Chapter 5Amino Acids, Peptides, and ProteinsRed blood cells contain the oxygen transporting protein hemoglobinLecture 1 of 2Learn Pages 115 – 129Learn to draw 20 standard amino acids (page 119)Proteinsv Abundant biomoleculev Occur in all cells and all parts of cellsv Occur in great variety (small to really large) v Polymers of amino acidsv Joined by a specific type of covalent bondv All proteins are built from the same set of 20 amino acidsv Covalently linked in characteristic linear sequence v Exhibit diversity in biological functionsGeneral Structure of an Amino AcidAll amino acids have this general structure except proline which is cyclicSide chain is different in each amino acidAmino acidRα carbon is bound to four different groupsα carbon atom is a chiral centerStereoisomerism in α-amino acidsThe object and its mirror image are not superimposableProject out of the plane of the paperbehind itAssumed to project out of the plane of the paperbehind itSteric relationship of the stereoisomers of alanine to the absolute configuration of L- and D- glyceraldehydeH l and d designations were used historically for levo and dextro rototaryH Not all L-amino acids are levorotatoryR group below α CL-amino acids have α amino group on leftαThe amino acid residues in proteins are exclusively L stereoisomers• Formation of stable repeating substructures in proteins require the amino acids to be only one stereo isomer• Cells can specifically synthesize L isomers amino acid because the active site of the enzymes are asymmetric causing the reactions catalyzed to be stereospecificNeed to know the single & 3 letter abbreviationR GroupS containingR groups are nonpolar(hydrophobic)Can form H bondsCyclicS containingReversible formation of a disulfide bond by the oxidation of two molecules of cysteineDisulfide bonds stabilize the structure of many proteinsNonstandard amino acidsSome of these are found in proteinsNo need to learn to drawForms of amino acidsAmino acids can act as acids and basesPredominates at neutral pHDoes not occur in significant amounts in aqueous solutionAmino acids have characteristic titration curves1 2 3At low pH predominant species is species 1At midpoint in the first stage of titration [species 1] = [species 2]At this pH predominant species is species 2 (dipolar ion)pKaof Carboxyl group = 2.34pKaof amino group = 9.6Formation of a peptide bondSer-Gly-Tyr-Ala-Leuor SerylglycyltyrosylalanylleucineThe PentapeptideN or C orNutraSweetLevels of structure in proteinsLinking of aaStable arrangement of aa give rise to structural patterns3-dimensional folding of polypeptidesArrangement in space of 2 or more polypeptide
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