amino acids, peptides, proteins - chemistrychemistry.creighton.edu/~jksoukup/lec5-aminoacids.pdf ·...
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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
Nonpolar, aliphatic R group Glycine Alanine Proline Valine Leucine Isoleucine Methionine
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
Amino Acid Abbreviations
Gly G Ala A Pro P Val V Leu L Ile I Met M
Phe F Tyr Y Trp W
Ser S Thr T Cys C Asn N Gln Q
Lys K His H Arg R
Asp D Glu E
Amino Acids Additional properties of Amino Acids Numbering of R group carbons
CH2
+NH3
CH COO- CH2 CH2 CH2
+NH3
α 2
β 3
γ 4
δ 5
ε 6 1
Aromatic side chains absorb UV light
Disulfide bond formation with cysteine
oxidation
reduction
Trp 280 nm Tyr 280 nm Phe 260 nm Peptide bond 210-214 nm
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 (Zn2+, Fe2+)
Negatively charged R groups Asp, Glu
General acids/bases in catalysis (lysozyme) Chelate divalent metal ions (Mg, Ca, Mn, Zn)
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 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 = α carbon
All AA except Gly
Gly
Amino Acids Optical Activity of Amino Acids 1 Chiral center = ___ stereoisomers
Stereoisomer found in proteins =
Amino Acids Optical Activity of Amino Acids 1 Chiral center = 2 stereoisomers
Stereoisomer found in proteins = L-amino acids
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
pK1 = a carboxyl group pK2 = 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 = (pK1 + pK2)/2
2 buffer regions
Titration Curve of Histidine
Histidine R group has pKa = 6.0 No other AA side chain has a pKa 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
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)