Chapter 15 Biomolecules: Amino Acids, Peptides, and Proteins Suggested Problems: 22,24,29,32-3,44-5,47

Proteins – Amides from Amino Acids Amino acids contain a basic amino group and an acidic carboxyl group Joined as amides between the –NH2 of one amino acid and the –CO2H of the next amino acid Chains with fewer than 50 units are called peptides Protein: larger chains that have structural or catalytic functions in biology

26.1 Structures of Amino Acids In neutral solution, the —COOH is ionized and the — NH2 is protonated The resulting structures have “+” and “–” charges (a dipolar ion, or zwitterion) They are internal salts and resemble ionic salts in solution

React with Acid or Base Amino acids are amphiprotic Can react with either acids or bases

The Common Amino Acids 20 amino acids form amides in proteins All are -amino acids - the amino and carboxyl are connected to the same C They differ by the other substituent attached to the  carbon, called the side chain, with H as the fourth substituent Proline is a five-membered secondary amine, with N and the  C part of a five-membered ring Other side chains may be acidic, basic, or neutral All amino acids except proline contain primary amines.

Abbreviations and Codes Alanine A, Ala Arginine R, Arg Asparagine N, Asn Aspartic acid D, Asp Cysteine C, Cys Glutamine Q, Gln Glutamic Acid E, Glu Glycine G, Gly Histidine H, His Isoleucine I, Ile Leucine L, Leu Lysine K, Lys Methionine M, Met Phenylalanine F, Phe Proline P, Pro Serine S, Ser Threonine T, Thr Tryptophan W, Trp Tyrosine Y, Tyr Valine V, Val

The 20 Common Amino Acids in Proteins Glycine is the one amino acid which does not have a chiral center

The 20 Common Amino Acids in Proteins Structures of amino acids

The 20 Common Amino Acids in Proteins Structures of amino acids

The 20 Common Amino Acids in Proteins Structures of amino acids

Types of side chains Neutral: Fifteen of the twenty have neutral side chains Asp and Glu have a second COOH and are acidic Lys, Arg, His have additional basic amino groups side chains (the N in tryptophan is a very weak base) Cys, Ser, Tyr (OH and SH) are weak acids that are good nucleophiles

Histidine Contains an imidazole ring that is partially protonated in neutral solution Only the pyridine-like, doubly bonded nitrogen in histidine is basic. The pyrrole-like singly bonded nitrogen is nonbasic because its lone pair of electrons is part of the 6  electron aromatic imidazole ring.

Chirality of Amino Acids Glycine, 2-amino-acetic acid, is achiral In all the others, the  carbons of the amino acids are centers of chirality The stereochemical reference for amino acids is the Fischer projection of L-serine Proteins are derived exclusively from L-amino acids

Essential Amino Acids All 20 of the amino acids are necessary for protein synthesis Humans can synthesize only 10 (nonessential) of the 20 The other 10 (essential) must be obtained from food

15.2 Isoelectric Points In acid solution, the carboxylate and amine are in their conjugate acid forms, an overall cation In base solution, the groups are in their base forms, an overall anion In neutral solution cation and anion forms are present This pH where the overall charge is 0 is the isoelectric point, pI

15.3 Peptides and Proteins Proteins and peptides are amino acid polymers in which the individual amino acid units, called residues, are linked together by amide bonds, or peptide bonds An amino group from one residue forms an amide bond with the carboxyl of a second residue Since peptides and proteins are formed through the loss of water to form the peptide bond, the breakage of a peptide bond is referred to as hydrolysis.

Peptide Linkages Two dipeptides can result from reaction between A and S, depending on which COOH reacts with which NH2 we get AS or SA The long, repetitive sequence of NCHCO atoms that make up a continuous chain is called the protein’s backbone Peptides are always written with the N-terminal amino acid (the one with the free NH2 group) on the left and the C-terminal amino acid (the one with the free CO2H group) on the right Alanylserine is abbreviated Ala-Ser (or A-S), and serylalanine is abbreviated Ser-Ala (or S-A)

15.4 Covalent Bonding in Peptides Amide bond links in amino acids is the same as other amide bonds Restricted rotation Planar configuration with the N— H 180⁰ to the C=O

Disulfide Linkages RS—SR linkage between two cysteine residues Formed by mild oxidation of RSH groups Cleaved by mild reducing agents RS—SR linkages can be inter- or intra-molecular

Disulfide Linkages in Insulin

15.8 Protein Structure Proteins are classified as either fibrous or globular proteins Fibrous – polypeptide chains arranged side by side in long filaments Connective tissue Collagen in tendon Myosin in muscles Globular – coiled into compact spherical shapes Mobile within cells Soluble in water Enzymes

Levels of Structure The primary structure of a protein is simply the amino acid sequence. The secondary structure of a protein describes how segments of the peptide backbone orient into a regular pattern. (a-helix and b-pleated sheet) The tertiary structure describes how the entire protein molecule coils into an overall three-dimensional shape. The quaternary structure describes how different protein molecules come together to yield large aggregate structures (more than one chain)

-Helix -Helix stabilized by H-bonds between amide N–H groups and C=O groups four residues away

-Pleated Sheet b-pleated sheet secondary structure is exhibited by polypeptide chains lined up in a parallel arrangement, and held together by hydrogen bonds between chains

Forces Involved in Tertiary Structure Forces the same as those that act on all molecules Hydrophilic Hydrophobic Determine the orientation on the interior or the exterior of the protein in an aqueous environment Also important in stabilizing tertiary structure are Disulfide bridge – between cysteine residues Salt bridges – between positively and negatively charge amino acid residues

Forces Involved in Tertiary Structure

15.9 Enzymes and Coenzymes An enzyme is a protein that acts as a catalyst for a biological reaction. Most enzymes are specific for substrates while enzymes involved in digestion, such as papain attack many substrates

Types of Enzymes by Function Enzymes are usually grouped according to the kind of reaction they catalyze, not by their structures

26.11 How Do Enzymes Work? Citrate Synthase Citrate synthase catalyzes a mixed Claisen condensation of acetyl CoA and oxaloacetate to give citrate Normally Claisen condensations require a strong base in an alcohol solvent but citrate synthase operates in neutral solution

The Structure of Citrate Synthase Determined by X-ray crystallography Enzyme is very large compared to substrates, creating a complete environment for the reaction

Mechanism of Citrate Synthetase A cleft with functional groups binds oxaloacetate Another cleft opens for acetyl CoA with H 274 and D 375, which have carboxylate that abstract a proton from acetyl CoA The enolate (stabilized by a cation) adds to the carbonyl group of oxaloacetate The thiol ester in citryl CoA is hydrolyzed

Mechanism of Citrate Synthetase

Sickle Cell Anemia In sickle cell anemia a single amino acid difference is responsible for the sickling. A valine replaces a glutamate at position 6 on the surface of the beta chain. Under conditions of low oxygen concentration, the hemoglobin sickles and clumps together in the smaller vessels.