Chapter2 - You are here Chapter 2: Protein Composition and...

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Unformatted text preview: You are here Chapter 2: Protein Composition and Structure The unique structure of the this protein allows it to sense visible light, and allows you to see this slide While the unique structure of this protein unfortunately produces a fatal and untreatable neurological disease Proteins are: Important constituents of living systems The most abundant macromolecule of life Important component of our food and nutrition Source of amino acids Sometimes a source of energy Insulin is a hormone that is two polypeptides linked by disulfide bonds “Children dying from diabetic keto-acidosis were kept in large wards, often with 50 or more patients in a ward, mostly comatose. Grieving family members were often in attendance, awaiting the (until then, inevitable) death. In one of medicine's more dramatic moments Banting, Best, and Collip went from bed to bed, injecting an entire ward with the new purified extract. Before they had reached the last dying child, the first few were awakening from their coma, to the joyous exclamations of their families.” The basic structure of proteins; polymers of amino acids • Molecule with at least one carboxyl group (organic acid) and at least one amino group (organic base) • Hundreds of amino acids found in nature; only 20 commonly used to make proteins An amino acid is: melamine D and L Amino Acids • Amino acids exist as enantiomers – Mirror images – D and L forms – Optical activity (D and L forms will rotate plane-polarized light in opposite directions) • Only L isomers are incorporated in proteins by ribosomes (normal peptide synthesis) (S absolute configuration) (R absolute configuration) Stereoisomers; which is which? Important Chemical Properties • Soluble in water • Amphoteric- can be a base or an acid • Exist as zwitterions at physiological pH (pH = 7.4). Same molecule has positive AND negative charge. The amino acid “backbone” has two titratable functional groups The amino and carboxyl groups Amino acids have at least two ionizable groups: alanine as an example Proteins have a P I (or pH I ) which is the pH at which their charges balance to a net = 0. What are the options for “R” groups? The simple… The aliphatic… iso-propyl iso-butyl sec-butyl The aromatic… Amino acid optical absorbance in the UV contribution from “side chain” contribution from “main chain” as well The strange…. The hydroxylated… The one with a thiol group…. Titratable amine groups…. pKa ~ 6.0 Histidine is unique and important… • Histidine has an ionizable group with a pKa near physiological pH. – Can act as an acid (proton donor) – Can act as a base (proton acceptor) – Physiologically very significant!! The carboxylates and carboxamides…. Knowing the exact pKas of the amino acid sidechains is not necessary in this course … however , you should know whether the pKa is > 7 or < 7 and therefore whether the side-chain is ionized at neutral pH But local environment can change these dramatically… Recognize the animo acid abbreviations too.Recognize the animo acid abbreviations too....
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This note was uploaded on 09/25/2011 for the course BBMB 404 taught by Professor Staff during the Spring '08 term at Iowa State.

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Chapter2 - You are here Chapter 2: Protein Composition and...

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