Chapter 19 Amino Acids and Proteins 19.1 Proteins and Amino Acids Classify proteins by their functions in the body. Give the name and abbreviation of an amino acid and draw its ionized structure.

Functions of Proteins Function of protein p - 673 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 each contain a different side group (R)

Examples of Amino Acids

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

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

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

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

Learning Check Identify each of the following as a polar (P) or nonpolar (NP) amino acid.

Solution Identify each of the following as a polar (P) or nonpolar (NP) amino acid. Nonpolar (NP) Polar (P)

Chapter 19 Amino Acids and Proteins 19.2 Amino Acids as Zwitterions Draw the zwitterion of an amino acid at its isoelectric point and its ionized structure at pH values above or below its isoelectric point.

Zwitterions and Isoelectric Points A zwitterion has an equal number of —NH3+ and COO– groups forms when the H from —COOH in an amino acid transfers to the —NH2

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 Glycine, 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.

pI, pH, and Charge

Summary of pH, pI, and Ionization

Ionized Forms of 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.

Electrophoresis Used to screen for sickle cell trait in newborns

Learning Check Which structure represents: A. alanine at a pH above its pI? B. alanine at a pH below its pI?

Solution Which structure represents: A. alanine at a pH above its pI? (2) B. alanine at a pH below its pI? (1)

Chapter 19 Amino Acids and Proteins 19.3 Formation of Peptides Draw the structure of a dipeptide.

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

Structure of protein p - 680 Naming Dipeptides Structure of protein p - 680 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

Learning Check Draw the condensed structural formula, and give the name and abbreviation for the dipeptide Ser-Thr.

Solution

Chapter 19 Amino Acids and Proteins 19.4 Protein Structure: Primary and Secondary Levels Describe the primary and secondary structures of a protein.

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 + Ala─Leu─Cys─Met

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

Polypeptide in the body H.N. P 684 Polypeptide in the body The nonapeptides oxytocin and vasopressin have similar primary structures differ only in the amino acids at positions 3 and 8

Secondary Structure: Alpha Helix The secondary structures of proteins indicate the three-dimensional spatial arrangements of the 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 consists of polypeptide chains arranged side by side 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 Primary & secondary Structure p 684

Essential Amino Acids Essential amino acids H.N. P 687 Essential amino acids are the ten amino acids not synthesized by the body must be obtained from the diet are in meat and diary products are missing (one or more) in grains and vegetables

Learning Check Indicate the type of protein structure as: 1) primary 2) alpha helix 3) beta-pleated sheet 4) triple helix A. polypeptide chains held side by side by H bonds B. sequence of amino acids in a polypeptide chain C. corkscrew shape with H bonds between amino acids D. three peptide chains woven like a rope

Solution Indicate the type of protein structure as: 1) primary 2) alpha helix 3) beta-pleated sheet 4) triple helix 3 A. polypeptide chains held side by side by H bonds 1 B. sequence of amino acids in a polypeptide chain 2 C. corkscrew shape with H bonds between amino acids 4 D. three peptide chains woven like a rope

Chapter 19 Amino Acids and Proteins 19.5 Protein Structure: Tertiary and Quaternary Levels Describe the tertiary and quaternary structures of a protein.

Tertiary Structure The tertiary structure of a protein gives a specific three-dimensional shape to the polypeptide chain involves interactions and cross-links between different parts of the peptide chain is stabilized by: hydrophobic and hydrophilic interactions salt bridges hydrogen bonds disulfide bonds

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

Cross-Links in Tertiary Structures

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

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

Learning Check Select the type of tertiary interaction as: 1) disulfide 2) ionic 3) H bonds 4) hydrophobic A. leucine and valine B. two cysteines C. aspartic acid and lysine D. serine and threonine

Solution Select the type of tertiary interaction as: 1) disulfide 2) ionic 3) H bonds 4) hydrophobic 4 A. leucine and valine 1 B. two cysteines 2 C. aspartic acid and lysine 3 D. serine and threonine

Tertiary & Quaternary Structure of proteins p 687 The quaternary structure is the combination of two or more tertiary units 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

Hemoglobin & Myoglobin Have similar biological functions. Hemoglobin carry oxygen in blood while Myoglobin carry oxygen in muscle. Myoglobin, a single polypeptide is identical to tertiary structure of each subunit of hemoglobin (4 units) Myoglobin stores one oxygen whereas hemoglobin stores 4 oxygen.

Summary of Protein Structures (continued)

Summary of Protein Structures

Sickle – Cell Anemia H.N. P 692 Caused by abnormality in the shape of one of the hemoglobin subunits. (β-chain) The shape of RBC changed. Less oxygen is transported.

Learning Check Identify the level of protein structure as: 1) primary 2) secondary 3) tertiary 4) quaternary A. beta-pleated sheet B. order of amino acids in a protein C. a protein with two or more peptide chains D. the shape of a globular protein E. disulfide bonds between R groups

Solution Identify the level of protein structure as: 1) primary 2) secondary 3) tertiary 4) quaternary 2 A. beta-pleated sheet 1 B. order of amino acids in a protein 4 C. a protein with two or more peptide chains 3 D. the shape of a globular protein 3 E. disulfide bonds between R groups

Chapter 19 Amino Acids and Proteins 19.6 Protein Hydrolysis and Denaturation Describe the hydrolysis and denaturation of proteins.

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 Denaturation Denaturation of protein occurs when an egg is cooked the skin is wiped with alcohol heat is used to cauterize blood vessels instruments are sterilized in an autoclave

Learning Check What are the products of the complete hydrolysis of the tripeptide Ala-Ser-Val?

Solution What are the products of the complete hydrolysis of the tripeptide Ala-Ser-Val? The products are the three amino acids: alanine, serine, and valine.

Learning Check Tannic acid is used to form a scab on a burn. An egg is hard-boiled by placing it in boiling water. What is similar about these two events?

Solution Tannic acid is used to form a scab on a burn. An egg is hard-boiled by placing it in boiling water. What is similar about these two events? Acid and heat cause the denaturation of protein. They both break bonds in the secondary and tertiary structures of proteins.