Download (a) (b) (c) Valine H3N O- O Phenylalanine H3N O

CHAPTER 23
Practice exercises
23.1
(a)
(b)
Only glycine does not contain a stereocentre.
Isoleucine and threonine each contain two stereocentres.
23.3
At pH 6.0, glutamic acid (pI 3.08) has a net negative charge and migrates toward the
positive electrode. At this pH, arginine (pI 10.76) has a net positive charge and moves
toward the negative electrode. Valine (pI 6.00) is neutral (has no net charge) and
therefore, remains at the origin.
23.5
At pH 7.4, the only negatively charged side chains are the carboxylates of glutamic acid
and aspartic acid. Therefore, these are the amino acid side chains that form salt bridges
with lysine side chains.
Review questions
23.1
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Phenylalanine
Serine
Aspartic acid
Glutamine
Histidine
Glycine
Tyrosine
23.3
H 3N
(a)
O
O
O
O-
H 3N
H 3N
O-
(b)
O-
(c)
O
Valine
Phenylalanine
NH2
Glutamine
23.5
Arg is referred to as a ‘basic amino acid’ because its side chain contains a basic group (an
–NH2 group). His and Lys also have basic side chains and are also considered basic
amino acids.
23.7
In β-amino acids, the amino group is shifted to the carbon that is β to the carboxyl group.
β
O
α
+
O
α
H 3N
+ NH3
O
β-Alanine
α-Alanine
23.9
O
β
Note: This question should read: “Classify the following amino acids as nonpolar, polar,
neutral, acidic or basic.”
(a)
Basic
(b)
Nonpolar
(c)
Acidic
(d)
Polar
(e)
Acidic
(f)
Nonpolar
(g)
Nonpolar
23.11
(a) Threonine
(b) Arginine
NH2+
OH
COOH
H 2N
NH3+
(c) Methionine
(d) Tyrosine
NH3+
23.13
COO-
COONH3+
23.15
COOH
NH3+
HO
1 mole NaOH
NH3+
(b)
NH3+
COOH
S
(a)
COOH
N
H
COO-
+ Na+ + H2O
NH2
1 mole HCl
COOH
+ Cl-
NH3+
At pH 1.0, the most prevalent form of aspartic acid has the α-amino group protonated and
a total charge of +1.
(a) HO
COOH
1 mole NaOH
COO-
HO
NH3+
O
NH3+
O
pH 1.0
(b) HO
COOH
2 mole NaOH
-O
NH3+
O
COONH3+
O
pH 1.0
(c) HO
COOH
3 mole NaOH
-O
NH3+
O
COOO
NH2
pH 1.0
23.17
(a)
COO-
NaOH(aq)
NH3+
(b)
COO-
NH2
HCl(aq)
NH3+
(c)
COOH
COO-
COOH
NH3+
CH3CH2OH
H2SO4
NH3+
(d)
COONH3+
O
COCH2CH3
NH3
O
(CH3C)2O
CH3COO-Na+
+
COONHCCH3
O
23.19 (a)
(b)
(c)
pH 7.64
pH 3.03
pH 6.04
+
H2O
23.21
2
23.23
(a)
(b)
23.25
3
23.27
(a)
Met-Leu-Glu
The glutamic acid residue has a free α−carboxyl group and is therefore the Cterminal end. The methionine residue has a free α−amino group is therefore the
N-terminal end.
N-terminal AA
+
C-terminal AA
O
H3N
N
H
H
N
O
O
O
NH2
peptide bonds
O
Phe-Val-Asn
(b)
N-terminal AA
+
H3N
O
N
H
C-terminal AA
H
N
O
O
O
peptide bonds
H2N
Leu-Val-Gln
O
23.29
Hydrolysis yields the individual amino acids: glycine (Gly), cysteine (Cys) and valine
(Val).
23.31
Intramolecular hydrogen bonding is responsible for the formation and stability of the αhelix secondary structures. Intermolecular hydrogen bonding is possible in β-sheet
secondary structures.
23.33
The denatured proteins precipitate or coagulate into the solid that is observed in a hard
boiled egg. The consistency does not return to its original form when cooled as the
denaturation of the protein cannot be reversed.
Review problems
23.35
S configuration
23.37
COOH3 N S H
+
H
R
OH
CH3
COOH3 N C H
+
H C OH
CH3
OH
R
O
S
O-
NH3+
Threonine
23.39
COOH
NH3+
CH(CH3)2
COOH C NH3+
CH(CH3)2
D-Valine
CH2CNH2
O
D-Asparagine
Fischer
projection
H
NH2
+
Line-angle
drawing
Fischer
projection
COOH
NH3+
-OOC
O
COOH C NH3+
CH2CNH2
O
NH2
-OOC
H
NH2
+
Line-angle
drawing
COOH
NH3+
CH2CH2COO-
D-Glutamic
O
COOH C NH3+
-OOC
H
CH2CH2COO-
acid
NH2
O-
+
Line-angle
drawing
Fischer
projection
23.41
Histidine is the biological precursor to histamine and a biosynthetic decarboxylation is
involved in the conversion of histidine to histamine.
23.43
Histidine, aspartic acid and serine are all polar and ionisable residues. These are best able
to catalyse biochemical reactions. Valine and leucine however, are nonpolar and are
therefore unable to participate in the largely ionic biochemical reactions that enzymes
catalyse.
23.45
O
O
COO-
-O
pH 7.0
COO-
H2N
NH3+
pH 7.0
Glutamic acid
NH3+
Glutamine
Amino acids have no net charge at their pI. Glutamic acid possesses an acidic side chain,
so in order for Glu to be neutral, the net charge on the carboxyls must be –1. Thus the pI
for glutamic acid is between the pKa values of each carboxyl group:
pI =
pI =
1
2
1
2
(pKa α-COOH + pKa –COOH)
(2.10 + 4.07) = 3.08
The amide side chain on glutamine is at near neutral pH, so the pI of the amino acid is
determined by the pKa values for the only ionisable groups, namely the α-carboxyl and
the α-amino groups. The pI of glutamine is determined by the following equation:
pI = 12 (pKa α-COOH + pKa α-NH3+)
pI =
23.47
1
2
(2.17 + 9.03) = 5.65
The basicity of guanidine and the guanidino group is due to the large resonance
stabilisation of the protonated guanidinium ion.
+
NH
R
N
H
NH2
+
H+
-
H+
NH2
R
N
H
NH2
R
NH2
N
H
NH2
NH2
+
R +
N
H
23.49
No.
Amino acid residues with
carboxylic acid side chains
Amino acid residues
with amino side chains
Aspartic acid (Asp)
Glutamic Acid (Glu)
0
4
Histidine (His)
Lysine (Lys)
Arginine (Arg)
2
1
1
Insulin has an equal number of acidic (4) and basic (4) side chains. Its isoelectric point
must be at a pH in which all the acidic groups are deprotonated (pH > 4) and all the basic
groups are protonated (pH<6). The pI of insulin is therefore between these two values,
which is similar to that of a neutral amino acid. The experimentally determined pI value
for insulin is 5.30–5.35.
23.51
(a)
Aspartame is composed of aspartic acid (Asp) attached via a peptide bond to the
methyl ester of phenylalanine (Phe).
Asp
O
-O
H
N
+
H3N
O
O
CH3
O
Phe
Aspartame
1
2
(b)
pI =
(c)
Esters are far more easily hydrolysed than amides, and therefore, the ester is
selectively hydrolysed while leaving the amide intact. If the reaction is run under
more forcing conditions, the amide bond will also hydrolyse, yielding the amino
acids Phe and Asp.
(9.82 + 3.86) = 6.84
NH2
O
O
-O
H
N
+
H3N
HO
O
O
CH3 1.0 M HCl
+
H3N
O
OH + CH3OH
O
O
23.53
H
N
Polar, acidic and basic side chains prefer to be on the outside of the protein surface, in
contact with the aqueous environment to maximise hydrophilic interactions. Of the
choices, (b) Arg, (c) Ser and (d) Lys best fit this criterion.
Nonpolar side chains prefer to avoid contact with the aqueous environment, turning
inward to the protein to maximise hydrophobic interactions. Of the choices, these are (a)
Leu and (e) Phe.
Additional exercises
23.55
continuing
chain
O
O
N
H
COO
O
H3N
COO
OH
Homoserine
residue
Lactone
O
Truncated Protein
O
N
H
COO
no reaction
O
H3N
COO
OH
Serine residue
Too Strained
The homoserine residue undergoes a cyclisation to form a lactone and a truncated protein.
The proposed cyclisation for the serine residue involves the formation of a fourmembered lactone ring, which is highly unstable due to ring strain and therefore, unlikely
to occur.
23.57
Reaction A
E
Reaction A + protein
E
Reaction coordinate
Reaction coordinate
The process shown in the second energy diagram is a lowering of activation energy and is
associated with catalysis and is a function of proteins known as enzyme activity.