Il.Structure of the Amino Acids UNCHARGED POLAR SIDE CHAINS C00HK1=22 COOH COOH 'HgN-C-H 'HgN-C-H pK2=9. H-C-OH H-C-OH 0HpK=10.1 Serine Threonine Tyrosine COOH COOH 'HgN-C-H 'HgN-C-H Co0H·pK,=17 % CHa pk=08 SH--pK2=8.3 Asparagine Glutamine Cysteine ACIDIC SIDE CHAINS pK,=21 COOH pK=98 pK3=9.7--HgN-C CH, CH, pK2=39 pk2=4.3 Aspartic acid BASIC SIDE CHAINS pK=22 pK=92 pK2=92 COOH pK29.0 COOH 'HjN-C -H 'HgN-C-H CH2 CH2 CH2 K2=6.0 -pK3=10.5 pK3=125 Histidine Lysine Arginine Figure 1.3 Stiosa8atsea8de8月rsn0eroHadyetnsacordng1ohechargeandpoaiyC +H3N COOH H CH2 O OH CH2 C ACIDIC SIDE CHAINS pK1 = 2.1 pK2 = 4.3 C pK3 = 9.7 +H3N COOH CH2 Aspartic acid O OH C pK2 = 3.9 pK3 = 9.8 C +H3N CH2 CH2 BASIC SIDE CHAINS C +H3N CH2 pK3 = 9.2 CH NH C H +HN C pK2 = 6.0 CH2 CH2 NH3 + pK3 = 10.5 C +H3N CH2 CH2 CH2 N pK3 = 12.5 H C NH2 NH2 + C +H3N H CH2 O NH2 C +H3N CH2 Asparagine Glutamine O NH2 CH2 C UNCHARGED POLAR SIDE CHAINS C +H3N COOH H CH2 Cysteine SH pK3 = 10.8 pK2 = 8.3 pK1 = 1.7 C +H3N COOH H Serine C H H OH C +H3N Threonine C CH3 H OH C +H3N COOH H CH2 Tyrosine OH pK2 = 9.1 pK3 = 10.1 pK1 = 2.2 C CH2 C ACIDIC SIDE CHAINS H O C pK2 = 3.9 C +H3N COOH H CH2 CH2 BASIC SIDE CHAINS pK1 = 2.2 C +H3N COOH H CH2 Histidine pK3 = 9.2 CH NH C H +HN C pK1 = 1.8 pK2 = 6.0 CH2 CH2 NH3 + pK3 = 10.5 Lysine pK2 = 9.0 C +H3N COOH H CH2 CH2 CH2 N pK3 = 12.5 Arginine H C NH2 NH2 + C +H3N COOH C +H3N COOH H CH2 O NH2 CH2 C UNCHARGED POLAR SIDE CHAINS CH2 C C OH C +H3N COOH H C CH3 H OH +H3N Tyrosine pK3 = 10.1 C Figure 1.3 Classification of the 20 amino acids commonly found in proteins, according to the charge and polarity of their side chains at acidic pH (continued from Figure 1.2). pK2 = 9.2 II. Structure of the Amino Acids 3 168397_P001-012.qxd7.0:02 Protein structure 5-20-04 2010.4.4 9:45 AM Page 3