2. 05.9.091 The Amino Acid as Zwitterion Has at least one acidic and one basic groupHas at least one acidic and one basic group The R-group may also contribute chargeThe R-group may also contribute charge The isoelectric point (pI) is the pH at which the overall electric charge is zeroThe isoelectric point (pI) is the pH at which the overall electric charge is zero

3. 05.9.092 Titration of an Amino Acid With two titratable groups, the pI is the arithmetic mean between the two pK a ’s

4. 05.9.093 Complex Titration Curves If the Sidechain is Charged

5. 05.9.094 Amino acids are buffers The NH 3 + group(s) and –CO 2 H group(s) react with OH The NH 2 group(s) and –CO 2 group(s) react with H +

6. 05.9.095 Functional Groups in Macromolecules Can Provide Buffering Capacity

7. 05.9.096 But Enzyme Catalysis is Also pH-sensitive Why might Why might this be? Think about it… Think about it…

8. 05.9.097 3-6

9. 05.9.098 Proteins Result from Peptide Bond Formation (a Condensation Reaction) * Peptide bond

10. 05.9.099 For example, a pentapeptide: Fig. 3-14 What is the approximate pI?

11. 05.9.0910 In Summary… There are 20 standard amino acids that cells incorporate into proteinThere are 20 standard amino acids that cells incorporate into protein They fall into five groups, but each has its own individual characteristicsThey fall into five groups, but each has its own individual characteristics These (and other amino acids) have many important roles in cellular metabolismThese (and other amino acids) have many important roles in cellular metabolism One essential role is as building blocks for the construction of peptides and proteinsOne essential role is as building blocks for the construction of peptides and proteins