Lecture 3 Kumar Amino acids and AA Transport 1 Alpha carbon

Mirror Images of Amino Acid   Mirror image Same chemical properties Stereo isomers 2

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pKa = 1.8~2.4 pKa = 3.9~4.3 pKa = 6.0 pKa = 8.3 pKa = 10 pKa = 8.8~11 pKa = 10~ H+H+ + H+H+ + H+H+ + H+H+ + H+H+ + H+H+ + H+H+ -COOH-COO - aa -COOH-COO - RR -Imidazole·H + -Imidazole His -SH-S - Cys -OH-O - Tyr -NH 3 + -NH 2 aa -NH 3 + -NH 2 RR Smaller pKa releases proton easier Residues on amino acids can release or accept protons pKa of Amino Acid Residues Only His has the residue with a neutral pKa (imidazole) pKa of  carboxylic or amino groups is lower than pKa of the R residues 10

COOH NH 2 H + COO - R - C - HR - C - H NH 2 H + R - C - HR - C - H COO - NH 2 R - C - HR - C - H Acidic environmentNeutral environmentAlkaline environment +1 0 pK 1 ~ 2 pK 2 ~ 9 Isoelectric point

Amino acids -COOH-NH 2 -R GlyG AlaA ValV LeuL IleI SerS ThrT MetM PheF TrpW AsnN GlnQ ProP AspD GluE HisH CysC TyrY LysK ArgR pK1pK1 pK1pK1 pK2pK2 pK2pK2 pK3pK3 [OH - ] pH pI pI ? pK 1 + pK 2 2 three pKa two pKa ? ? p K a of Amino Acids 12

HOOC-CH 2 -C-COOH NH3+NH3+ H HOOC-CH 2 -C-COO - NH3+NH3+ H - OOC-CH 2 -C-COO - NH3+NH3+ H NH 2 H pK 1 = 2.1 pK 2 = 3.9 pK 3 = = 3.0 first second third Isoelectric point Isoelectric point is the average of the two pKa flanking the zero net-charged form pK1pK1 pK2pK2 pK3pK3 Aspartic acid [OH] 13

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Learning objectives for lecture 3 Know the structures of simple sugars Understand the difference between D and L forms Know the structural differences between isomers and enantiomers Know the basis for denoting amino acids as neutral, acid or basic Understand how pKa of amino acids are defined and how are they used to calculate isoelectric points Know what makes amino acids good buffers at many pH values Understand how amino acids are generally transported from intestinal lumen. 17