Amino Acids, Peptides, Proteins Functions of proteins: Enzymes Transport and Storage Motion, muscle contraction Hormones Mechanical support Immune protection (Antibodies) Generate and transmit nerve impulses Control growth and differentiation Lens protein in eye Feathers Spider webs Horns Milk proteins Antibiotics Mushroom poison.. Luciferin, luciferase Hemoglobin
Amino Acids General structure α carbon, side chain (R group)
Amino Acids
Amino Acids
Amino Acids
Amino Acids
Amino Acids
Amino Acids Amino Acid Nonpolar, aliphatic R group Glycine Alanine Proline Valine Leucine Isoleucine Methionine Abbreviations Gly G Ala A Pro P Val V Leu L Ile I Met M Aromatic R groups Phenylalanine Tyrosine Tryptophan Polar, uncharged R groups Serine Threonine Cysteine Asparagine Glutamine Positively charge R groups Lysine Histidine Arginine Negatively charged R groups Aspartate Glutamate Phe Tyr Trp Ser Thr Cys Asn Gln Lys His Arg Asp Glu F Y W S T C N Q K H R D E
Amino Acids
Amino Acids Additional properties of Amino Acids Numbering of R group carbons ε δ γ β α 6 5 4 3 2 1 CH 2 CH 2 CH 2 CH 2 CH COO - + NH 3 + NH 3 Aromatic side chains absorb UV light Trp 280 nm Tyr 280 nm Phe 260 nm Peptide bond 210-214 nm Disulfide bond formation with cysteine oxidation reduction
Amino Acids Nonpolar, aliphatic R group Gly, Ala, Pro, Val, Leu, Ile, Met Gly - no steric hindrance Pro - hinders backbone flexibility hydrophobic core of soluble proteins found in transbilayer part of membrane proteins Aromatic R groups Phe, Tyr, Trp hydrophobic, Stacking Tyr/Trp - H-bonding Tyr - site of phosphorylation Polar, uncharged R groups Ser, Thr, Cys, Asn, Gln Ser/Thr - H-bonding; phosphorylated, glycosylated; enzyme active sites Cys - disulfide bonds; enzyme active sites; metal ion binding Asn/Gln - very polar, H-bonding Positively charge R groups Lys, His, Arg His - pka close to neutrality (catalysis); ligand for metal ions (Zn 2+, Fe 2+ ) Negatively charged R groups Asp, Glu General acids/bases in catalysis (lysozyme) Chelate divalent metal ions (Mg, Ca, Mn, Zn)
Uncommon Amino Acids collagen prothrombin myosin elastin
Amino Acids Optical Activity of Amino Acids For all AA except glycine the α carbon is bonded to 4 different groups: Carboxyl, amino, hydrogen, and R group (in Gly, R group is hydrogen) Chiral center = All AA except Gly Gly
Amino Acids Optical Activity of Amino Acids 1 Chiral center = stereoisomers Stereoisomer found in proteins =
Amino acids act as acids and bases zwitterion amphoteric Base = proton acceptor, electron pair donor Acid = proton donor, electron pair acceptor
Amino acid titration Curve 2 buffer regions pk 1 = a carboxyl group pk 2 = a amino group isoelectric point (pi)- ph where there is an equal amount of (+) and (-) charges (overall charge of zero) isoelectric point (pi) for glycine is at ph = 5.97 pi = (pk 1 + pk 2 )/2
Chemical environment influences pka
Titration Curve of Histidine Histidine R group has pk a = 6.0 No other AA side chain has a pk a near neutral ph So Histidine is really the only AA that can be: an effective buffer at physiological ph (7.0)
Peptides and Proteins Peptide - two amino acids joined covalently by a peptide bond Polypeptide - many AA joined together by peptide bond (M.W.<10,000) Protein - macromolecule with one or more polypeptide chains condensation
Peptides Ionization
Biologically active Peptides & Polypeptides Dipeptide (Nutrasweet) Other small peptides Oxytocin (9 aa) - stimulates uterine contractions Bradykinin (9 aa) - inhibits tissue inflammation Amanitin - mushroom poison Polypeptides Insulin - pancreatic hormone, needed for sugar metabolism, 2 polypeptide chains (30 aa and 21 aa) Glucagon - pancreatic hormone, opposes action of insulin (29 aa) Corticotropin - anterior pituitary gland hormone, stimulates adrenal cortex (39 aa)
Proteins contain other prosthetic groups
Protein Structure