Amino Acid Chemistry Lecture Notes PDF

Lippincott’s illustrated reviews Chapter 1, Page 1 Lecture 11 Amino Acid Chemistry 1  Specific Objectives By the end of this lecture students can able to: Differentiate between hydrophobic and hydrophilic amino acids Differentiate between Es...

Lippincott’s illustrated reviews Chapter 1, Page 1 Lecture 11 Amino Acid Chemistry 1  Specific Objectives By the end of this lecture students can able to: Differentiate between hydrophobic and hydrophilic amino acids Differentiate between Essential and nonessential amino acids Know optical properties of amino acids Explain the buffering action of amino acids 2  Amino acids Amino acid is a building blocks of protein Although more than 300 amino acids have been described in nature, only 20 are commonly found as constituents of mammalian proteins 3 4  Classification of amino acids According to the properties of their side chains, that is, whether they are: A. Amino acids with nonpolar side chains B. Amino acids with uncharged polar side chains C. Amino acids with acidic side chains D. Amino acids with basic side chains 5 A. Amino acids with nonpolar side chains These groups are hydrophobic and lipophilic. Each of these amino acids has a nonpolar side chain that does not gain or lose protons or participate in hydrogen or ionic bonds. As, Glycine, Alanine, Valine, Leucine, Isoleucine, Phenylalanine, Tryptophan, Methionine, Proline. The side chains of these amino acids can be thought of as 6 ‘oily’ or lipid-like, 7 Location of nonpolar amino acids in proteins: In proteins found in aqueous solutions- a polar environment- the side chains of the nonpolar amino acids tend to cluster together in the interior of the protein. However, for protein that are located in a hydrophobic environment, such as a membrane, the nonpolar R-groups are found on the outside surface of the protein, interacting with the lipid environment. 8 9 Proline: It is frequently referred to as imino acid. The unique geometry of proline contributes to the formation of the fibrous structure of collagen, and often interrupts the α-helices found in globular proteins. 10  B. Amino acids with uncharged polar side chains These groups are hydrophilic in nature. As, Serine, Threonine, Tyrosine, Asparagine, glutamine, Cysteine. These amino acids have zero net charge at neutral pH. By this group of amino acid protein can bind to oligosaccharide to form glycoprotein and bind to phosphate group to form phosphoprotein. 11 12  C. Amino acids with acidic side chains These groups are hydrophilic in nature. The amino acids aspartic and glutamic acid are proton donors. They are negatively charged at physiologic pH. 13 14  D. Amino acids with basic side chains These groups are hydrophilic in nature. The side chains with basic amino acids accept protons. Ex., arginine, lysine, histidine. At physiologic the side chains of lysine and arginine are fully ionized and positively charged. 15  Amino acids with basic chains 16  Clinical significance of amino acid properties In Sickle cell anemia there is HbS, where the 6th amino acid in the beta chain, the normal hydrophilic glutamic acid is replaced by hydrophobic valine. 17  Classification of amino acids according to nutritional requirement 1- Essential or indispensable: These groups are essential to be found in diet. Their carbon skeleton of these amino acids cannot be synthesized by human being. These include, Isoleucine, Leucine, Threonine, Lysine, Methionine, Phenylalanine, Tryptophan and Valine. 18 2- Nonessential or dispensable: These groups are not essential to be found in diet. Their carbon skeleton can be synthesized by metabolic pathways. These include, aspartic acid, glutamic acid, glycine, alanine, proline, serine, cystein, tyrosine, glutamine and asparagine. 3- Partially essential or semi-essential: Growing children require them in food, but they are not essential for the adult individual. These include, Histidine and Arginine. 19  Optical properties of amino acids Amino acids that have asymmetric center at the α-carbon can exist in two forms, designated D and L forms. They termed stereoisomers, optical isomers or enantiomers All amino acids found in proteins are of the L-configuration. D-amino acids are found in some antibiotics and in plant and bacterial cell walls. Glycine is the only amino acids that hasn't optical activity since it has symmetric carbon. 20 21  Acidic and basic properties of amino acids Amino acids in aqueous solution contain weakly acidic α-carboxyl groups and weakly basic α-amino groups. Thus, both free amino acids and some amino acids combined in peptide linkages can act as buffers. A buffer is a solution that resist change in pH following the addition of an acid or base. 22 Reference Book: Champe, P. C., Harvey, R. A. and Ferrier, D. R., 2005. Biochemistry “Lippincott’s Illustrated Reviews”, 5th or 6th Edition 23