Chapter 19: Proteins CHEM 1152 Dr. Sheppard

Outline Amino Acids Amino Acids as Acids and Bases Formation of Peptides Functions of Proteins Protein Structure: Primary, Secondary, Tertiary and Quaternary Structure Protein Hydrolysis and Denaturation

19.1 Amino Acids Amino acids: The building blocks of proteins. Two functional groups: carboxylic acid group amino group on the alpha () carbon. Have different side groups (R) that give each amino acid unique characteristics.

Nonpolar Amino Acids Amino acids are classified as nonpolar when the R groups are H, alkyl, or aromatic Note three-letter abbreviations

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

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

Essential Amino Acids Essential amino acids: Ten amino acids that are not synthesized by the body Must be obtained from the diet

Essential Amino Acids Complete proteins contain all 10 essential amino acids (animal products) Incomplete proteins are deficient in one or more (plant proteins) Obtained by combining two or more vegetables that provide complementary proteins

Learning Check Classify each amino acid as polar or nonpolar: A. Glycine NH3+—CH2—COO- CH3 | CH—OH B. Threonine NH3+—CH—COO-

Learning Check Classify the following amino acids as hydrophobic (nonpolar), hydrophilic (polar, neutral), acidic, or basic: A. Lysine R = CH2CH2CH2CH2NH2 B. Aspartate R = CH2CO2H C. Leucine R = CH2CH(CH3)2 D. Serine R = CH2OH

Learning Check Give the name of the amino acid represented by each of the following three-letter abbreviations: A. Trp B. Met C. Pro D. Gly

19.2 Zwitterions Both the –NH2 and the –COOH groups in an amino acid undergo ionization in water A zwitterion forms that has + and – charge Melting points of amino acids = very high

Isoelectric Point At the isoelectric point (pI), the + and – charges in the zwitterion are equal (no net charge). At pH values above the isoelectric point, the amino acid has a net negative value. At pH values below the isoelectric point, the amino acid has a net positive value.

Amino Acids as Acids In solutions more basic (higher pH) than the pI, the —NH3+ in the amino acid loses a proton to become —NH2 + OH− H3N—CH2—COO– H2N—CH2—COO– Zwitterion Negative ion at pI Higher pH

Amino Acids as Bases In solution more acidic (lower pH) than the pI, the —COO- in the amino acid gains a proton to become —COOH + H+ + H3N—CH2—COO– H3N—CH2—COOH Zwitterion Positive ion at pI Low pH

pH and Ionization Alanine

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

Learning Check CH3 CH3 + | | H3N—CH—COOH H2N—CH—COO– (1) (2) + | | 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?

Learning Check Would the following ions of serine exist at a pH above, below, or at pI?

Learning Check Write the zwitterion of each of the following amino acids: A. Phenylalanine B. Methionine

19.3 Reactions of Amino Acids Two reactions of interest: Oxidation of cysteine Peptide bond formation

Oxidation of Cysteine Cys side chain = ─CH2─SH Remember oxidation of thiols to form disulfide bonds (section 13.9)? R─SH + R─SH R─S─S─R Two Cys amino acids can react to form a disulfide bond (or disulfide bridge) [O]

Disulfide Bridge/bond

Review Carboxylic acid + amine = ? Structure of amino acid

The Peptide Bond A peptide bond is an amide bond between the carboxyl group of one amino acid and the amino group of the next amino acid O CH3 O + || + | || H3N—CH2—C—O– + H3N—CH—C—O– O H CH3 O + || | | || H3N—CH2—C—N—CH—C—O– + H2O peptide bond

Peptides Dipeptide = two amino acids; tri = 3; poly = many Protein = polypeptide with >50 amino acid residues Peptide is named from the N-terminal end (left; NH3+) Use -yl endings for the names of the amino acids The C-terminal amino acid (right; COO-) uses its amino acid name

Peptides

Learning Check Use the three-letter abbreviations of the amino acids to write structures for all possible tripeptides that could form from two glycine and one alanine. Name each tripeptide.

Learning Check Write the name of the following tetrapeptide using amino acid names and three-letter abbreviations.

Learning Check Draw the structural formula of each of the following peptides. A. Methionylaspartic acid B. Alanyltryptophan

19.5 Characteristics of Proteins Size: Varies from 6000 to several million g/mol Hemoglobin = C2952H4664O832S8Fe4

Characteristics of Proteins Function:

Characteristics of Proteins Function:

Example Classify each of the following proteins according to its function: A. Insulin, a hormone needed for glucose utilization B. Antibodies, proteins that disable foreign proteins C. Casein, milk protein D. Lipases that hydrolyze lipids

Characteristics of Proteins Classification: Fibrous vs. globular (based on structural shape) Fibrous proteins: Long rods or strings; fibers Water-insoluble Connective tissue, hair, skin Globular proteins: Spherical Water-soluble Synthesis, transport and metabolism in cells

Protein Structure Chain of amino acids folded into 3D structure Levels of structure: Primary Secondary Tertiary Quaternary