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The Amino Acid as Zwitterion • Has at least one acidic and one basic group • The R-group may also contribute charge • The isoelectric point (pI) is the pH at which the overall electric charge is zero 1 05.9.09 Titration of an Amino Acid With two titratable groups, the pI is the arithmetic mean between the two pKa’s 2 05.9.09 Complex Titration Curves If the Sidechain is Charged 3 05.9.09 Amino acids are buffers The NH2 group(s) and –CO2 group(s) react with H+ The NH3+ group(s) and –CO2H group(s) react with OH 4 05.9.09 Functional Groups in Macromolecules Can Provide Buffering Capacity 5 05.9.09 But Enzyme Catalysis is Also pH-sensitive Why might this be? Think about it… 6 05.9.09 3-6 7 05.9.09 Proteins Result from Peptide Bond * Formation (a Condensation Reaction) 8 Peptide bond 05.9.09 For example, a pentapeptide: What is the approximate pI? 9 Fig. 3-14 05.9.09 In Summary… • There are 20 standard amino acids that cells incorporate into protein • They fall into five groups, but each has its own individual characteristics • These (and other amino acids) have many important roles in cellular metabolism • One essential role is as building blocks for the construction of peptides and proteins 10 05.9.09
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