Aula2_Aminoacidos_2018

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Universidade Federal do Rio de Janeiro 
Profa	
  Gisele	
  Amorim	
  
	
  
Bioquímica	
  
Aula	
  2:	
  Aminoácidos	
  
Proteínas	
  
ü Enzimas,	
  hormônios,	
  an=corpos,	
  transportadores,	
  fibras	
  
musculares	
  etc.	
  
ü An=bió=cos,	
  venenos	
  etc.	
  
	
  
ü Polímeros	
  de	
  aminoácidos	
  ligados	
  por	
  um	
  =po	
  especifico	
  de	
  
ligação	
  covalente.	
  	
  
	
  
ü  Todas	
  as	
  proteínas,	
  mesmo	
  as	
  pertencentes	
  às	
  mais	
  an=gas	
  
linhagens	
  de	
  bactérias,	
  ou	
  aos	
  organismos	
  mais	
  complexos,	
  
são	
  formadas	
  por	
  repe=ções	
  dos	
  mesmos	
  20	
  aminoácidos.	
  	
  
Apolares	
  alifá1cos	
  
Apolares aromáticos 
O grupamento hidroxila da tirosina pode formar pontes de hidrogênio, é um 
importante grupamento funcional no sítio catalítico de algumas enzimas. 
Tirosina e triptofano são significativamente mais polares do que fenilalanina, 
devido a hidroxila fenólica da tirosina e ao nitrogênio indólico do triptofano 
São mais hidrofílicos que os aminoácidos apolares porque contém grupamentos 
funcionais capazes de fazer ligações de hidrogênio com a água 
A polaridade da serina e da 
treonina é devida à hidroxila (–OH) 
A polaridade da cisteína é 
devida à sulfidrila (-SH) 
Os grupamentos amida 
conferem polaridade às 
cadeias laterais da 
asparagina e da glutamina 
Polares não carregados 
Grupamentos R com carga positiva (básicos) 
Muito hidrofílicos. Apresentam carga elétrica positiva 
em pH fisiológico. 
 
Lisina contém uma amina primária 
Arginina contém um grupamento 
guanidino 
Histidina contém um grupamento 
imidazol 
amina 
guanidino 
imidazol 
Possuem carga negativa em pH fisiológico 
Grupamentos R com carga negativa (ácidos) 
Cisteínas	
  -­‐	
  Ligações	
  Dissulfeto	
  
A cisteína é facilmente oxidada para formar uma cistina através de uma 
ligação covalente do tipo dissulfeto 
As ligações dissulfeto tem um papel muito importante na estrutura de muitas 
proteínas formando ligações covalentes entre diferentes partes de uma molécula de 
proteína ou entre duas cadeias polipeptídicas diferentes. 
Aminoácidos	
  “não-­‐padrão”	
  
Presentes	
  em	
  certas	
  proteínas	
  
	
  
Aminoácidos	
  “não-­‐padrão”	
  
Não	
  ocorrem	
  em	
  proteínas	
  
Nomenclatura	
  dos	
  átomos	
  da	
  cadeia	
  lateral	
  
G	
  -­‐	
  Glycine	
  
(Gly)	
  
P	
  -­‐	
  Proline	
  
(Pro)	
  
A	
  -­‐	
  Alanine	
  
(Ala)	
  
V	
  -­‐	
  Valine	
  
(Val)	
  
L	
  -­‐	
  Leucine	
  
(Leu)	
  
I	
  -­‐	
  Isoleucine	
  
(Ile)	
  
M	
  -­‐	
  
Methionine	
  
(Met)	
  
C	
  -­‐	
  Cysteine	
  
(Cys)	
  
F	
  -­‐	
  
Phenylalanine	
  
(Phe)	
  
Y	
  -­‐	
  Tyrosine	
  
(Tyr)	
  
W	
  -­‐	
  
Tryptophan	
  
(Trp)	
  
H	
  -­‐	
  His=dine	
  
(His)	
  
K	
  -­‐	
  Lysine	
  (Lys)	
   R	
  -­‐	
  Arginine	
  (Arg)	
  
Q	
  -­‐	
  Glutamine	
  
(Gln)	
  
N	
  -­‐	
  Asparagine	
  
(Asn)	
  
E	
  -­‐	
  Glutamic	
  
Acid	
  (Glu)	
  
D	
  -­‐	
  Aspar=c	
  
Acid	
  (Asp)	
   S	
  -­‐	
  Serine	
  (Ser)	
  
T	
  -­‐	
  Threonine	
  
(Thr)	
  
Código	
  de	
  três	
  letras	
  e	
  uma	
  letra	
  para	
  os	
  aminoácidos	
  
Podem	
  ionizar?	
  
Ionização	
  dos	
  Aminoácidos	
  
São	
  anfóteros,	
  como	
  a	
  água	
  
pH	
  fisiológico?	
  
pKa	
  COOH:	
  1,8-­‐2,7	
  
	
  
	
  
pKa	
  NH2:	
  	
  8,8-­‐10,6	
  
pKa	
  reflete	
  a	
  tendência	
  de	
  um	
  acido	
  fraco	
  em	
  doar/perder	
  um	
  próton	
  
Ionização	
  dos	
  grupos	
  amino	
  e	
  
carboxila	
  depende	
  do	
  pH	
  
Propriedades	
  Acido-­‐Básicas	
  dos	
  Aminoácidos	
  
Curva	
  de	
  1tulação	
  da	
  glicina	
  
Curva	
  de	
  1tulação	
  da	
  glicina	
  
group in the range of 1.8 to 2.4, and pKa of the ONH3!
group in the range of 8.8 to 11.0 (Table 3–1).
Second, amino acids with an ionizable R group have
more complex titration curves, with three stages corre-
sponding to the three possible ionization steps; thus
they have three pKa values. The additional stage for the
titration of the ionizable R group merges to some extent
with the other two. The titration curves for two amino
acids of this type, glutamate and histidine, are shown in
Figure 3–12. The isoelectric points reflect the nature of
the ionizing R groups present. For example, glutamate
Chapter 3 Amino Acids, Peptides, and Proteins84
10
8
6
4
2
0
Glutamate
H3N
!
N
!
N
!
C
COOH
C
C
COOH
H2
H2
H
pK1
H3 C
COO"
C
C
COOH
H2
H2
H
pKR
H3 C
COO"
C
C
COO"
H2
H2
H
pK2
H2N C
COO"
C
C
COO"
H2
H2
H
pK2 #
9.67
pKR #
4.25
pK1 #
2.19
1.0 2.0 3.0
pH
OH" (equivalents)
(a)
FIGURE 3–12 Titration curves for (a) glutamate and (b) histidine. The
pKa of the R group is designated here as pKR.
The second piece of information provided by the
titration curve of glycine is that this amino acid has two
regions of buffering power. One of these is the relatively
flat portion of the curve, extending for approximately 
1 pH unit on either side of the first pKa of 2.34, indi-
cating that glycine is a good buffer near this pH. The
other buffering zone is centered around pH 9.60. (Note
that glycine is not a good buffer at the pH of intracel-
lular fluid or blood, about 7.4.) Within the buffering
ranges of glycine, the Henderson-Hasselbalch equation
(see Box 2–3) can be used to calculate the proportions
of proton-donor and proton-acceptor species of glycine
required to make a buffer at a given pH.
Titration Curves Predict the Electric Charge 
of Amino Acids
Another important piece of information derived from
the titration curve of an amino acid is the relationship
between its net electric charge and the pH of the solu-
tion. At pH 5.97, the point of inflection between the
two stages in its titration curve, glycine is present pre-
dominantly as its dipolar form, fully ionized but with no
net electric charge (Fig. 3–10). The characteristic pH
at which the net electric charge is zero is called the
isoelectric point or isoelectric pH, designated pI.
For glycine, which has no ionizable group in its side
chain, the isoelectric point is simply the arithmetic mean
of the two pKa values:
pI # $
1
2$ (pK1 ! pK2) # $
1
2$ (2.34 ! 9.60) # 5.97
As is evident in Figure 3–10, glycine has a net negative
charge at any pH above its pI and will thus move toward
the positive electrode (the anode) when placed in an
electric field. At any pH below its pI, glycine has a net
positive charge and will move toward the negative elec-
trode (the cathode). The farther the pH of a glycine so-
lution is from its isoelectric point, the greater the net
electric charge of the population of glycine molecules.
At pH 1.0, for example, glycine exists almost entirely as
the form !H3NOCH2OCOOH, with a net positive
charge of 1.0. At pH 2.34, where there is an equal mix-
ture of !H3NOCH2OCOOH and !H3NOCH2OCOO",
the average or net positive charge is 0.5. The sign and
the magnitude of the net charge of any amino acid at
any pH can be predicted in the same way.
Amino Acids Differ in Their Acid-Base Properties
The shared properties of many amino acids permit some
simplifying generalizations about their acid-base behav-
iors. First, all amino acids with a single !-amino group,
a single !-carboxyl group, and an R group that does not
ionize have titration curves resembling that of glycine
(Fig. 3–10). These amino acids have very similar, al-
though not identical, pKa values: pKa of the OCOOH
C
H3N
!
C
COOH
C
CH
C
H
N
H2
H H3N
!
C
COO"
CH2
H H3N
!
C
COO"
CH2
H H2N C
CH2
H
pK1 #
1.82
pKR #
6.0
pK2 #
9.17
C
H
N
CH
C
H
N
!
H
C
H
N
CH
C
H
N
!
H
C
H
N
CH
C
H
N
10
8
6
4
2
0 1.0 2.0 3.0
pH
OH" (equivalents)
(b)
COO"
H
N
Histidine
pK2pKRpK1
8885d_c03_084 12/23/03 10:21 AM Page 84 mac111 mac111:reb:
group in the range of 1.8 to 2.4, and pKa of the ONH3!
group in the range of 8.8 to 11.0 (Table 3–1).
Second, amino acids with an ionizable R group have
more complextitration curves, with three stages corre-
sponding to the three possible ionization steps; thus
they have three pKa values. The additional stage for the
titration of the ionizable R group merges to some extent
with the other two. The titration curves for two amino
acids of this type, glutamate and histidine, are shown in
Figure 3–12. The isoelectric points reflect the nature of
the ionizing R groups present. For example, glutamate
Chapter 3 Amino Acids, Peptides, and Proteins84
10
8
6
4
2
0
Glutamate
H3N
!
N
!
N
!
C
COOH
C
C
COOH
H2
H2
H
pK1
H3 C
COO"
C
C
COOH
H2
H2
H
pKR
H3 C
COO"
C
C
COO"
H2
H2
H
pK2
H2N C
COO"
C
C
COO"
H2
H2
H
pK2 #
9.67
pKR #
4.25
pK1 #
2.19
1.0 2.0 3.0
pH
OH" (equivalents)
(a)
FIGURE 3–12 Titration curves for (a) glutamate and (b) histidine. The
pKa of the R group is designated here as pKR.
The second piece of information provided by the
titration curve of glycine is that this amino acid has two
regions of buffering power. One of these is the relatively
flat portion of the curve, extending for approximately 
1 pH unit on either side of the first pKa of 2.34, indi-
cating that glycine is a good buffer near this pH. The
other buffering zone is centered around pH 9.60. (Note
that glycine is not a good buffer at the pH of intracel-
lular fluid or blood, about 7.4.) Within the buffering
ranges of glycine, the Henderson-Hasselbalch equation
(see Box 2–3) can be used to calculate the proportions
of proton-donor and proton-acceptor species of glycine
required to make a buffer at a given pH.
Titration Curves Predict the Electric Charge 
of Amino Acids
Another important piece of information derived from
the titration curve of an amino acid is the relationship
between its net electric charge and the pH of the solu-
tion. At pH 5.97, the point of inflection between the
two stages in its titration curve, glycine is present pre-
dominantly as its dipolar form, fully ionized but with no
net electric charge (Fig. 3–10). The characteristic pH
at which the net electric charge is zero is called the
isoelectric point or isoelectric pH, designated pI.
For glycine, which has no ionizable group in its side
chain, the isoelectric point is simply the arithmetic mean
of the two pKa values:
pI # $
1
2$ (pK1 ! pK2) # $
1
2$ (2.34 ! 9.60) # 5.97
As is evident in Figure 3–10, glycine has a net negative
charge at any pH above its pI and will thus move toward
the positive electrode (the anode) when placed in an
electric field. At any pH below its pI, glycine has a net
positive charge and will move toward the negative elec-
trode (the cathode). The farther the pH of a glycine so-
lution is from its isoelectric point, the greater the net
electric charge of the population of glycine molecules.
At pH 1.0, for example, glycine exists almost entirely as
the form !H3NOCH2OCOOH, with a net positive
charge of 1.0. At pH 2.34, where there is an equal mix-
ture of !H3NOCH2OCOOH and !H3NOCH2OCOO",
the average or net positive charge is 0.5. The sign and
the magnitude of the net charge of any amino acid at
any pH can be predicted in the same way.
Amino Acids Differ in Their Acid-Base Properties
The shared properties of many amino acids permit some
simplifying generalizations about their acid-base behav-
iors. First, all amino acids with a single !-amino group,
a single !-carboxyl group, and an R group that does not
ionize have titration curves resembling that of glycine
(Fig. 3–10). These amino acids have very similar, al-
though not identical, pKa values: pKa of the OCOOH
C
H3N
!
C
COOH
C
CH
C
H
N
H2
H H3N
!
C
COO"
CH2
H H3N
!
C
COO"
CH2
H H2N C
CH2
H
pK1 #
1.82
pKR #
6.0
pK2 #
9.17
C
H
N
CH
C
H
N
!
H
C
H
N
CH
C
H
N
!
H
C
H
N
CH
C
H
N
10
8
6
4
2
0 1.0 2.0 3.0
pH
OH" (equivalents)
(b)
COO"
H
N
Histidine
pK2pKRpK1
8885d_c03_084 12/23/03 10:21 AM Page 84 mac111 mac111:reb:
Ponto	
  isoelétrico	
  (pI)	
  
Para	
  a	
  glicina	
  
pH	
  onde	
  a	
  carga	
  global	
  é	
  igual	
  a	
  zero	
  
pH	
  abaixo	
  do	
  pI	
   pH	
  =	
  pI	
   pH	
  acima	
  do	
  pI	
  
group in the range of 1.8 to 2.4, and pKa of the ONH3!
group in the range of 8.8 to 11.0 (Table 3–1).
Second, amino acids with an ionizable R group have
more complex titration curves, with three stages corre-
sponding to the three possible ionization steps; thus
they have three pKa values. The additional stage for the
titration of the ionizable R group merges to some extent
with the other two. The titration curves for two amino
acids of this type, glutamate and histidine, are shown in
Figure 3–12. The isoelectric points reflect the nature of
the ionizing R groups present. For example, glutamate
Chapter 3 Amino Acids, Peptides, and Proteins84
10
8
6
4
2
0
Glutamate
H3N
!
N
!
N
!
C
COOH
C
C
COOH
H2
H2
H
pK1
H3 C
COO"
C
C
COOH
H2
H2
H
pKR
H3 C
COO"
C
C
COO"
H2
H2
H
pK2
H2N C
COO"
C
C
COO"
H2
H2
H
pK2 #
9.67
pKR #
4.25
pK1 #
2.19
1.0 2.0 3.0
pH
OH" (equivalents)
(a)
FIGURE 3–12 Titration curves for (a) glutamate and (b) histidine. The
pKa of the R group is designated here as pKR.
The second piece of information provided by the
titration curve of glycine is that this amino acid has two
regions of buffering power. One of these is the relatively
flat portion of the curve, extending for approximately 
1 pH unit on either side of the first pKa of 2.34, indi-
cating that glycine is a good buffer near this pH. The
other buffering zone is centered around pH 9.60. (Note
that glycine is not a good buffer at the pH of intracel-
lular fluid or blood, about 7.4.) Within the buffering
ranges of glycine, the Henderson-Hasselbalch equation
(see Box 2–3) can be used to calculate the proportions
of proton-donor and proton-acceptor species of glycine
required to make a buffer at a given pH.
Titration Curves Predict the Electric Charge 
of Amino Acids
Another important piece of information derived from
the titration curve of an amino acid is the relationship
between its net electric charge and the pH of the solu-
tion. At pH 5.97, the point of inflection between the
two stages in its titration curve, glycine is present pre-
dominantly as its dipolar form, fully ionized but with no
net electric charge (Fig. 3–10). The characteristic pH
at which the net electric charge is zero is called the
isoelectric point or isoelectric pH, designated pI.
For glycine, which has no ionizable group in its side
chain, the isoelectric point is simply the arithmetic mean
of the two pKa values:
pI # $
1
2$ (pK1 ! pK2) # $
1
2$ (2.34 ! 9.60) # 5.97
As is evident in Figure 3–10, glycine has a net negative
charge at any pH above its pI and will thus move toward
the positive electrode (the anode) when placed in an
electric field. At any pH below its pI, glycine has a net
positive charge and will move toward the negative elec-
trode (the cathode). The farther the pH of a glycine so-
lution is from its isoelectric point, the greater the net
electric charge of the population of glycine molecules.
At pH 1.0, for example, glycine exists almost entirely as
the form !H3NOCH2OCOOH, with a net positive
charge of 1.0. At pH 2.34, where there is an equal mix-
ture of !H3NOCH2OCOOH and !H3NOCH2OCOO",
the average or net positive charge is 0.5. The sign and
the magnitude of the net charge of any amino acid at
any pH can be predicted in the same way.
Amino Acids Differ in Their Acid-Base Properties
The shared properties of many amino acids permit some
simplifying generalizations about their acid-base behav-
iors. First, all amino acids with a single !-amino group,
a single !-carboxyl group, and an R group that does not
ionize have titration curves resembling that of glycine
(Fig. 3–10). These amino acids have very similar, al-
though not identical, pKa values: pKa of the OCOOH
C
H3N
!
C
COOH
C
CH
C
H
N
H2
H H3N
!
C
COO"
CH2
H H3N
!
C
COO"
CH2
H H2N C
CH2
H
pK1 #
1.82
pKR #
6.0
pK2 #
9.17
C
H
N
CH
C
H
N
!
H
C
H
N
CH
C
HN
!
H
C
H
N
CH
C
H
N
10
8
6
4
2
0 1.0 2.0 3.0
pH
OH" (equivalents)
(b)
COO"
H
N
Histidine
pK2pKRpK1
8885d_c03_084 12/23/03 10:21 AM Page 84 mac111 mac111:reb:
group in the range of 1.8 to 2.4, and pKa of the ONH3!
group in the range of 8.8 to 11.0 (Table 3–1).
Second, amino acids with an ionizable R group have
more complex titration curves, with three stages corre-
sponding to the three possible ionization steps; thus
they have three pKa values. The additional stage for the
titration of the ionizable R group merges to some extent
with the other two. The titration curves for two amino
acids of this type, glutamate and histidine, are shown in
Figure 3–12. The isoelectric points reflect the nature of
the ionizing R groups present. For example, glutamate
Chapter 3 Amino Acids, Peptides, and Proteins84
10
8
6
4
2
0
Glutamate
H3N
!
N
!
N
!
C
COOH
C
C
COOH
H2
H2
H
pK1
H3 C
COO"
C
C
COOH
H2
H2
H
pKR
H3 C
COO"
C
C
COO"
H2
H2
H
pK2
H2N C
COO"
C
C
COO"
H2
H2
H
pK2 #
9.67
pKR #
4.25
pK1 #
2.19
1.0 2.0 3.0
pH
OH" (equivalents)
(a)
FIGURE 3–12 Titration curves for (a) glutamate and (b) histidine. The
pKa of the R group is designated here as pKR.
The second piece of information provided by the
titration curve of glycine is that this amino acid has two
regions of buffering power. One of these is the relatively
flat portion of the curve, extending for approximately 
1 pH unit on either side of the first pKa of 2.34, indi-
cating that glycine is a good buffer near this pH. The
other buffering zone is centered around pH 9.60. (Note
that glycine is not a good buffer at the pH of intracel-
lular fluid or blood, about 7.4.) Within the buffering
ranges of glycine, the Henderson-Hasselbalch equation
(see Box 2–3) can be used to calculate the proportions
of proton-donor and proton-acceptor species of glycine
required to make a buffer at a given pH.
Titration Curves Predict the Electric Charge 
of Amino Acids
Another important piece of information derived from
the titration curve of an amino acid is the relationship
between its net electric charge and the pH of the solu-
tion. At pH 5.97, the point of inflection between the
two stages in its titration curve, glycine is present pre-
dominantly as its dipolar form, fully ionized but with no
net electric charge (Fig. 3–10). The characteristic pH
at which the net electric charge is zero is called the
isoelectric point or isoelectric pH, designated pI.
For glycine, which has no ionizable group in its side
chain, the isoelectric point is simply the arithmetic mean
of the two pKa values:
pI # $
1
2$ (pK1 ! pK2) # $
1
2$ (2.34 ! 9.60) # 5.97
As is evident in Figure 3–10, glycine has a net negative
charge at any pH above its pI and will thus move toward
the positive electrode (the anode) when placed in an
electric field. At any pH below its pI, glycine has a net
positive charge and will move toward the negative elec-
trode (the cathode). The farther the pH of a glycine so-
lution is from its isoelectric point, the greater the net
electric charge of the population of glycine molecules.
At pH 1.0, for example, glycine exists almost entirely as
the form !H3NOCH2OCOOH, with a net positive
charge of 1.0. At pH 2.34, where there is an equal mix-
ture of !H3NOCH2OCOOH and !H3NOCH2OCOO",
the average or net positive charge is 0.5. The sign and
the magnitude of the net charge of any amino acid at
any pH can be predicted in the same way.
Amino Acids Differ in Their Acid-Base Properties
The shared properties of many amino acids permit some
simplifying generalizations about their acid-base behav-
iors. First, all amino acids with a single !-amino group,
a single !-carboxyl group, and an R group that does not
ionize have titration curves resembling that of glycine
(Fig. 3–10). These amino acids have very similar, al-
though not identical, pKa values: pKa of the OCOOH
C
H3N
!
C
COOH
C
CH
C
H
N
H2
H H3N
!
C
COO"
CH2
H H3N
!
C
COO"
CH2
H H2N C
CH2
H
pK1 #
1.82
pKR #
6.0
pK2 #
9.17
C
H
N
CH
C
H
N
!
H
C
H
N
CH
C
H
N
!
H
C
H
N
CH
C
H
N
10
8
6
4
2
0 1.0 2.0 3.0
pH
OH" (equivalents)
(b)
COO"
H
N
Histidine
pK2pKRpK1
8885d_c03_084 12/23/03 10:21 AM Page 84 mac111 mac111:reb:
Ponto	
  isoelétrico	
  (pI)	
  
Para	
  a	
  glicina	
  
pH	
  onde	
  a	
  carga	
  global	
  é	
  igual	
  a	
  zero	
  
pH	
  abaixo	
  do	
  pI	
   pH	
  =	
  pI	
   pH	
  acima	
  do	
  pI	
  
Section 4-1. The Amino Acids of Proteins 69
Table 4-1 (Continued)
Name Residue Average 
Three-Letter Symbol, Structural Mass Occurrence pK1 pK2 pKR
and One-Letter Symbol Formulaa (D)b in Proteins (%)c !-COOHd !-d Side Chaind
Amino acids with uncharged polar side chains
Serine 87.1 6.5 2.19 9.21
Ser
S
Threonine 101.1 5.3 2.09 9.10
Thr
T
Asparaginef 114.1 4.0 2.14 8.72
Asn
N
Glutaminef 128.1 3.9 2.17 9.13
Gln
Q
Tyrosine 163.2 2.9 2.20 9.21 10.46 (phenol)
Tyr
Y
Cysteine 103.1 1.4 1.92 10.70 8.37 (sulfhydryl)
Cys
C
Amino acids with charged polar side chains
Lysine 128.2 5.9 2.16 9.06 10.54 (ε-NH"3 )
Lys
K
Arginine 156.2 5.5 1.82 8.99 12.48 (guanidino)
Arg
R
Histidinee 137.1 2.3 1.80 9.33 6.04 (imidazole)
His
H
Aspartic acidf 115.1 5.4 1.99 9.90 3.90 (#-COOH)
Asp
D
Glutamic acidf 129.1 6.8 2.10 9.47 4.07 ($-COOH)
Glu
E
NH"d3
C
COO%
O%
C
O
CH2 CH2H
NH3"
C
COO%
CCH2 CH2 CH2 NHH
NH2
NH3" NH2
"
N
H
1 2
3
4
5C
COO%
NH"
CH2H
NH3"
C
COO%
O%
C
O
CH2H
NH3"
C
COO%
CH2 OHH
NH3"
C
COO%
CH2 SHH
NH3"
C
COO%
CH2 CH2 CH2 CH2H
NH3"
NH3"
C
COO%
NH2
C
O
CH2CH2H
NH3"
C
COO%
NH2
C
O
CH2H
NH3"
C
COO%
OHCH2H
NH3"
C
COO%
C CH3
H
OH
H *
NH3"
JWCL281_c04_065-081.qxd 5/31/10 1:37 PM Page 69
Section 4-1. The Amino Acids of Proteins 69
Table 4-1 (Continued)
Name Residue Average 
Three-Letter Symbol, Structural Mass Occurrence pK1 pK2 pKR
and One-Letter Symbol Formulaa (D)b in Proteins (%)c !-COOHd !-d Side Chaind
Amino acids with uncharged polar side chains
Serine 87.1 6.5 2.19 9.21
Ser
S
Threonine 101.1 5.3 2.09 9.10
Thr
T
Asparaginef 114.1 4.0 2.14 8.72
Asn
N
Glutaminef 128.1 3.9 2.17 9.13
Gln
Q
Tyrosine 163.2 2.9 2.20 9.21 10.46 (phenol)
Tyr
Y
Cysteine 103.1 1.4 1.92 10.70 8.37 (sulfhydryl)
Cys
C
Amino acids with charged polar side chains
Lysine 128.2 5.9 2.16 9.06 10.54 (ε-NH"3 )
Lys
K
Arginine 156.2 5.5 1.82 8.99 12.48 (guanidino)
Arg
R
Histidinee 137.1 2.3 1.80 9.33 6.04 (imidazole)
His
H
Aspartic acidf 115.1 5.4 1.99 9.90 3.90 (#-COOH)
Asp
D
Glutamic acidf 129.1 6.8 2.10 9.47 4.07 ($-COOH)
Glu
E
NH"d3
C
COO%
O%
C
O
CH2 CH2H
NH3"
C
COO%
CCH2 CH2 CH2 NHH
NH2
NH3" NH2
"
N
H
1 2
3
4
5C
COO%
NH"
CH2H
NH3"
C
COO%
O%
C
O
CH2H
NH3"
C
COO%
CH2 OHH
NH3"
C
COO%
CH2 SHH
NH3"
C
COO%
CH2 CH2 CH2 CH2H
NH3"
NH3"
C
COO%
NH2
C
O
CH2CH2H
NH3"
C
COO%
NH2
C
O
CH2H
NH3"
C
COO%
OHCH2H
NH3"
C
COO%
C CH3
H
OH
H *
NH3"
JWCL281_c04_065-081.qxd 5/31/10 1:37 PM Page 69
Curva	
  de	
  1tulação	
  do	
  glutamato	
  
Curva	
  de	
  1tulação	
  da	
  his1dina	
  
Observe	
  a	
  curva	
  de	
  =tulação	
  do	
  aminoácido	
  his=dina	
  (His).	
  	
  
U=lizando	
  a	
  informação	
  fornecida	
  abaixo,	
  iden=fique	
  o	
  pK	
  para	
  todos	
  os	
  grupos	
  ionizáveis	
  
e	
  as	
  espécies	
  predominantes	
  da	
  his=dina	
  em	
  cada	
  posição	
  indicada	
  na	
  curva	
  pelas	
  letras	
  
de	
  A	
  a	
  G.	
  	
  
	
  	
  
OBS:	
  Algumas	
  posições	
  tem	
  mais	
  de	
  uma	
  resposta.	
  Nem	
  todas	
  as	
  informações	
  disponíveis	
  
deverão	
  ser	
  u=lizadas.	
  
	
  
Exercício	
  
His+2	
  
His+	
  
His-­‐2	
  
His0	
  
His-­‐	
  
[His+2]=	
  [His+]	
  
[His+]=	
  [His0]	
  
[His0]=	
  [His-­‐]	
  
pI	
  
pK	
  do	
  grupo	
  carboxila	
  
pK	
  do	
  grupo	
  amino	
  
pK	
  da	
  cadeia	
  lateral