Chapter 3 Amino Acids PDF

Chapter 3 Amino Acids  Learning Objectives Be able to describe the structure of an amino acid and how it can vary with changes in pH. Be able to explain how the R groups of amino acids differ. Be able to recognize common functional groups seen in amino acids. Be able to cla...

Chapter 3 Amino Acids  Learning Objectives Be able to describe the structure of an amino acid and how it can vary with changes in pH. Be able to explain how the R groups of amino acids differ. Be able to recognize common functional groups seen in amino acids. Be able to classify the amino acids into four groups and know which ones belong to each group. Be able to identify and provide the name of all 20 amino acids. Be able to indicate the 7 amino acids with ionizable side chains. Be able to describe the difference between essential and nonessential amino acids and know which amino acids belong to each group.  3.1 Proteins Are Built from a Repertoire of 20 Amino Acids α-amino acids α carbon in center with four bonds to: Amino group Carboxyl group Hydrogen atom R group Have two charged groups at neutral pH Dipolar ions or zwitterions Protonated amino group (-NH3+) Deprotonated carboxyl group (-COO-) Change in pH changes charges on amino acids Acidic pH – (-COOH) Basic pH – (-NH2) 3.2 Amino Acids Contain a Wide Array of Functional Groups Variations in R group Functional groups include: based on: Alcohols Size Thiols Shape Thioethers Charge Carboxylic acids Hydrogen-bonding capacity Carboxamides Hydrophobic character Variety of basic groups Chemical reactivity 3.2 Amino Acids Contain a Wide Array of Functional Groups Classifications of amino acids based on chemical characteristics 1. Hydrophobic amino acids with nonpolar R groups 2. Polar amino acids with neutral R groups but the charge is not evenly distributed 3. Positively charged amino acids with R groups that have a positive charge at physiological pH (7.4) 4. Negatively charged amino acids with R groups that have a negative charge at physiological pH 3.2 Amino Acids Contain a Wide Array of Functional Groups Hydrophobic Amino Acids Have Mainly Hydrocarbon Side Chains Glycine – single hydrogen atom (-H) Alanine – methyl group (-CH3) Valine Leucine Isoleucine Methionine – thioether Proline – aliphatic side chain bonded to α carbon and nitrogen of amino group Phenylalanine – phenyl ring in place of one of the methyl hydrogen atoms of alanine Tryptophan – indole ring joined to methylene (-CH2-) Indole is two fused rings and an NH group 3.2 Amino Acids Contain a Wide Array of Functional Groups Polar Amino Acids Have Side Chains That Contain an Electronegative Atom Neutral overall, but R group contains electronegative atom Serine – like alanine, but hydroxyl group in place of a methyl hydrogen atom Threonine – like valine, but with a hydroxyl group in place of a methyl group Tyrosine – like phenylalanine, but with hydroxyl group attached to aromatic ring Cysteine – similar to serine, but with thiol (-SH) in place of hydroxyl group Asparagine – contain carboxamide Glutamine – contain carboxamide 3.2 Amino Acids Contain a Wide Array of Functional Groups Positively Charged Amino Acids Are Hydrophilic Lysine – ends in amino group Arginine – ends in guanidinium group Histidine – ends in imidazole group (aromatic ring) Charged or uncharged near neutral pH Often in active site of enzymes where it can bind and release protons during chemical reactions 3.2 Amino Acids Contain a Wide Array of Functional Groups Negatively Charged Amino Acids Have Acidic Side Chains Acidic side chains are usually negatively charged Aspartic acid (aspartate) Glutamic acid (glutamate) Both have carboxylic acid groups in place of carboxamide groups in asparagine and glutamine, respectively 3.2 Amino Acids Contain a Wide Array of Functional Groups The Ionizable Side Chains Enhance Reactivity and Bonding 7 amino acids with ionizable side chains 1. Tyrosine 2. Cysteine 3. Arginine 4. Lysine 5. Histidine 6. Aspartic acid 7. Glutamic acid Can form ionic bonds Acid-base catalysis – able to donate and accept protons  3.3 Essential Amino Acids Must Be Obtained from the Diet Essential amino acids – cannot be made by the body, so must be obtained in the diet Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine Nonessential amino acids – can be made by the body Alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, tyrosine