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Chapter 3 - Chapter 3 Amino Acids I Introduction and...

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Chapter 3: Amino Acids I. Introduction and Background a. Biochemists have been studying proteins since the 1800s i. Mulder is responsible for discovering proteins (Greek Word Protios ) b. Abundance in Cell i. Water 70% ii. Proteins 14% iii. Nucleic Acids 7% iv. Carbohydrates 3% c. Proteins play a role in just about every biological process i. Structure (Cytoskeleton, Mircotubules) ii. Enzymes Reaction Catalysts iii. Immune response (Immunoglobulins) iv. Oxygen Transport (Hemoglobin) II. Proteins are built from amino acids a. General Structure of an Amino Acid i. Amino acids are tetrahedral structures with a positively charged amino group at one end and a negatively charged carboxyl group at the other b. Amino acids are zwitterions at physiological pH (7.0) i. Definition of zwitterion 1. Molecule that possesses a positive and negative charge c. The charge on an amino acid depends on its pH NOTE: The pKa of an amino acid in a protein is not always the same as the pKa of the individual amino acid i. Amino acids are weak polyprotic acids 1. They donate their protons in a stepwise fashion that is dependent on pH 2. As you increase the concentration of OH- there are several differing pKa values that are reached: a. pKa 1 : At this point there is 50% -COOH and 50% -COO - i. There is an equilibrium at first but as the OH- concentration increases, the percentage of deprotonated COOH increases ii. Most amino acids have a pKa 1 near 2 b. pKa 2: At this point the carboxylic acid is completely deprotonated, but the amino group is 50% NH3+ and 50% NH2
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i. Most amino acids have a Ka 2 near 9 c. pI At this point the net charge of the molecule is zero there is a simple equation it is equal to ½(pK i + pK j ) i.
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