ch24 - Chapter 24 Proteins Introduction The three major...

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Chapter 24 Proteins
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Chapter 24 2 Introduction The three major groups of biological polymers are polysaccharides, proteins and nucleic acids Proteins have many diverse functions; they are major components of the following biomolecules Enzymes and hormones which catalyze and regulate biological reactions Muscles and tendons which provide the body with means for movement Hemoglobin which carries oxygen to all parts of the body Antibodies they are integral parts of the immune system All proteins are polyamides Their monomeric units are one of about 20 α - amino acids
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Chapter 24 3 Proteins have several levels of structure Primary structure refers to the exact sequence of amino acids along a protein chain Secondary and tertiary structures refer to the further bending and folding of the primary structure Quaternary structure refers to the aggregation of more than one polyamide chain All amino acids except glycine are chiral and have the L configuration (as related to glyceraldhyde) at the α carbon
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Chapter 24 4 Amino acids Structure and Names 22 amino acids but only 20 amino acids comprise the building blocks for synthesis of proteins The remaining 2 amino acids are derived by modification after biosynthesis of the protein Hydroxyproline and cystine are synthesized from proline and cysteine, respectively, after the protein chain has been synthesized Cysteine is oxidized under mild conditions to the dissulfide cystine The reaction is reversible This linkage is important in maintaining the overall shape of a protein Essential Amino Acids Essential amino acids are not made by higher animals and must be part of the diet There are 8 essential amino acids for adult humans (see Table 24.1)
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Chapter 24 5
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Chapter 24 6
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Chapter 24 7
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Chapter 24 8 Amino Acids as Dipolar Ions In the dry solid state amino acids exist as dipolar ions (zwitterions) In aqueous solution an equilibrium exists between the dipolar ion, the cationic and the anionic forms of the amino acid The predominant form depends on the pH of the solution At low pH the amino acid exists primarily in the cationic form At high pH the amino acid exists primarily in the anionic form At some intermediate pH called the p I ( isoelectric point ), the concentration of the dipolar ion is at a maximum and the concentrations of anionic and cationic forms are equal Each individual amino acid has a characteristic p I (see Table 24.1) Entire proteins also have a characteristic p I
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9 The amino acid alanine has a neutral side chain and can be used to illustrate the fundamental behavior of an amino acid at various pHs At low pH alanine exist as the cation p K a1 of alanine (for ionization of the carboxylic acid proton) is 2.3, considerably lower than the p K a of a normal carboxylic acid p K a2 of alanine (for ionization of a proton from the protonated amino group) is 9.7
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This note was uploaded on 03/14/2011 for the course CHEM 20A3 taught by Professor Brooke during the Spring '10 term at McMaster University.

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ch24 - Chapter 24 Proteins Introduction The three major...

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