6. Amino acids… The 20 amino acids in proteins encoded by DNA are shown in; each can be designated by a three-letter abbreviation and one-letter symbol 6 Table 1
  7. 7
  8. Amino acids… Classify amino acids 1. Based on nature of side chain 1.1 Amino acids with hydrophobic properties Non-polar amino acids: glycin, Alanine , Valine ,Leucine isoleucine , proline & some aromatic side chains It contains indole ring 8
  9. Amino acids… 1.2. Amino acids with Hydrophilic properties on the side chains Polar uncharged (neutral) amino acids: Serine, Threonine, Tyrosine (OH containing alcohol side group) Methionine , Asparagine & Glutamine Note: Methionine can be considered nonpolar or polar because it has a sulfur in it. . 9
  10. Positively charged R-group Basic amino acids: Arginine: It contain guanidino group and is monocarboxylic acid. It is an intermediate in urea cycle and is precursor for nitric oxide. Lysine: It is an essential amino acid. These are strongly polar. Histidine :It contains imidazole ring. 10
  11. Negatively charged R-group Acidic amino acids: • Aspartic acid, Glutamic acid • Are capable of forming ionic bonds and involved in chemical reactions Proline Chemically, proline is α-imino acid (not α-amino acid). Its side chain pyrrolidine ring includes both α-amino group and side chain methyl group. It forces a bend in a polypeptide chain 11
  12. Specific Roles of Some Side Chains • Properties of the amino acids are determined by the functional groups in their side chains . • In addition, the functional groups may perform some specific roles, thereby imparting special properties to proteins. 1. The hydroxyl groups of serine ,threonine and tyrosine & even cysteine is exist at the catalytic sites of certain kinds of enzyme e.g. the enzymes that regulate energy metabolism and fuel storage in the body. 12
  13. Specific Roles of Some Side Chains… They can be phosphorylated, hence their charge changes from neutral to negative, which changes the shape and function of the enzyme proteins. Thus, these (hydroxyl) groups are important in the regulation of activities of these enzymes The –OH groups of serine and threonine also similarly form linkage with reducing end of oligosaccharide, called O- glycosidic linkage 2. Thiol group of a cysteine can form a disulphide bond with another cysteine through the oxidation of the two thiol groups The dimeric compound so formed, called cystine, is important in cross-linking the adjacent polypeptide chains. . 13
  14. Specific Roles of Some Side Chains …. 3. Amide bearing side chain of asparagine forms covalent linkage with oligosaccharide unit (called N- glycosidiclinkage) in glycoproteins 4. Aromatic side chains are responsible for the ultraviolet absorption of most proteins, which have absorption maxima between 275 nm and 285 nm. Tryptophan has greater absorption in this region than the other two aromatic amino acids. Since nearly all proteins contain aromatic amino acids, the amount of light absorbed at 280 nm by a protein is used as an indirect measure of protein concentration. 14
  15. Specific Roles of Some Side Chains …. 5. Imidazole group of histidine is important in the buffering activities of proteins. 6. Cyclic pyrrolidine group of proline introduces bend in the peptide chain. 8. Butylammonium side chain of lysine binds with the co- enzymes pyridoxal phosphate and biotin. 9. R group of glycine provides little stearic hindrance because of its small size, so that proteins can bend or rotate easily wherever glycine forms part of their structure 15
  16. Amino acids… 2. Based on nutritional requirement a) Essential amino acids: Essential amino acids cannot be endogenously synthesized, and therefore, their dietary intake is essential • Methionine, Arginine, Threonine, Tryptophan, Valine, Leucine , Isoleucine,, Phenylalanine, Histidine, Lysine b)Non-essential amino acids: The nonessential amino acids, on the other hand, can be endo- genously synthesized in adequate quantities so as to meet the body’s requirements. Glutamic acid, Aspartic acid, Glutamine, Asparagine, Glycine, Alanine 16
  17. Amino acids… (c) Semi-essential amino acids: These are growth promoting factors since they are not synthesized in sufficient quantity during growth of children, pregnancy and lactating women.  include arginine and histidine But they are not essential for the adult individual. Occurrence of amino acids: All the standard amino acids mentioned above occur in almost all proteins Cereals are rich in acidic amino acids Asp and Glu while collagen is rich in basic amino acids , proline and hydroxyproline 17
  18. 18
  19. Amino acids… 3. Based on metabolic fate • Glucogenic amino acids: • Ketogenic amino acids • Both glucogenic and ketogenic:I. Glucogenic Amino Acids • Those amino acids in which their carbon skeleton gets degraded to pyrurate, α ketoglutarate, succinyl CoA, fumrate and oxaloacetate and then converted to Glucose and Glycogen, are called as Glucogenic amino acids. • These include:- Glutamic acid, Aspartic acid, Glutamine, Asparagine, Glycine, Alanine , cysteine, tyrosine, Arginine, Isoleucine, 19
  20. II. Ketogenic Amino Acids • Those amino acids in which their carbon skeleton is degraded to Acetoacetyl CoA, or acetyl CoA. • then converted to acetone and β-hydroxy butyrate which are the main ketone bodies are called ketogenic amino acids. • These includes:- leucine, and lysine. • These amino acids have ability to form ketone bodies which is particularly evident in untreated diabetes mellitus in which large amounts of ketone bodies are produced by the liver (i.e. not only from fatty acids but also from ketogenic amino acids) • Degradation of Leucine which is an exclusively ketogenic amino acid makes a substantial contribution to ketone bodies during starvation. 20
  21. III. Ketogenic and glucogenic Amino Acids The division between ketogenic and glucogenic amino acids is not sharp for amino acids • (Tryptophan, phenylalanine, tyrosine and Isoleucine are both ketogenic and glucogenic) 21
  22. Fig . Ketogenic, Glucogenic and Glucogenic-Ketogenic amino acids 22
  23. PROPERTIES OF AMINO ACIDs A) Aminoacid are amphoteric compounds, as they contain both acidic (COOH) and basic (NH2) groups. -NH2and-COOH groups of amino acids are ionizable groups. • Further, charged polar side chains of few amino acids also ionise. • Depending on the pH of the solution these groups act as proton donors (acids) or proton acceptors(bases). • On the acidic side of its pI amino acids exist as a cation by accepting a proton and on alkaline as anion by donating a proton. • This property is called as amphoteric and therefore amino acids are called as ampholytes. 23
  24. PROPERTIES OF AMINO ACIDs • At a specific pH the a/ acid carries both equal number charges and exists as dipolar ion or “Zwitterion”. • At this point the net charge on it is zero, i.e. positive charges and negative charges on the protein/amino acid molecule equalizes. all the groups are ionized so , the charges will cancel each other. 24
  25. PROPERTIES OF AMINO ACIDs… • The net charge on an amino acid—the algebraic sum of all the positively and negatively charged groups present—depends upon; – the pKa Values of its functional groups and on – the pH of the surrounding medium. • Altering the charge on amino acids and their derivatives by varying the pH facilitates the physical separation of amino acids, peptides, and proteins 25
  26. PROPERTIES OF AMINO ACIDS B. Isomerism: Two types of isomerism are shown by amino acids basically due to the presence of asymmetric carbon atom Glycine has no asymmetric carbon atom in its structure hence is optically inactive. (a) Stereoisomerism: All amino acids except glycine exist in D and L isomers. In D-amino acids – NH2 group is on the right hand while in L- amino acids it is oriented to the left. Natural proteins of animals and plants generally contain L-amino acids but D- amino acids occur in bacteria 26
  27. PROPERTIES OF AMINO ACIDS… (b) Optical Isomerism: All amino acids except glycine have asymmetric carbon atom. • all but glycine exhibit ‘optical’ activities and rotate the plane of plane polarized light and exist as dextrorotatory (d) or laevorotatory (l) isomers optical activity depends on the pH and side chain 27
  28. PROPERTIES OF AMINO ACIDS… C. Physical Properties: They are colourless, crystalline substances, more soluble in water than in polar solvents. Tyrosine is soluble in hot water. They have high melting point usually more than 200°C. They have a high dielectric constant. They possess a large dipole moment. 28
  29. PROPERTIES OF AMINO ACIDS… D. Chemical Properties I. Due to Carboxylic (—COOH) Group 1. Formation of esters: They can form esters with alcohols 2. Formation of amines by decarboxylation: Action of specific amino acid decarboxylases, COOH group and changes the amino acid into its amine Fig. 6.2:Formation of amine (decarboxylation) 29
  30. II. Properties Due to Amino (–NH2) Group 1. Salt formation with acids: The basic amino group reacts with mineral acids such as HCl to form salts like hydrochlorides (Fig. 6.4). Fig. 6.4: Salt formation of amino acid 30
  31. III. Properties of Amino acids due to Both NH2and COOH Groups • In addition to the property of reacting with both cation and anion, the amino acids form chelated, co-ordination complexes with certain heavy metals and other ions. • These include Cu++, Co++, Mn++ and Ca++ An example of chelated complex of Ca++ and glycine is given in below Fig. : Chelation of glycine with Ca++ Name of compound is calcium diglycinate 31
  32. CLINICAL APPLICATION • Chelates are non-ionic and therefore amino acids may be used to remove calcium from bones and teeth • It is possible that the amino acids resulting from the breakdown of enamel and dentine could in this way form soluble calcium complexes thereby causing a loss of calcium and the development of caries 32
  33. Non-standard Amino Acids • In addition to the 20 primary amino acids found in protein, some “non-standard amino acids” are present in small amounts in specialized structures. • They may also occur in free or combined states and independently play a variety of biological roles. 33
  34. Non-standard Amino Acids… 1. Non-standard amino acids found in proteins: These amino acids are produced by specific modification of a primary amino acid residue after the polypeptide chain has been synthesized. They are essential for the function of the protein. Some examples of modified amino acids are: hydroxylysine and hydroxyproline in collagen 34
  35. Non-standard Amino Acids… 2. Non-standard amino acids not found in proteins: Termed non-protein amino acids, they occur in free state in cells e.g. Citrulline that are intermediates of urea cycle. The most abundant amino acid in human organism, taurine, also occurs in a free state in bile and plays an important role in fat digestion and absorption. 35
  36. Non-standard Amino Acids.. 3. Biologically active amino acids: These amino acids are used for functions other than protein synthesis. Examples: (a) as chemical messengers for communication between cells, e.g. glycine, dopamine (tyrosine derivative), ϒ-aminobutyric acid (glutamate derivative), (b) as local mediator of allergic reactions, e.g. histamine (histidine derivative), and (c) as a hormone, e.g. thyroxine (another tyrosine derivative) 36
  37. New Amino Acids In addition to 20 L-amino acids that take part in protein synthesis, recently two more new amino acids described. They are: A . Selenocysteine - 21st amino acids B. Pyrrolysine - 22nd amino acid A. Selenocysteine : is recently introduced as 21st amino acid. • Selenocysteine occurs at the “active site” of several enzymes e.g :Include: Thioredoxin reductase Glutathione peroxidase which scavenges peroxides B. Pyrrolysine : the 22nd Amino Acid  Recently it has been claimed as 22nd amino acid by some scientists  The STOP codon UAG can code for pyrrolysine 37
  38. THANKS!! 38