Chapter 16 Amino Acids, Proteins, and Enzymes

1. Chapter 16 Amino Acids, Proteins, and Enzymes 16.1 Functions of Proteins 16.2 Amino Acids 16.3 Amino Acids as Acids and Bases

2. Functions of Proteins Proteins perform many different functions in the body.

3. Amino Acids Amino acids • are the building blocks of proteins. • contain a carboxylic acid group and an amino group on the alpha () carbon. • are ionized in solution. • each contain a different side group (R). R side chainR │ + │ H2N—C —COOHH3N—C —COO− │ │ H H ionized form

4. Examples of Amino Acids H + │ H3N—C—COO− │ H glycine CH3 +│ H3N—C—COO− │ H alanine

5. Types of Amino Acids Amino acids are classified as • nonpolar (hydrophobic) with hydrocarbon side chains. • polar (hydrophilic) with polar or ionic side chains. • acidic (hydrophilic) with acidic side chains. • basic (hydrophilic) with –NH2 side chains. Nonpolar Polar Acidic Basic

6. Nonpolar Amino Acids An amino acid is nonpolar when the R group is H, alkyl, or aromatic.

7. Polar Amino Acids An amino acid is polar when the R group is an alcohol, thiol, or amide.

8. Acidic and Basic Amino Acids An amino acid is • acidic when the R group is a carboxylic acid. • basic when the R group is an amine.

9. Learning Check Identify each as (1) polar or (2) nonpolar. + A. H3N–CH2–COO− (Glycine) CH3 | CH–OH + │ B. H3N–CH–COO − (Threonine)

10. Solution Identify each as (1) polar or (2) nonpolar. + A. H3N–CH2–COO− (Glycine) (2) nonpolar CH3 | CH–OH + │ B. H3N–CH–COO − (Threonine) (1) polar

11. C O O H C O O H C O O H C O O H H N H H N H H N H H N H 2 2 2 2 C H C H C H S H C H S H 3 3 2 2 Fischer Projections of Amino Acids Amino acids • are chiral except glycine. • have Fischer projections that are stereoisomers. • that are L are the only amino acids used in proteins. L-Alanine D-Alanine L-Cysteine D-Cysteine

12. Zwitterions A zwitterion • has charged−NH3+ and COO– groups. • forms when both the –NH2 and the –COOH groups in an amino acid ionize in water. • has equal + and – charges at the isoelectric point (pI). O O ║+║ NH2—CH2—C—OH H3N—CH2—C—O– glycine zwitterion of glycine

13. Amino Acids as Acids In solutions more basic than the pI, • the —NH3+ in the amino acid donates a proton. + OH– H3N—CH2—COO–H2N—CH2—COO– zwitterion Negative ion at pI pH > pI Charge: 0 Charge: 1-

14. Amino Acids as Bases In solution more acidic than the pI, • the COO- in the amino acid accepts a proton. + H+ + H3N—CH2—COO–H3N—CH2—COOH zwitterion Positive ion at pI pH< pI Charge: 0 Charge: 1+

15. pH and ionization H+ OH– + + H3N–CH2–COOH H3N–CH2–COO–H2N–CH2–COO– positive ion zwitterion negative ion low pH pI high pH

16. Separation of Amino Acids When an electric current is used to separate a mixture of amino acids • the positively charged amino acids move towards the negative electrode. • the negatively charged amino acids move toward the positive electrode. • an amino acid at its pI does not migrate. • the amino acids are identified as separate bands on the filter paper or thin layer plate.

17. Separation of Amino Acids With an electric current, a mixture of lysine, aspartate, and valine are separated.

18. Learning Check CH3 CH3 +|| H3N—CH—COOH H2N—CH—COO– (1) (2) Which structure represents: A. Alanine at a pH above its pI? B. Alanine at a pH below its pI?

19. Solution CH3 CH3 +|| H3N—CH—COOH H2N—CH—COO– (1) (2) Which structure represents: A. Alanine at a pH above its pI? (2) B. Alanine at a pH below its pI? (1)