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Lecture note on Proteins

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Arizona State University
BIO 181
Kenneth Mossman

Proteins  Amino Acids (AA’s)- General Structure  All proteins are composed of chains of roughly 21 different amino acids o Numbers vary by species  All amino acids have a central carbon atom attached by covalent bonds to an amine group (NH ), 2 carboxylic group (COOH), a hydrogen atom (H), and a variable side chain (R) o The amine group attached to a carbon atom attached to an acid group forms the backbone  Side chains differ in size, shape, chemical reactivity o Those with hydroxyl, amino acid, carboxylic acid, or sulfhydryl function have more reactivity than those composed of hydrogen and carbon  Also dictate interaction with water o Polar R groups form hydrogen bonds  Dissolve in water o Nonpolar R groups can’t form hydrogen bonds 3+  In water (pH 7), amino and carboxylic acid groups ionize to NH and COO- respectively o Makes protein more reactive o Nonzwitterionic- no charges o Zwitterionic- acid gives hydrogen and amine takes the hydrogen  Isomers- amino acids (AA’s)  Same molecular formula, different structure  Isomers that don’t apply to AA’s o Structural- differ in order of atoms o Geometric- differ in arrangements of atoms around double bond  Isomer that applies to AA’s o Optical- differ in arrangement around a carbon atom that has four different groups attached o With one exception, all AA have optical isomers  Cells primarily use “left-handed” forms for protein synthesis  Condensation and Hydrolysis Reactions  AA’s are monomers that can polymerize (join together) to form protein polymers  Polymerization requires energy and is NOT spontaneous o Monomers polymerize through special chemical reactions called condensation reactions that release water molecules o In the reverse situation, hydrolysis reactions require the addition of water molecules for a polymer to release a monomer  The principle of entropy suggests that hydrolytic processes should predominate over condensations because they are energetically favorable o However, biology often contradicts these chemical and physical properties  Peptide bonds  Condensation reactions bond the carboxylic acid group of one amino acid to the amine group of another to form a pep
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