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Transcript
Chem 191: Biochemistry Lecture 9 – Proteins I. Introduction A. Protein means “first in importance” II. The Amino Acids A. Polymers of amino acids B. Only 20 are commonly found in proteins C. They are α-amino acids Figure 19.1 - General Structure of α-amino acids 1. 2. D. They contain a carboxylic acid group. The carbon adjacent to the carboxylic acid group is called the α(alpha)carbon. 3. Attached to the α-carbon is an amino group. Characteristic side chains. 1. In addition to the carboxylic acid and amino groups, there is also an Rgroup attached to the α-carbon. 2. Each amino acid has its own name a. They also have a three-letter and a one-letter abreviation Table 19.1 - The Common Amino Acids b. c. 3. The R-group is referred to as the amino acid sidechain. Each of the 20 commonly occurring amino acids has a different sidechain. d. Each amino acid sidechain has its own set of chemical and physical properties. Physical properties a. Non polar sidechains: i. Glycine ii. Alanine iii. Valine iv. Leucine v. Isoleucine vi. Proline vii. Phenylanine viii. Methioine b. Polar but not charged at pH7: i. Serine ii. Threonine iii. Tyrosine iv. Tryptophan v. Asparagine vi. Glutamine 1 Chem191 Lecture 9 c. E. F. Proteins vii. Cysteine Charged at pH7: i. Aspartic Acid (-) ii. Glutamic Acid (-) iii. Lysine (+) iv. Arginine (+) v. Histidine (0/+) Abbreviations Stereochemistery 1. With the exception of glycine, the α-carbon atom is chiral 2. All of the common amino acids used to make proteins have the Lconfiguration. a. These are absolute configurations analogous to L-glyceraldehyde. i. Using a Fisher-like projection, place the carboxylic acid group at the top and the R-group at the bottom. ii. The L-configuration places the amino group on the left and the hydrogen on the right. III. Zwitterions A. The carboxylic acid group is a stronger acid than water and the amino group is a stronger base than water, therefore at pH 7 both are in their ionic form (-COOand -NH3+) 1. This form is called the zwitterion which means “between ions” 2. The zwitterion has a net charge of 0 a. At lower pH values the carbolylate group becomes protonated and the amino acid has a net charge of +1. b. At higher pH values the amino group becomes unprotonated and the amino acid has a net charge of -1. c. There is no pH value for which both the carboxyl group and the amino group are uncharged state. 3. The isoelectric point is the pH value where the net charge on a molecule is 0 IV. Reactions of Amino Acids A. Oxidation of cysteine to form disulfide bonds (cystine) 1. Two cysteine sidechain thiol groups (-SH) react to form a disulfide bond (-S-S-) a. This is an oxidation reaction. b. This is an important reaction in proteins 2. The two cysteines linked together by the disulfide bond are called cystine Peptide formation B. Peptide bond formation 1. As you learned in Chapter 16, amines and carboxylic acid can react to form amides. a. When amino acids do this it is called a peptide bond. 2 Chem191 Lecture 9 2. C. D. E. Proteins When this reaction is done in a test tube acid chorides or and acid anhydrides are used instead of the carboxylic acid. a. This is done to make the reaction more favorable. 3. It is instructive, however, to write the reaction using the carboxylic acid and the amine. Forming peptides 1. When two amino acids are bonded together by a peptide bond it is called a dipeptide. a. Alanylglycine 2. When several amino acids are bonded together by peptide bonds they are called an oligopeptide. 3. When many amino acids are bonded together by peptide bonds polypeptides a. Your book suggests that the terms polypeptide and protein are interchangeable. i. I prefer to use the term polypeptide to refer to a long chain of amino acids connected by peptide bonds and to reserver the term protein to refer to polypeptides that form a well defined 3-dimensional structure and have a well-defined function. 1. This distinction may become clearer to you when we discuss enzymes in the next chapter and when we discuss how proteins fold later in this chapter. 4. When the peptide bond forms, a carboxyl group and amino group The N and C terminus of Amino acid residues V. Important Peptides A. Peptide hormones 1. Met and Leu enkephalin 2. Oxytocin (9 aa) a. CYLQNCPLGNH2 b. Smooth muscles of uterus to contract c. Stimulates milk ejection 3. Vasopressin (9 aa) a. Raises blood pressure b. Antidiuretic hormone c. CYFQNCPRGNH2. 4. ACTH (adrenocorticotropic hormone) from pituitary (39 aa) 5. Insulin 6. Human growth hormone 7. somatostatin VI. Characteristics of Proteins A. Size 1. 6000 u to several million u B. Classification 3 Chem191 Lecture 9 1. 2. VII. Proteins Shape a. Fibrous proteins b. Globular proteins Structure a. Simple b. Conjugated i. Prosthetic groups The Primary structure of Proteins A. The primary structure is defined by a protein’s amino acid sequence 1. With the exception of disulfide bonds between cyteine sidechains, this defines all the covalent bonds in a protein VIII. The Secondary Structure of Proteins A. The protein backbone can form local periodic structures. B. The two most common ones are the α-helix and the β-sheet 1. Both these structure lead to hydrogen bond formation between the peptide carbonyl oxygen and the peptide nitrogen. IX. The Tertiary Structure of Proteins A. Defines the arrangement of the elements of secondary structure. 1. The α-helices and β-sheets are connected by loops B. Stabilized primarily by interactions between the sidechains; these interactions include: 1. Disulfide bonds 2. Salt bridges 3. Hydrogen bonds 4. Hydrophobic interactions X. The Quaternary Structure of Proteins A. Proteins can assemble into bigger proteins. 1. The proteins making up the bigger protein are called subunits. XI. Protein Hydrolysis and Denaturation A. Hydrolysis 1. Heat 2. Denaturants a. Alcohol b. Soaps c. Heavy metals Ag+, Hg+2, Pb+2 3. pH 4