lecture 27 - Amino Acids & Proteins Alanine and glycine...

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Amino Acids Amino Acids & Proteins & Proteins Chapter 27 Alanine and glycine are major components of spider silk Amino Acids ± Amino acid: Amino acid: a compound that contains both an amino group and a carboxyl group α - Amino ac i d : an amino acid in which the amino group is on the carbon adjacent to the carboxyl group Amino Acids Amino Acids although α -a mino acids are commonly written in the unionized form, they are more properly written in the zwitterion (internal salt) form RCHCO - NH 3 + O RCHCOH 2 O Chirality of Amino Acids of Amino Acids ± With the exception of glycine, all protein- derived amino acids have at least one stereocenter (the α -carbon) and are chiral Chirality Chirality of Amino Acids the vast majority have the L- configuration (S-stereocenter) at their α - carbon COO - CH 3 H H 3 N L-Alanine - 3 HN H 3 + D-Alanine
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Nonpolar Nonpolar side chains side chains NH 3 + COO - 3 + - 3 + - N HH - Alanine (Ala, A) Glycine (Gly, G) Phenylalanine (Phe, F) Proline (Pro, P) Nonpolar Nonpolar side chains side chains 3 + - 3 + - 3 + - S 3 + - N H - 3 + Isoleucine (Ile, I) Leucine (Leu, L) M ethionine (Met, M) Tryptophan (Trp, W) Valine (Val, V) Polar side chains 3 + - H 2 N O 3 + - HO Asparagine (Asn, N) Serine (Ser, S) 3 + - H 2 N O 3 + - OH Glutamine (Gln, Q) Threonine (Thr, T) Acidic Side Chains Acidic Side Chains 3 + - - O O 3 + - - O O 3 + - HS 3 + - HO Cysteine (Cys, C) Tyrosine (Tyr, Y) Glutamic acid (Glu, E) Aspartic acid (Asp, D) Basic Side Chains 3 + - N H H 2 N 2 + 3 + - N N H 3 + - H 3 N Histidine (His, H) Lysine (Lys, K) Arginine (Arg, R) + Other Amino Acids Other Amino Acids NH 3 + COO - N H H 2 N O NH 3 + COO - H 3 N HO O CH 2 CHCOO - NH 3 + II I I NH 3 + - O O L-Citrulline L-Ornithine L-Thyroxine, T 4 4-Aminobutanoic acid ( γ -Aminobutyric acid, GABA) +
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Acid - Base Base Proper Proper - ties p K a of p K a valine 2.29 9.72 tryptophan 2.38 9.39 9.10 2.09 threonine serine 2.21 9.15 10.60 2.00 proline phenylalanine 2.58 9.24 9.21 2.28 methionine 9.74 2.33 leucine isoleucine 2.32 9.76 glycine 2.35 9.78 9.13 2.17 glutamine 8.80 2.02 asparagine 9.87 2.35 alanine Nonpolar & polar side chains α− NH 3 + COOH Acid Acid -Base Properties Base Properties p K a p K a p K a 10.07 9.11 2.20 tyrosine glutamic acid 2.10 9.47 4.07 8.00 10.25 2.05 cysteine aspartic acid 2.10 9.82 3.86 Side Ch ain Acidic Ch ains NH 3 + carboxyl carboxyl sufhydryl phenolic Side Chain Group Acid -Base Properties Base Properties lysine 2.18 8.95 10.53 6.10 9.18 1.77 histidine arginine 2.01 9.04 12.48 p K a p K a p K a Ch ain Basic Ch ains NH 3 + guanidino imidazole 1° amino Chain Acidity: Acidity: α -COOH Groups COOH Groups ± The average p K a of an α -c arboxyl group is 2.19, which makes them considerably stronger acids than acetic acid (p K a 4.76) the greater acidity is accounted for by the electron- withdrawing inductive effect of the adjacent -NH 3 + group Acidity: side chain Acidity: side chain -COOH COOH ± Due to the electron -w ithdrawing inductive effect of the α -N H 3 + group, side chain -COOH groups are also stronger than acetic acid (pKa 4.76) the effect decreases with distance from the α -NH 3 + group. Compare: α -COOH group of alanine ( p K a 2.35 ) β -COOH group of aspartic acid ( p K a 3.86 ) γ -COOH group of glutamic acid ( p K a 4.07 )
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Acidity: Acidity: α -NH NH 3 + groups groups ± The average value of p K a for an α -N H 3 +
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lecture 27 - Amino Acids & Proteins Alanine and glycine...

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