Functions of Proteins 20.2 Amino Acids 20.3 Amino Acids as Acids and Bases Chapter 20 Amino Acids and Proteins

2 Functions of Proteins Proteins perform many different functions.

3 Amino acids: Are the building blocks of proteins. Contain a carboxylic acid group and an amino group on the alpha (  ) carbon. Have different side groups R that give each amino acid unique characteristics. Rside chain | H 2 N—C —COOH General structure of an |  -amino acid H Amino Acids

4 Nonpolar Amino Acids Amino acids are classified as nonpolar when the R groups are H, alkyl, or aromatic.

5 Polar Amino Acids Amino acids are classified as polar when the R groups are alcohols, thiols, or amides.

6 Acidic and Basic Amino Acids Amino acids are classified as acidic when the R group is a carboxylic acid. Amino acids are classified as basic when the R group is an amine.

7 Fischer Projections of Amino Acids All amino acids except glycine are chiral. Amino acids have stereoisomers. In biological systems, only L amino acids are used in proteins. L-Alanine D-Alanine L-Cysteine D-Cysteine

8 Both the –NH 2 and the –COOH groups in an amino acid undergo ionization in water. A zwitterion forms that has + and – charge. At the isoelectric point (pI), the + and – charges in the zwitterion are equal. + NH 2 —CH 2 —COOH H 3 N—CH 2 —COO – Glycine Zwitterion of glycine Zwitterions

9 In solutions more basic than the pI, the —NH 3 + in the amino acid donates a proton. + OH – H 3 N—CH 2 —COO – H 2 N—CH 2 —COO – ZwitterionNegative ion at pI Higher pH Amino Acids as Acids

10 In solution more acidic than the pI, the COO - in the amino acid accepts a proton. + H + + H 3 N—CH 2 —COO – H 3 N—CH 2 —COOH Zwitterion Positive ion at pI Low pH Amino Acids as Bases

11 pH and Ionization Acidic amino acids such as aspartic acid have a second carboxyl group that can donate and accept protons. The pI for aspartic acid occurs at a pH of 2.8.

12 Electrophoresis Electrophoresis separates amino acids according to their isoelectric points. 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 will not migrate in either direction.

13 Separation of Amino Acids When electrophoresis is completed, the amino acids are identified as separate bands on the filter paper or thin layer plate.