Chapter3_BCH461

Chapter3_BCH461 - BCH 461 Chapter 3 Amino acids, peptides,...

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Unformatted text preview: BCH 461 Chapter 3 Amino acids, peptides, and proteins General properties of proteins • Most abundant biomolecule; accounts for 50% of dry weight. • Built by assembling long chains of amino acids (monomers), following by intricate folding. • Final shape of protein is very specific. Unless correctly folded, is not functional • Several 1000 different types of proteins in any cell; millions of protein molecules. • All proteins are composed of 20 “standard” amino acids. Proteins do most of the work of the cell • Each protein is specialized to do a certain job. • Some proteins are structural: control shapes of cells and bind cells together. E.g Keratin • Chemical reactions of the cells are controlled by protein enzymes. • Protein pumps move things across the cell memrane. • Proteins give mobility : muscle, flagella, molecular motors The common amino acids are known as α-amino acids because they have a primary amino group (-NH 2 ) as a substituent of the α carbon atom, the carbon next to the carboxylic acid group. The 20 standard amino acids differ in the structure of their side chains (R groups). 3.1 Amino Acids α Nonpolar Aliphatic Amino Acids Gly, G Ala, A Pro, P Val, V Leu, L Ile, I Met, M Polar Uncharged Amino Acids Ser, S Thr, T Cys, C Asn, N Gln, Q Aromatic Amino Acids Phe, F Tyr, Y Trp, W Negatively charged Amino Acids Asp, D Glu, E Positively charged Amino Acids Lys, K Arg, R His, H Ultraviolet Spectra of Aromatic Amino Acids • Tyrosine and tryptophan absorb at approximately 280 nm . • Phenylanaline absorbs at approximately 260 nm, similar to nucleic acids • Determination of protein concentration by ultraviolet absorption at 280 nm depends on the presence of tyr and trp residues in proteins. Amino Acids are Zwitterions (Twin Ions) at Physiological pH Ionic Forms of Amino Acids pH 1, Net charge +1 pH 7, Net charge 0 pH 13, Net charge -1 Cationic form Zwitterion (neutral) form Anionic form pK a Values of the Amino Acids You should know these numbers and know what they mean! • Alpha carboxyl group: pK 1 ≈ 2 • Alpha amino group: pK 2 ≈ 9 • These numbers are approximate, but entirely suitable for our purposes. Ionization of Side Chains: Acidic aa pK R = 3.9 pK R = 4.3 asp glu Ionization of Side Chains: Basic aa lys arg his pK R = 10.5 pK R = 12.5 pK R = 6.0 Ionization of Side Chains: -OH Groups ser thr tyr pK R ~ 10.5 pK R ~ 13 pK R = 10.1 Ionization of Side Chains: Cysteine pK R = 8.2 Ampholites and Isoelectric Point Ampholites are molecules that contain both acidic and basic pK a values (e.g. amino acids)....
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This note was uploaded on 10/10/2009 for the course BCH 461 taught by Professor Lefler during the Spring '08 term at ASU.

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Chapter3_BCH461 - BCH 461 Chapter 3 Amino acids, peptides,...

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