BISC 330L Biochemistry Spring 2015 USC Lecture 4 Wednesday Jan 21 2015 1 Chapter 2 Introduction 2 Amino acid structures and properties Chapter 2 Protein Composition and Structure Introduction to proteins The 20 amino acids Levels of protein structure Example Insulin is a protein hormone Electron density map from crystallography Different modes of presentation Chapter 2 Protein Composition and Structure Building blocks are amino acids Contain functional groups Proteins interact with one another and with other biological macromolecules Proteins can be rigid or flexible Protein function is dependent on 3 D structure The eukaryotic replication fork many specialized proteins Proteins can be flexible Example Lactoferrin binding to iron Proteins can bind DNA to control transcription read out of genetic information Proteins can bind small molecules to transduce neuronal signals L and D isomers are mirror images of each other ON ward across bridge R group on the left ON ward across bridge R group on the right Ionization state varies with pH Zwitterion Simple amino acids Nonpolar R R H R CH3 Nonpolar R continued Aliphatic Hydrophobic Larger hydrocarbon side chains Thioether group Non polar R continued Aliphatic Ring R CH2 3 imino acid Proline is unique because its side chain is bonded to both nitrogen and carbon atoms Non polar R continued Aromatic Phe has a phenyl ring Tyr has a OH group Ala Trp has an indole group joined to a methylene CH2 group Note Tyr is very weakly acidic Polar R uncharged neutral nonionizable R groups make more hydrophilic and reactive Ala Val Polar R uncharged neutral nonionizable Asn and Gln contains a terminal carboxamide instead of carboxylic acid Polar R Acidic AH A H Asp and Glu have carboxylic acid as R group At phys pH lack proton Polar R Weakly Acidic SH S H R group is sulfhydryl group 2 Cys can come together to form disulfide bonds Polar R Basic B H BH Lys R is amino group Arg R is guanidinium group His R is imidazole group Ionization of imidazole ring in Histidine R group The amino acids Free AA pKa 2 1 0 3 Asp 3 9 Glu 4 1 9 6 0 6 6 0 8 3 10 9 10 8 12 5 Please know these End of Lecture 4
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