19.1 Proteins and Amino Acids General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake Chapter 19 Amino Acids and Proteins 19.1 Proteins and Amino Acids Lectures © 2013 Pearson Education, Inc.

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

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. There are 20 common amino acids found in human proteins.

Ionization of Amino Acids At the pH of most bodily fluids, amino acids are ionized. The carboxylic acid group (−COOH) donates an H+ to the amino group (−NH2) to give a carboxylate (−COO−) and and ammonium group (−NH3+). The ionized form is called a zwitterion.

Classification of Amino Acids Amino acids are classified according to their R groups.

Classification of Amino Acids continued… Amino acids are classified as nonpolar with hydrocarbon side chains (hydrophobic). polar (neutral) with polar side chains (hydrophilic). polar with charged R groups acidic side chains (negatively charged) while basic chains are positively charged. Nonpolar Polar (neutral) Acidic Basic

Nonpolar Amino Acids A nonpolar amino acid has an R group that is H, an alkyl group, or aromatic. The R group is always neutral.

Polar (Neutral) Amino Acids A polar amino acid has an R group that is either an alcohol, thiol, or amide.

Amino Acids with Neutral R Groups: Polar (Acidic) and Polar (Basic) An amino acid is acidic with a carboxyl R group (COO–). basic with an amino R group (NH3+).

Amino Acid Stereoisomers: Fischer Projections of Amino Acids are chiral except glycine, which has two H atoms attached to the alpha carbon atom. have Fischer projections that are stereoisomers. that are L isomers are used in proteins.

Amino Acids as Zwitterions A zwitterion has an equal number of —NH3+ and COO– groups. The pI values for nonpolar and polar neutral amino acids are from pH 5.1 to 6.3.

Isoelectric Point (pI) The isoelectric points (pI) are the pH at which zwitterions have an overall zero charge. of nonpolar and polar (neutral) amino acids exist at pH values from 5.1 to 6.3.

Zwitterions in Acidic Solutions In solutions that are more acidic than the pI, the COO– in the zwitterion accepts a proton. the amino acid has a positive charge. Alanine, with a pI of 6.0, has a 1+ charge in solutions that have a pH below pH 6.0.

Zwitterions in Basic Solutions In solutions that are more basic than the pI, the NH3+ in the zwitterion loses a proton. the amino acid has a negative charge. Glycine, with a pI of 6.0, has a 1– charge in solutions that have a pH above pH 6.0.

Summary of pH, pI, and Ionization

Ionized Forms of Polar (Acidic) and Polar (Basic) Amino Acids Polar (acidic) and polar (basic) amino acids also ionize the COO and NH3+ in their polar R groups. Zwitterions of polar (acidic) amino acids exist at pH values from 2.8 to 3.2. Zwitterions of polar (basic) amino acids exist at pH values from 7.6 to 10.8.

Zwitterions of Aspartic Acid Aspartic acid, a polar (acidic) amino acid, has a pI of 2.8. forms a zwitterion at pH 2.8. forms negative ions with charges 1– and 2– at pH values greater than pH 2.8.

Electrophoresis: Separation of Amino Acids In electrophoresis, an electric current is used to separate a mixture of amino acids, and the positively charged amino acids move toward 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.

Electrophoresis With an electric current, a mixture of lysine, aspartate, and valine are separated.

The Peptide Bond A peptide bond is an amide bond. forms between the carboxyl group of one amino acid and the amino group of the next amino acid. contains an N (free H3N+) terminal written on the left. contains a C (free COO–) terminal written on the right.

Formation of a Dipeptide

Naming Dipeptides A dipeptide is named with a -yl ending for the N-terminal (free H3N+) amino acid. the full amino acid name of the free carboxyl group (COO–) at the C-terminal end.

Guide to Drawing a Peptide

Sample Problem 19.4 Drawing a Peptide Draw the condensed structural formula for the tripeptide Gly-Ser-Met, GSM. Solution Analyze the Problem The name Gly-Ser-Met gives the order of the amino acids. The N-terminal amino acid drawn on the left is glycine, the middle amino acid is serine, and the C-terminal amino acid drawn on the right is methionine. We can obtain the R groups from Table 19.2. Step 1 Draw the structures for each amino acid in the peptide, starting with the N-terminal amino acid on the left. 24

Sample Problem 19.4 Drawing a Peptide Continued Step 2 Remove the O atom from the carboxylate group of the N-terminal amino acid and two H atoms from the adjacent amino acid. Repeat this process until the C-terminal amino acid is reached. 25

Sample Problem 19.4 Drawing a Peptide Continued Step 3 Connect the remaining parts of the amino acids by forming amide (peptide) bonds. Study Guide 19.4 Draw the condensed structural formula for the dipeptide Phe-Thr, part of the peptide glucagon, which increases blood glucose levels. 26

Primary Structure of Proteins The primary structure of a protein is the particular sequence of amino acids. the backbone of a peptide chain or protein.

Primary Structure of Insulin was the first protein to have its primary structure determined. has a primary structure of two polypeptide chains linked by disulfide bonds. has an A chain with 21 amino acids and a B chain with 30 amino acids.

Secondary Structure: Alpha Helix The secondary structures of proteins describes the type of structure that forms when amino acids form hydrogen bonds within a single polypeptide chain or between polypeptide chains An alpha helix (α-helix) has a coiled shape held in place by hydrogen bonds between the amide groups and the carbonyl groups of the amino acids along the chain. hydrogen bonds between the H of an —NH group and the O of C═O of the fourth amino acid down the chain.

Secondary Structure: Alpha Helix (continued)

Secondary Structure: Beta-Pleated Sheet A beta-pleated sheet (β-pleated sheet) is a secondary structure that can form between adjacent polypeptide chains or within the same polypeptide chain when the rigid structure of the amino acid proline causes a bend in the polypeptide chain. has hydrogen bonds between chains. has R groups above and below the sheet. is typical of fibrous proteins, such as silk.

Secondary Structure: Beta-Pleated Sheet (continued)

Secondary Structure: Triple Helix A triple helix consists of three alpha helix chains woven together. contains large amounts of glycine, proline, hydroxyproline, and hydroxylysine that contain –OH groups for hydrogen bonding. is found in collagen, connective tissue, skin, tendons, and cartilage.

Tertiary Structure The tertiary structure of a protein gives a specific three-dimensional shape to the polypeptide chain. involves the attractions and repulsions of the R groups of the amino acids of the peptide chain. is stabilized by: hydrophobic and hydrophilic interactions, salt bridges, hydrogen bonds, and disulfide bonds.

Tertiary Structure (continued) The interactions of the R groups give a protein its specific three-dimensional tertiary structure.

R Group Interactions in Tertiary Structures

Globular Proteins Globular proteins Myoglobin have compact, spherical shapes. carry out synthesis, transport, and metabolism in the cells. such as myoglobin store and transport oxygen in muscle. Myoglobin

Fibrous Proteins Fibrous proteins consist of long, fiber-like shapes. such as alpha keratins make up hair, wool, skin, and nails. such as feathers contain beta keratins with large amounts of beta-pleated sheet structures.

Quaternary Structure The quaternary structure Hemoglobin is the combination of two or more polypeptide chains. is stabilized by the same interactions found in tertiary structures. of hemoglobin consists of two alpha chains and two beta chains with heme groups in each subunit that pick up oxygen for transport in the blood to the tissues. Hemoglobin

Summary of Protein Structures

Summary of Protein Structures (continued)

Protein Hydrolysis Protein hydrolysis splits the peptide bonds to give smaller peptides and amino acids. occurs in the digestion of proteins. occurs in cells when amino acids are needed to synthesize new proteins and repair tissues.

Hydrolysis of a Dipeptide In the lab, the hydrolysis of a peptide requires acid or base, water, and heat. In the body, enzymes catalyze the hydrolysis of proteins.

Denaturation Denaturation involves the disruption of bonds in the secondary, tertiary, and quaternary protein structures. heat and organic compounds that break apart H bonds and disrupt hydrophobic interactions. acids and bases that break H bonds between polar R groups and disrupt ionic bonds. heavy metal ions that react with S—S bonds to form solids. agitation, such as whipping, that stretches peptide chains until bonds break.

Applications of Protein Denaturation