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Chapter 5 Amino Acids, Peptides, and Proteins. Lecture 1 of 2 Learn Pages 115 – 129 Learn to draw 20 standard amino acids (page 119). Red blood cells contain the oxygen transporting protein hemoglobin. Proteins Abundant biomolecule Occur in all cells and all parts of cells
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Chapter 5 Amino Acids, Peptides, and Proteins Lecture 1 of 2 Learn Pages 115 – 129 Learn to draw 20 standard amino acids (page 119) Red blood cells contain the oxygen transporting protein hemoglobin
Proteins • Abundant biomolecule • Occur in all cells and all parts of cells • Occur in great variety (small to really large) • Polymers of amino acids • Joined by a specific type of covalent bond • All proteins are built from the same set of 20 amino acids • Covalently linked in characteristic linear sequence • Exhibit diversity in biological functions
General Structure of an Amino Acid Side chain is different in each amino acid All amino acids have this general structure except proline which is cyclic
Amino acid R • carbon is bound to four different groups • carbon atom is a chiral center
Stereoisomerism in a-amino acids The object and its mirror image are not superimposable
Project out of the plane of the paper behind it
Steric relationship of the stereoisomers of alanine to the absolute configuration of L- and D- glyceraldehyde L-amino acids have a amino group on left a R group below a C • l and d designations were used historically for levo and dextro rototary • Not all L-amino acids are levorotatory
The 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 stereospecific
R Group S containing
R groups are nonpolar (hydrophobic) Can form H bonds
S containing Cyclic
Reversible formation of a disulfide bond by the oxidation of two molecules of cysteine Disulfide bonds stabilize the structure of many proteins
Nonstandard amino acids No need to learn to draw Some of these are found in proteins
Amino acids can act as acids and bases Forms of amino acids Does not occur in significant amounts in aqueous solution Predominates at neutral pH
Amino acids have characteristic titration curves At this pH predominant species is species 2 (dipolar ion) 1 2 3 At midpoint in the first stage of titration [species 1] = [species 2] At low pH predominant species is species 1 pKa of Carboxyl group = 2.34 pKa of amino group = 9.6
The Pentapeptide Ser-Gly-Tyr-Ala-Leu or Serylglycyltyrosylalanylleucine N or C or
Levels of structure in proteins Linking of aa Stable arrangement of aa give rise to structural patterns 3-dimensional folding of polypeptides Arrangement in space of 2 or more polypeptide subunits