Fundamentals While their name implies that amino acids are compounds that contain an 2 group and a 2 group, these groups are actually present as 3 and 2 respectively. They are classified as α, β, γ, etc.. amino acids according the carbon that bears the nitrogen. α 3 2 3 2 2 2 β an α-amino acid that is an intermediate in the biosynthesis of ethylene a β-amino acid that is one of the structural units present in coenzyme A a γ-amino acid involved in 3 2 2 2 2 the transmission of nerve γ impulses The 20 (22) Key More than 700 amino acids occur naturally, but 20 (22?)of them are especially important. These 22 amino acids are the building blocks of proteins. All are α-amino acids. They differ in respect to the group attached to the α carbon. See andout. 3 R Glycine (Gly or G) 3 The amino acids obtained by hydrolysis of proteins differ in respect to R (the side chain). The properties of the amino acid vary as the structure of R varies. Glycine is the simplest amino acid. It is the only one in the table that is achiral. In all of the other amino acids in the table the α carbon is a stereogenic center.
3 3 3 Alanine (Ala or A) ( 3 ) 2 Valine (Val or V) 3 3 2 ( 3 ) 2 Leucine (Leu or L) 3 2 3 Isoleucine (Ile or I) 3 2 3 S 2 2 Methionine (Met or M) 2 2 2 Proline (Pro or P)
3 3 2 2 Phenylalanine (Phe or F) Tryptophan (Trp or W) 3 3 2 2 Asparagine (Asn or ) 2 2 2 Glutamine (Gln or Q) 3 3 2 Serine (Ser or S) 3 Threonine (Thr or T)
3 2 3 2 2 Aspartic Acid (Asp or D) Glutamic Acid (Glu or E) 3 3 2 2 S Tyrosine (Tyr or Y) ysteine (ys or ) 3 2 2 2 2 3 3 2 2 2 2 Lysine (Lys or K) Arginine (Arg or R) 2
: #21 (2001) 3 2 istidine (is or ) Selenocysteine : #22 (2002) : #22 (2002) 2 2 X= X 3 2 Pyrrolysine (4 R, 5 R) Pyrrolysine with eutral Side hains Acid-Base Behavior of Glycine 3 pk a1 = 2.34 pk a2 = 9.60 pi = 5.97
with eutral Side hains with eutral Side hains Alanine 3 3 pk a1 = 2.34 pk a2 = 9.69 pi = 6.00 Valine 3 ( 3 ) 2 pk a1 = 2.32 pk a2 = 9.62 pi = 5.96 with eutral Side hains with eutral Side hains Leucine 3 2 ( 3 ) 2 pk a1 = 2.36 pk a2 = 9.60 pi = 5.98 Isoleucine 3 3 2 3 pk a1 = 2.36 pk a2 = 9.60 pi = 5.98 with eutral Side hains with eutral Side hains Methionine 3 3 S 2 2 pk a1 = 2.28 pk a2 = 9.21 pi = 5.74 Proline 2 2 2 2 pk a1 = 1.99 pk a2 = 10.60 pi = 6.30
Side hains with eutral Side hains Phenylalanine 3 2 pk a1 = 1.83 pk a2 = 9.13 pi = 5.48 Tryptophan 3 2 pk a1 = 2.83 pk a2 = 9.39 pi = 5.89 with eutral Side hains with eutral Side hains Asparagine 3 2 2 pk a1 = 2.02 pk a2 = 8.80 pi = 5.41 Glutamine 3 2 2 2 pk a1 = 2.17 pk a2 = 9.13 pi = 5.65 with eutral Side hains with eutral Side hains Serine 3 2 pk a1 = 2.21 pk a2 = 9.15 pi = 5.68 Threonine 3 3 pk a1 = 2.09 pk a2 = 9.10 pi = 5.60
with Ionizable Side hains with Ionizable Side hains Aspartic acid 3 2 pk a1 = 1.88 pk a2 = 3.65 pk a3 = 9.60 pi = 2.77 Glutamic acid 3 2 2 pk a1 = 2.19 pk a2 = 4.25 pk a3 = 9.67 pi = 3.22 For amino acids with acidic side chains, pi is the average of pkp a1 and pkp a2. with Ionizable Side hains with Ionizable Side hains Tyrosine 3 2 pk a1 = 2.20 pk a2 = 9.11 pk a3 = 10.07 pi = 5.66 ysteine 3 pk a1 = 1.96 pk a2 = 8.18 pk a3 = 10.28 2 S pi = 5.07 with Ionizable Side hains with Ionizable Side hains 3 2 2 2 2 3 Lysine pk a1 = 2.18 pk a2 = 8.95 pk a3 = 10.53 pi = 9.74 For amino acids with basic side chains, pi is the average of pkp a2 and pkp a3. 3 Arginine 2 2 2 2 2 pk a1 = 2.17 pk a2 = 9.04 pk a3 = 12.48 pi = 10.76
with Ionizable Side hains istidine 3 2 pk a1 = 1.82 pk a2 = 6.00 pk a3 = 9.17 pi = 7.59