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Chapter 14 Proteins Proteins Proteins serve many functions, including the following: ◦ 1. Structure: Collagen and keratin are the chief constituents of skin, bone, hair, and nails. ◦ 2. Catalysts: Virtually all reactions in living systems are catalyzed by proteins called enzymes. ◦ 3. Movement: Muscles are made up of proteins called myosin and actin. ◦ 4. Transport: Hemoglobin transports oxygen from the lungs to cells; other proteins transport molecules across cell membranes. ◦ 5. Hormones: Many hormones are proteins, among them insulin, oxytocin, and human growth hormone. Proteins ◦ 6. Protection: Blood clotting involves the protein fibrinogen; the body used proteins called antibodies to fight disease. ◦ 7. Storage: Casein in milk and ovalbumin in eggs store nutrients for newborn infants and birds. Ferritin, a protein in the liver, stores iron. ◦ 8. Regulation: Certain proteins not only control the expression of genes, but also control when gene expression takes place. Proteins are divided into two types: ◦ Fibrous proteins ◦ Globular proteins Proteins Proteins are divided into two types: ◦ Fibrous proteins: insoluble in water and are used mainly for structural purposes ◦ long fibers or sheets formed by parallel polypeptide chains ◦ dominated mostly by secondary structure ◦ mostly water insoluble ◦ great strength and/or stretchiness from affects of regular H-bonds ◦ examples: ◦ collagen in connective tissue ◦ actin and myosin in muscle tissue Proteins Globular proteins: more or less soluble in water and are used for nonstructural purpose ◦ folded into complex 3-D irregular spherical shape ◦ dominated mostly by tertiary structure ◦ mostly water soluble ◦ functions determined by 3-D shape ◦ examples: ◦ enzymes such as amylase ◦ hormones such as insulin ◦ transport such as hemoglobin ◦ protective, such as immunoglobulins Amino Acids Amino acid: A compound that contains both an amino group and a carboxyl group. ◦ -Amino acid: An amino acid in which the amino group is on the carbon adjacent to the carboxyl group. Table 14.1 The 20 amino acids commonly found in proteins Chirality of -Amino Acids With the exception of glycine, all protein-derived amino acids have at least one stereocenter (the -carbon) and are chiral. ◦ The vast majority of -amino acids have the L-configuration at the -carbon. Chirality of -Amino Acids A comparison of the configuration of L-alanine and Dglyceraldehyde (as Fischer projections): Protein-Derived -Amino Acids Nonpolar side chains. Each ionizable group is shown in the form present in highest concentration at pH 7.0). Protein-Derived -Amino Acids Polar side chains (at pH 7.0) Protein-Derived -Amino Acids Acidic and basic side chains (at pH 7.0) Protein-Derived -Amino Acids 1. For 19 of the 20, the -amino group is primary; for proline, it is secondary. 2. With the exception of glycine, the -carbon of each is a stereocenter. 3. Isoleucine (left) and threonine (right) contain a second stereocenter. Ionization vs. pH The net charge on an amino acid depends on the pH of the solution in which it is dissolved. ◦ If we dissolve an amino acid in water, it is present in the aqueous solution as its zwitterion. Ionization vs. pH To summarize pH = 0 : a zwitterion pH = 7 pH = ~ 14 Isoelectric Point (pI) Isoelectric point, pI: The pH at which the majority of molecules of a compound in solution have no net charge. Nonpolar & polar side chains alanine asparagine cysteine glutamine glycine isoleucine leucine methionine phenylalanine proline serine threonine tyrosine tryptophan valine pI 6.01 5.41 5.07 5.65 5.97 6.02 5.98 5.74 5.48 6.48 5.68 5.87 5.66 5.88 5.97 Acidic pI Side Chains aspartic acid 2.77 glutamic acid 3.22 Basic pI Side Chains 10.76 arginine histidine 7.59 lysine 9.74 What determines the characteristic of amino acid Cystine The -SH (sulfhydryl) group of cysteine is easily oxidized to an -S-S(disulfide). Hair is made up by a protein called karetin that contains a large number of cysteine residues Phe, Trp, and Tyr The amino acids phenylalanine, tryptophan, and tyrosine have aromatic rings on their side chains. Tryptophan is the precursor to the neurotransmitter serotonin. Tyr and Phe Phenylalanine and tyrosine are precursors to norepinephrine and epinephrine, both of which are stimulatory. Other Amino Acids Figure 14.3 Hydroxylation (oxidation) of proline, lysine, and tyrosine, respectively and iodination for tyrosine, give these uncommon amino acids. Peptides In 1902, Emil Fischer proposed that proteins are long chains of amino acids joined by amide bonds. ◦ Peptide bond (peptide linkage): The special name given to the amide bond between the -carboxyl group of one amino acid and the -amino group of another. Peptides ◦ Peptide: A short polymer of amino acids joined by peptide bonds; they are classified by the number of amino acids in the chain. ◦ Dipeptide: A molecule containing two amino acids joined by a peptide bond. ◦ Tripeptide: A molecule containing three amino acids joined by peptide bonds. ◦ Polypeptide: A macromolecule containing many amino acids joined by peptide bonds. ◦ Protein: A biological macromolecule containing at least 30 to 50 amino acids joined by peptide bonds. ◦ The individual amino acid units are often referred to as “residues”. Peptide Bond A peptide bond is typically written as a carbonyl group bonded to an N-H group. Linus Pauling, however, discovered that there is about 40% double bond character to the C-N bond and that a peptide bond between two amino acids is planar, which Pauling explained using the concept of resonance. Peptide bond Writing Peptides By convention, peptides are written from the left to right, beginning with the free -NH3+ group and ending with the free COO- group. ◦ C-terminal amino acid: The amino acid at the end of the chain having the free -COO- group. ◦ N-terminal amino acid: The amino acid at the end of the chain having the free -NH3+ group. Writing Peptide Bond Example Show how to form the dipeptide Gly-Val Draw the tetrapeptide Ala-Thr-Asp-Asn and indicate the peptide bond