Transcript Amino Acids
Amino Acids, Peptides, Protein Primary Structure Chapter 3 Amino Acids • Basic struct’l units of prot’s • 3 common funct’l grps: – -NH2, -COOH, -H • Indiv aa’s each have diff R grp • 4 grps att’d to a C – Is this a chiral C? • At neutral pH: dipole (zwitterion) – Amino grp as NH3+ – Carboxyl grp as COO- • Chiral aC, so D,L stereoisomers – L aa’s polymerize prot’s • Side chains vary: size, shape, charge, reactivity, H-bond capacity • Five aa groups, based on R grp similarities • Some notes: – Glycine – only optically inactive aa – Cysteine – highly reactive sulfhydryl grp – Histidine – R grp may be proton donor or acceptor at physio pH • 1) Nonpolar w/ aliphatic R grps – – – – – – – Glycine (Gly, G) Alanine (Ala, A) Proline (Pro, P) Valine (Val, V) Leucine (Leu, L) Isoleucine (Ile, I) Methionine (Met, M) • 2) Aromatic R grps – Phenylalanine (Phe, F) – Tyrosine (Tyr, Y) – Tryptophan (Trp, W) – Hydrophobic • 3) Polar w/ uncharged R grps – – – – – Serine (Ser, S) Threonine (Thr, T) Cysteine (Cys, C) Asparagine (Asn, N) Glutamine (Gln, Q) • 4) Polar w/ + charged R grps at physio pH – Lysine (Lys, K) – Histidine (His, H) – Arginine (Arg, R) • 5) Polar w/ - charged R grps at physio pH – Aspartate (Asp, D) – Glutamate (Glu, E) Cysteine/Cystine • Cys reactive SH grp oxidizes disulfide bond • Forms cystine – Hydrophobic – Impt to protein 3D structure Amino Acid Titration Curves • Aa’s – weak acids – Titration curves for each – REMEMBER: Meas D pH as titrate w/ OH-. Plot OH- added on x axis vs. pH on y axis • 2 abstractable H’s, both on grps att’d to a C (bottom p. 81) – Carboxyl grp – Amino grp • 2 inflection pts – Shape of each sim to inflection seen w/ monoprotic acid (Chpt 2) – Each aa has at least 2 pKa’s • At titration midpoint([OH-]=1 eq), cmpd fully dipolar – No net electrical charge – “Isoelectric point” – Isoelectric pH = pI • Each aa has characteristic pI – At any pH<pI, aa has net + charge – At any pH>pI, aa has net - charge • pKa1 <<<< pKa2 – First H+ released: much more easily given up than second H+ • 2 pKa’s = 2 regions of buffering capacity • Aa’s w/ ionizable R grps (lys, arg, his) have 3rd pKa Peptide Bonds • Link two aa’s – Dipeptide – Condensation rxn • What mol is removed?? – Endothermic – Stable under physio cond’s • Broken w/ boiling in strong acid/base a carboxyl of aa1 joined to a amino of aa2 • In cells, bond form’n assisted by ribosomes during translation • Oligopeptide = several aa’s joined by peptide bonds • Polypeptide = many aa’s = small protein – Protein commonly MW > 10,000 • Amino terminus = aa residue w/ free amino grp • Carboxy terminus = aa residue w/ free carboxyl grp • At neutral pH, all peptides have 1 free NH3+ and 1 free COO• BUT R grps on indiv aa may be ionized – Characteristic pI for each peptide • Peptide ionization = sum of all R grp charges of aa’s in peptide