V.Chapter Summary Amino acids are composed of when protonated can a-Carboxyl group a-Amino group Side chains Release H' (-COOH) -NH2) (20 different ones) pn9aoeg0pR2 grouped as Weak acids Henderso polar Buffering capacity ent e inte pH=pKa when [HA]=[A] 82 roteins,mos hus,the how the the role that the cid playV. Chapter Summary 11 Figure 1.14 Key concept map for amino acids. Deprotonated (COO– ) at physiologic pH On the outside of proteins that function in an aqueous environment and in the interior of membrane-associated proteins In the interior of proteins that function in an aqueous environment and on the surface of proteins (such as membrane proteins) that interact with lipids Weak acids Release H+ pH = pKa when [HA] = [A–] Buffering occurs ±1 pH unit of pKa Buffering capacity Maximal buffer when pH = pKa Protonated (NH3 + ) at physiologic pH described by grouped as and act as is is In proteins, most α-COO– and α-NH3 + of amino acids are combined through peptide bonds. Therefore, these groups are not available for chemical reaction. Thus, the chemical nature of the side chain determines the role that the amino acid plays in a protein, particularly . . . Nonpolar side chains Alanine Glycine Isoleucine Leucine Methionine Phenylalanine Proline Tryptophan Valine Uncharged polar side chains Asparagine Cysteine Glutamine Serine Threonine Tyrosine Henderson-Hasselbalch equation: [A–] [HA] Amino acids Side chain dissociates to –COO– at physiologic pH Side chain is pro￾tonated and generally has a positive charge at physiologic pH characterized by characterized by found found found found predicts predicts predicts predicts Acidic side chains Aspartic acid Glutamic acid Basic side chains Arginine Histidine Lysine α-Carboxyl group (–COOH) α-Amino group (–NH2) Side chains (20 different ones) are composed of when protonated can . . . how the protein folds into its native conformation. Structure of Proteins 2 pH = pKa + log 168397_P001-012.qxd7.0:02 Protein structure 5-20-04 2010.4.4 9:45 AM Page 11