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The Amino Acid as ZwitterionThe Amino Acid as Zwitterion

• Has at least one acidic and Has at least one acidic and one basic groupone basic group

• The R-group may also The R-group may also contribute chargecontribute charge

• The The isoelectric pointisoelectric point (pI) (pI) is the pH at which the overall is the pH at which the overall electric charge is zeroelectric charge is zero

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Titration of an Amino AcidTitration of an Amino Acid

With two titratable groups,the pI is the arithmetic mean between the two pKa’s

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Complex Titration Curves If the Complex Titration Curves If the Sidechain is ChargedSidechain is Charged

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Amino acids are buffersAmino acids are buffers

The NH3+ group(s) and –CO2H group(s) react with OH

The NH2 group(s) and –CO2 group(s) react with H+

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Functional Groups in Macromolecules Functional Groups in Macromolecules Can Provide Buffering CapacityCan Provide Buffering Capacity

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But Enzyme Catalysis is AlsoBut Enzyme Catalysis is AlsopH-sensitivepH-sensitive

Why might Why might

this be?this be?

Think about it…Think about it…

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3-6

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Proteins Result from Peptide Bond Proteins Result from Peptide Bond Formation (a Condensation Reaction)Formation (a Condensation Reaction)

*

Peptide bond

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For example, a pentapeptide:For example, a pentapeptide:

Fig. 3-14What is the approximate pI?

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In Summary…In Summary…

• There are 20 standard amino acids that There are 20 standard amino acids that cells incorporate into proteincells incorporate into protein

• They fall into five groups, but each has its They fall into five groups, but each has its own individual characteristicsown individual characteristics

• These (and other amino acids) have many These (and other amino acids) have many important roles in cellular metabolismimportant roles in cellular metabolism

• One essential role is as building blocks for One essential role is as building blocks for the construction of peptides and proteinsthe construction of peptides and proteins