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Amino Acids• In 1839, the Swedish chemist Jacob Berzelius coined
the term protein (Greek: proteios, ) for thisclass of compounds
• The first amino acids had been isolated in 1830
• Scientists began to consider the nutritive qualities ofAmino Acids ( )
• The central role of amino acids in biochemistry:
1. Several amino acids are among the organiccompounds believed to have appeared early in theearth’s history
2. Amino acids have been co-opted by evolution for avariety of purposes in living system
Amino AcidsAmino Acid Structure
• All proteins are composed of 20“standard” amino acids
• Not every protein contains all 20types of amino acids
• The common amino acids areknown as α-amino acids
• The sole exception is proline,which has a secondary aminogroup (—NH—), although foruniformity we will refer to prolineas an α-amino acid
Amino AcidsAmino Acid Structure
A. Amino Acids Are Dipolar Ions
• The amino and carboxylic acid groupsof amino acids ionize readily
• The pK values of the carboxylic acidgroups lie in a small range around 2.2
• The pK values of the α-amino groups(pK2) are near 9.4
• At physiological pH (∼7.4), the aminogroups are protonated and thecarboxylic acid groups are in theirconjugate base (carboxylate) form
See Table 4-1page 82
Amino AcidsAmino Acid Structure
A. Amino Acids Are Dipolar Ions
• An amino acid can therefore act as both an acidand a base
• Molecules such as amino acids, which bear chargedgroups of opposite polarity, are known as dipolarions or zwitterions
• Amino acids, like other ionic compounds, are moresoluble in polar solvents than in nonpolar solvents
• The ionic properties of the side chains influencethe physical and chemical properties of free aminoacids and amino acids in proteins
Amino AcidsAmino Acid Structure
B. Peptide Bonds Link Amino Acids
• Amino acids can be polymerized to form chains
• This process can be represented as a condensationreaction, bond formation with the elimination of a watermolecule
• The resulting linkage, an , is knownas a
• Polymers composed of , , a , andamino acid units are known, respectively, as ,
, , and
• Amino acid residues & peptides (Terminologies to know!)
Amino AcidsAmino Acid Structure
B. Peptide Bonds Link Amino Acids
• Polypeptides are linear polymers rather thanbranched chains; that is, each amino acid residueparticipates in two peptide bonds and is linked to itsneighbours in a head-to-tail fashion
• The residues at the two ends of the polypeptide eachparticipate in just one peptide bond
• The residue with a free amino group is called theamino terminus or N-terminus
• The residue with a free carboxylate group (at theright) is called the carboxyl terminus or C-terminus
Amino AcidsAmino Acid Structure
C. Amino Acid Side Chains Are Nonpolar, Polar, or Charged
• The most useful way to classify the 20 standard aminoacids is by the polarities of their side chains
• According to the most common classification scheme,there are three major types of amino acids:
1. those with nonpolar R groups
2. those with uncharged polar R groups, and
3. those with charged polar R groups
Amino AcidsAmino Acid Structure
C. Amino Acid Side Chains Are Nonpolar, Polar, or Charged
• Nine amino acids are classified as having nonpolarside chains
• The three dimensional shapes of some of these aminoacids are shown in Fig. 4-4, page 85
• Glycine has the smallest possible side chain, an Hatom
, , , and haveside chains ranging in size
from a methyl group for to isomeric butylgroups for and
Amino AcidsAmino Acid Structure
C. Amino Acid Side Chains Are Nonpolar, Polar, or Charged
has a side chain thatresembles an n-butyl group in many of its physicalproperties
has a cyclic side group
(with its moiety) and(with its group) contain
, which are characterized bybulk as well as nonpolarity
Amino AcidsAmino Acid Structure
C. Amino Acid Side Chains Are Nonpolar, Polar, or Charged
Uncharged Polar Side Chains Have Hydroxyl, Amide, or Thiol Groups
• Six amino acids are commonly classified as having uncharged polarside chains
and bear of differentsizes
and haveof different sizes
has a (and, like phenylalanine andtryptophan, is aromatic)
is unique among the 20 amino acids in that it has athat can form a disulfide bond with another cysteine
through the oxidation of the two thiol groups
Amino AcidsAmino Acid Structure
C. Amino Acid Side Chains Are Nonpolar, Polar, or Charged
Charged Polar Side Chains Are Positively or Negatively Charged
• Five amino acids have charged side chains
• The side chains of the basic amino acids are positivelycharged at physiological pH values
has a side chain, andbears a group
carries an moiety
Amino AcidsAmino Acid Structure
C. Amino Acid Side Chains Are Nonpolar, Polar, or Charged
Charged Polar Side Chains Are Positively or Negatively Charged
• The side chains of the acidic amino acids,and , are negatively charged
above pH 3
and are, respectively, theamides of and
Amino AcidsAmino Acid Structure
D. The pK Values of Ionizable Groups Depend on Nearby Groups
• The α-amino acids have two or, for those withionizable side chains, three acid–base groups
• At values, these groups are fully, and at values, these groups
are
• At pH values, the tend tobe , and the tend to be
Amino AcidsAmino Acid Structure
D. The pK Values of Ionizable Groups Depend on Nearby Groups
• Thus, for the amino acid , below pH 2.35 (thepK value of its carboxylic acid group), the+H3NCH2COOH form predominates
• Above pH 2.35, the carboxylic acid is mostly ionizedbut the amino group is still mostly protonated(+H3NCH2COO−)
• Above pH 9.78 (the pK value of the amino group), theH2NCH2COO− form predominates
Amino AcidsAmino Acid Structure
D. The pK Values of Ionizable Groups Depend on Nearby Groups
• The at which a molecule carries no net electric charge isknown as its isoelectric point,
• where Ki and Kj are the dissociation constants of the twoionizations involving the neutral species
• For monoamino, monocarboxylic acids such as glycine, Ki andKj represent K1 and K2
• For aspartic and glutamic acids, Ki and Kj are K1 and KR
• For arginine, histidine, and lysine, these quantities are KR andK2
Amino AcidsAmino Acid Structure
D. The pK Values of Ionizable Groups Depend on Nearby Groups
Amino AcidsAmino Acid Structure
D. The pK Values of Ionizable Groups Depend on Nearby Groups
• Deprotonation of carboxylic acids gives
carboxylate anions
• these are resonance stabilized, because
the negative charge is delocalized over
the two oxygen atoms, increasing the
stability of the anion
• Each of the carbon–oxygen bonds in the
carboxylate anion has a partial double-
bond character
Amino AcidsAmino Acid Structure
D. The pK Values of Ionizable Groups Depend on Nearby Groups
• Amino acid residues in the interior of a polypeptidechain do not have free α-amino and α-carboxylgroups that can ionize
• These groups are joined in peptide bonds
Amino AcidsAmino Acid Structure
E. Amino Acid Names Are Abbreviated
• The three-letter abbreviations for the 20 standardamino acids given in Table 4-1, page 82, are widelyused in the biochemical literature
• Most of these abbreviations are taken from the firstthree letters of the name of the corresponding aminoacid and are pronounced as written
• The symbol Glx indicates Glu or Gln, and similarly, Asxmeans Asp or Asn
Amino AcidsAmino Acid Structure
E. Amino Acid Names Are Abbreviated
• The one-letter symbols for the amino acids are alsogiven in Table 4-1, page 82.
• This more compact code is often used whencomparing the amino acid sequences of severalsimilar proteins
• Note that the one-letter symbol is usually the firstletter of the amino acid’s name
• However, for sets of residues that have the same firstletter, this is true only of the most abundant residueof the set
Amino AcidsAmino Acid Structure
E. Amino Acid Names Are Abbreviated
• Amino acid residues in polypeptides are named bydropping the suffix, usually -ine, in the name of theamino acid and replacing it by -yl
• Polypeptide chains are described by starting at the N-terminus and proceeding to the C-terminus
• The amino acid at the C-terminus is given the name ofits parent amino acid
Amino AcidsStereochemistry
• Amino acids are chiral molecules whose configurations can be depicted by Fischer projections
• The amino acids in proteins all have the L stereochemical configuration
Amino AcidsStereochemistry