AMINO ACIDS
AND
PROTEIN
CHEMISTRY
GENERAL STRUCTURE/FORMULA OF AMINO ACIDS
COO- | +H3N- C – H | R
α- Carbon
Amino group
Carboxyl
group Hydrogen
Side chain
1. α-carbon bonded to an amino group, a
carboxyl group, a hydrogen atom and a
side-chain group.
2. The amino group bonded to the α-carbon
makes all AAs for that matter as α-amino
acids.
3. R group – determines the identity of the
AA and the role or biologic property of the
AA in a protein molecule.
4. Carbon atoms of the side chains are
sequentially labeled as β, γ, δ, ε, which
refer to carbons 3, 4, 5, and 6 respectively.
5. α-carbon is a chiral or asymmetric carbon
because there are 4 different groups
attached to it.
6. All AAs except glycine have chiral
carbons; glycine has 2 H atoms attached
to the α-carbon, hence a symmetric
molecule.
7. The amino and carboxyl groups
participate in chemical reactions, hence
constitute the functional groups of amino
acids and are unique to all amino acids.
7. At physiologic ph (7.4), the COOH group is
deprotonated, forming the negatively charged
carboxylate ion ( -CO0-) and the amino group
is protonated (-NH3+).
8. With each AA containing at least one amino
and carboxylic acid group, amino acids are
classified as amphoteric substances and react
with both acids and bases.
D & L FORMS (ISOMERS) OF AMINO ACIDS
1. Glyceraldehyde is used as a reference compound for D and L isomers of
amino acids (similar to carbohydrates).
2. In L-glyceraldehyde, the OH- group is on the left side of the molecule;
in D-glyceraldehyde, the OH- group is on the right side.
3. In L-amino acid as L-alanine, the amino group (NH3+) is on the left side
with the carboxyl group at the top of the structure; in D-alanine, the amino
group is on the right side with the carboxyl group at the top of the structure.
4. In proteins, all of the AAs are in the L-isomer; hence by convention, all AAs
are presumed to be in the L configuration unless specifically designated.
CLASSIFICATION OF AMINO ACIDS- based on the
chemical properties of the side chains
Grp I. AAs with Non-polar or Hydrophobic Side Chains
– have the tendency to cluster away from H2O (or H2O-hating)
1. Leucine – have 4-carbon side chain; the purely ketogenic amino acid.
2. Proline
a. Its 3-carbon side chain is bonded to the nitrogen of its α-amino group and to the α-
carbon, creating a cyclic or ring structure; this amino acid contains a secondary
rather than a primary amino group, hence an imino acid or a secondary amine.
b. This substituted α-amino group influences protein folding by forcing a bend in the
polypetide chain.
Imino
group
CLASSIFICATION OF AMINO ACIDS Grp I. AAs with Non-polar Side Chains – also
thought of as “oily” or lipid-like.
Alanine – has a methyl group side chain.
CLASSIFICATION OF AMINO ACIDS Grp I. AAs with Non-polar Side Chains
1. Methionine – side chain contains a sulfur group, similar to cysteine
2. Tryptophan – side chain contains an indole ring; classified as a neutral amino acid.
In Hartnup’s Disease, there is inability of the intestinal epithelial cells to absorb neutral amino acids
like tryptophan excessive amount in the urine impaired synthesis of niacinamide.
3. Phenylalanine – hydrocarbon group is aromatic; i.e., contains a cyclic group similar to a benzene
ring.
CLASSIFICATION OF AMINO ACIDS Grp 2. AAs with Neutral Polar Side Chains (Uncharged) – participate in
hydrogen bonding
1. Glycine – with very small side chain, hence it causes the least hindrance in a
protein (i.e., it does not significantly impinge on the space occupied by other atoms
or chemical groups); the simplest AA.
2. Serine – polar hydroxyl (OH-) group is bonded to aliphatic hydrocarbon groups.
3. Asparagine & Glutamine – with amide groups in their side chains.
CLASSIFICATION OF AMINO ACIDS Grp 2. AAs with Neutral Polar Side Chains (Uncharged)
Threonine – with 2 asymmetric
carbons
Cysteine – polar grp. consists of a –SH
(thiol) grp. which can react with
other cysteine –SH grps. to form
disulfide bridges (-S-S-) or bonds in
proteins.
Tyrosine – hydroxyl grp. is bonded to
an aromatic hydrocarbon grp.
CLASSIFICATION OF AMINO ACIDS Grp 3. AAs with Carboxyl Groups in their Side
Chains (Acidic)
The carboxyl grp. (in addition to the one present in all
AAs, hence dicarboxylic amino acids) makes these
amino acids negatively charged.
CLASSIFICATION OF AMINO ACIDS
Grp 4. AAs with Basic Side Chains – positively charged
1. Histidine – side chain consists of
an imidazole group
2. Arginine – side chain basic group
(guanidino group) is bonded to an
aliphatic hydrocarbon tail; the most
basic amino acid.
3. Lysine – side chain amino group
is attached to an aliphatic hydro-
carbon tail
FORMATION OF THE PEPTIDE BOND
Slide shows the
peptide bond (C-N)
formed between the
carboxyl group of
valine and the amino
group of alanine to
form a dipeptide, read
as valylalanine, not
alaninevaline.
Again, each component
AA in the dipeptide is
called a “residue” or
“moiety”.
PRIMARY STRUCTURE
Refers to the linear sequence of amino
acids in a polypeptide chain
LEU-GLY-THR-VAL-ARG-ASP-HIS
VAL-HIS-ASP-LEU-GLY-ARG-THR
1. Although these 2 peptides have the same number and kinds of
amino acids, they have different sequence of amino acids and
hence have different primary structure.
2. The primary structure determines its 3-dimensional structure,
properties and functions of a protein.
3. This primary structure is determined from the genetic information
encoded in DNA.
SECONDARY STRUCTURE Refers to the hydrogen-bonded arrangement of the backbone of the
polypeptide chain.
1. There are 2 bonds within an AA with
reasonably free rotation:
a. bond bet. the α-carbon and the amino
nitrogen of the residue
b. bond bet. the α-carbon and the
carboxyl carbon of that residue.
2. A peptide chain backbone can be visualized
as a series of playing cards, each card
representing a planar peptide group.
3. The cards are linked at opposite corners by
swivels, representing the bonds about which
there is a considerable rotation.
4. The angles φ (phi) and Ψ (psi), frequently
called Ramachandran angles, are used to
designate rotation around the C-N (α-carbon
and amino nitrogen) and C-C (α-carbon and
carboxyl carbon) bonds respectively.
5. The side chains also play vital role in
determining the 3-dimensional shape of the
protein, but only the backbone is considered
in the secondary structure.
TERTIARY STRUCTURE
OF PROTEINS
1. Refers to the shape of the fully folded
polypeptide chain, hence distant
portions of the secondary structures
are close together.
2. Exemplified by the structure of
MYOGLOBIN
a. Myoglobin is a compact structure
consisting of a single polypeptide
chain (153 AAs) and the prosthetic
group heme-containing Fe.
b. It consists of 8 helices (A to H)
stabilized by hydrogen bonds and
so are the AA side chains.
QUARTERNARY STRUCTURE
OF PROTEINS 1. Refers to the arrangement of 2 or more
polypeptide chains or subunits with
respect to one another to form a
multisubunit molecule.
2. Exemplified by the structure of
HEMOGLOBIN
a. Hemoglobin is a tetramer consisting of 4 polypeptide chains:
*2 α-chains (blue color) – each chain
is 141 residues long
*2 β-chains (green color) – each
chain is 146 residues long
b. The 2 α-chains are identical; the 2 β-
chains are likewise identical.
c. Hemoglobin has therefore this
overall structure: α2β2.
d. Buried in a crevice within each of
the α and β-chains are the prosthetic
groups hemes containing Fe+2.
DENATURATION OF PROTEINS
1. Refers to the unfolding of the protein
hence destruction of its native
conformation, esp. of the secondary
and tertiary structure.
2. This is not accompanied by
hydrolysis of the peptide bonds.
3. Agents that cause denaturation:
a. Heat causes vibrations disruption
of tertiary structure unfolding or
denaturation.
b. Extremes of pH decrease
electrostatic interactions that
maintain native, active form
denaturation.
c. Chemical Agents:
c1. Chaotropic – urea and
guanidinium salts
c2. Detergents – sodium didecyl
sulfate (DDS)
PROTEIN DIGESTION
AND AMINO ACID
ABSORPTION
OVERVIEW OF DIGESTION OF DIETARY PROTEINS
Food
Protein
Polypeptides HCO3
-
Trypsin
Chymotrypsin
Elastase
Carboxypeptidases
A & B
Aminopeptidases
Di- and tri- peptides
+ Amino acids
Intestinal epithelial cell
Di- and tri-
peptidases Amino
acids Amino
acids
Oligopeptides
HCl Pepsin
Stomach
Pancreas
Small
intestine
Blood
1. Proteins are too large to be absorbed,
hence they must be first hydrolyzed to
their constituent amino acids.
2. Digestion occurs via the sequential
action of proteolytic enzymes in the
stomach, pancreas & small intestines.
a. In the stomach, dietary proteins are
converted to polypeptides & some
free amino acids by pepsin.
b. In the small intestines, trypsin,
chymotrypsin, elastase & carboxy-
peptidases A & B produce oligopep-
tides in the presence of HCO3- (which
neutralizes the stomach acid &
raises the pH for activation of the
zymogens).
c. Final digestion occurs via amino-
peptidases to produce free AAs and
smaller peptides like di- and
tripeptides.
3. Amino acids are then absorbed by the
intestinal epithelial cells, brought to
the liver (via the portal system) for
metabolism or release into the
general circulation.
ACTIVATION OF THE GASTRIC AND PANCREATIC ZYMOGENS
Proenzymes Active Enzymes
Pepsinogen Pepsin
Trypsinogen Trypsin
Chymotrypsinogen Chymotrypsin
Proelastase Elastase
Procarboxypeptidase A & B Carboxypeptidae A & B
H+ (parietal cells)
Enteropeptidase (enterokinase)
trypsin
trypsin
1. The gastric and pancreatic zymogens involved in protein digestion are
secreted in their inactive precursors or zymogen forms initially and then
converted to their active forms.
2. Pepsinogen, secreted by the chief cells of the stomach, is converted to
pepsin via HCl (from gastric parietal cells) and autocatalytically by pepsin.
3. Trypsinogen is converted to the active form trypsin by enteropeptidase
(formerly enterokinase) from intestinal mucosal cells.
4. Thereafter, trypsin cleaves the other pancreatic zymogens, producing their
active forms.
5. Trypsin is therefore the master or common activator of the pancreatic
zymogens.
trypsin
ACTION OF THE DIGESTIVE
PROTEASES
+
NH3
|
H – C – R
|
C = O
|
NH
|
H – C –R
|
C = O
|
NH
|
H – C – R
|
C = O
|
NH
|
H – C – R
|
C = O
|
NH
|
H – C – R
|
C = O
|
NH
|
H – C – R
|
C = O
|
NH
|
H – C – R
|
COO-
N - terminus
Phe
Tyr
Glu
Asp
Arg
Lys
Phe
Tyr
Glu
Leu
Ala
Gly
Ser
carboxy-
peptidase A
(hydrophobic)
carboxy-
peptidase B
(Arg Lys)
amino-
peptidases
pepsin
trypsin
chymotrypsin
elastase
carboxy-
peptidases
C - terminus
A. Endopeptidases – hydrolyze peptide bonds
at various points within the polypeptide chain.
1. Pepsin preferentially cleaves peptide bonds
in which the carboxyl (carbonyl) group is
provided (or contributed) by aromatic AAs
(Phe, Tyr) and acidic AAs (Glu, Asp).
2. Trypsin specifically cleaves peptide bonds
in which the carboxyl group is provided by
positively charged AAs (Arg, Lys).
3. Chymotrypsin favors hydrolysis of peptide
bonds in which the carboxyl group is provided
by aromatic AAs (Phe,Tyr) and hydrophobic
AAs (Glu, Leu).
4. Elastase cleaves elastin (for which it was
named) and peptide bonds whose carboxyl
group is provided by AAs with small side
chains (Ala, Gly, Ser).
ACTION OF THE DIGESTIVE
PROTEASES
+
NH3
|
H – C – R
|
C = O
|
NH
|
H – C –R
|
C = O
|
NH
|
H – C – R
|
C = O
|
NH
|
H – C – R
|
C = O
|
NH
|
H – C – R
|
C = O
|
NH
|
H – C – R
|
C = O
|
NH
|
H – C – R
|
COO-
N - terminus
Phe
Tyr
Glu
Asp
Arg
Lys
Phe
Tyr
Glu
Leu
Ala
Gly
Ser
carboxy-
peptidase A
(hydrophobic)
carboxy-
peptidase B
(Arg Lys)
amino-
peptidases
pepsin
trypsin
chymotrypsin
elastase
carboxy-
peptidases
C - terminus
B. Exopeptidases – hydrolyze peptide bonds
at outer ends of a polypeptide chain.
1. Carboxypeptidase A – hydrolyzes
peptide bonds at the C-terminus of
hydrophobic AAs (Ala, Ile, Leu, Val).
2. Carboxypeptidase B – hydrolyze peptide
bonds at the C-terminus of Arg and Lys.
3. Aminopeptidases, coming from the
brush border of the epithelial mucosa,
complete protein digestion by
hydrolyzing one amino acid at a time
from the N-terminus of peptides to
produce free amino acids, di- and
tripeptides.
Amino
acid Na+
Amino
acid
Na+
Intestinal
lumen
Brush
border
Amino
acid
Na+
K+
K+
ATP
ADP
+Pi
Active
transporter
Facilitated
transporter
Serosal
side
Portal
vein
TRANSEPITHELIAL AMINO ACID TRANSPORT
1. It is an active process, similar to CHO
digestion in the intestinal epithelial cells.
2. In the brush border membrane is a
semi-specific Na+-dependent transport
protein that carries or pulls with it the AA
as it moves along concentration gradient
(since Na+ conc. in the lumen is > than its
concentration in the inside of the cell).
3. Na+ is then pumped into the serosal side
by the Na+-K+-ATPase system which is
counterbalance by K+ moving in the
opposite direction.
4. The accumulated AA inside the cell is then
transported down its concentration
gradient via facilitated transporters into
the serosal side brought to the liver
via the portal vein for metabolism or for
distribution to other tissues.
Amino
acid Na+
Amino
acid
Na+
Intestinal
lumen
Brush
border
Amino
acid
Na+
K+
K+
ATP
ADP
+Pi
Active
transporter
Facilitated
transporter
Serosal
side
Portal
vein
TRANSEPITHELIAL AMINO ACID TRANSPORT
5. There are different Na+-dependent AA
transport proteins in the apical brush
border membrane of the intestinal
epithelial cells, each transport system
transporting a group of closely related
amino acids:
a. Transport system for neutral AAs.
b. Transport system for proline and
hydroxyproline.
c. Transport system for acidic AAs.
d. Transport system for basic AAs.
(Lys, Arg, Ornithine) and cystine.
6. Some amino acids use facilitated
transport carriers.
7. Most amino acids are transported by more
than one transport system.
9. Ingested dietary amino acids by man are used primarily for synthesis of proteins.
BIOSYNTHESIS
OF THE
NUTRITIONALLY
NON-ESSENTIAL
AMINO ACIDS
CLASSIFICATION OF AMINO ACIDS
Glucogenic
(13 AAs)
Glucogenic
and
Ketogenic
(5 AAs)
Ketogenic
(2 AAs)
Alanine
Asparagine
Aspartate
Cysteine
Glutamate
Glutamine
Glycine
Proline
Serine
Tyrosine
Arginine
Histidine
Methionine
Valine
Isoleucine
Phenylalanine
Tyrptophan
Threonine
Leucine
Lysine
Es
se
nti
al
No
n-e
ss
en
tia
l
P V T T I M H A L L
OVERVIEW OF THE BIOSYNTHESIS OF THE
NON-ESSENTIAL AMINO ACIDS GLUCOSE
3-Phosphoglycerate
Pyruvate Alanine
Serine
Glycine
Cysteine
Methionine
Acetyl CoA
Oxaloacetate Aspartate
Asparagine
Glutamine Citrate
Isocitrate
-ketoglutarate Glutamate
Glutamine
Proline
Arginine
Phenylalanine Tyrosine
1. There are 11 non-essential amino acids of the 21common amino acids.
2. Non-essential because they can be synthesized in sufficient amounts by the body from:
a. Amphibolic intermediates of:
a1. Glycolysis
i. 3-phosphoglycerate - Cysteine, Serine, and Glycine
ii. Pyruvate - Alanine
a2. Citric Acid Cycle
i. -ketoglutarate – Glutamine, Glutamate, Proline and Arginine
ii. Oxaloacetate – Aspartate and Asparagine
b. Phenylalanine – Tyrosine
Glutamate
I. AMINO ACIDS DERIVED FROM GLYCOLYSIS
3-phosphoglycerate dehydrogenase
PLP
NAD+ NADH
+ H+
α-Ketoglutarate
Pi
Phosphoserine phosphatase
Glycolysis
Serine
A. Serine C1OO-
I
H – C2 – OH
I
C3H2
I
O - PO32-
3-Phosphoglycerate
C1OO-
I
C2 = O
I
C3H2
I
O - PO32-
3-Phosphohydroxypyruvate
COO-
|
H3N+ - C – H
|
CH2
|
CH2
|
COO-
Glutamate
C1OO-
I
H3N – C2 – H
I
C3H2
I
O - PO32-
3-Phosphoserine
+
C1OO-
I
H3N – C2 – H
I
C3H2
I
OH
+
Glucose
aminotransferase
(keto acid)
Glycine D- serine
(CNS neurotransmitters) Selenocysteine
1. 3-phosphoglycerate (from
glycolysis) is converted to 3-phos-
phohydroxypyruvate, a keto acid, via
3-phosphoglycerate dehydrogenase
with NAD+ as a hydrogen acceptor
reduced to NADH2.
2. Followed by transamination with
glutamate as the amino group donor
to 3-phosphohydroxypyruvate,
forming 3-phosphoserine and -
ketoglutarate in the presence of
PLP,a cofactor, via an
aminotransferase.
3. Finally, phosphoserine phosphatase
hydrolyzes the phosphate group
(dephosphorylation) to form serine.
4. Serine is a precursor of the neuro-
transmitters glycine and D-serine;
also a component of the unusual AA
selenocysteine found in glutathione
peroxidase.
I. AMINO ACIDS DERIVED FROM GLYCOLYSIS
Other Pathway For Serine
H2C – NH3+
|
COO-
Glycine
CH2OH
|
H – C – NH3+
|
COO-
Serine N5, N10 – CH2 – TH4 (N5-N10-Methylene THF)
TH4
(Tetrahydrofolate)
Serine hydroxymethyl
transferase
PLP
H20
H20
1. Serine may also be formed reversibly from glycine by transfer of a
hydroxymethyl group via serine hydroxymethyl transferase in the
presence of PLP and tetrahydrofolate (TH4), forming N5, N10-Methylene
TH4.
2. The demand for serine and glycine and the amount of N5, N10-Methylene
TH4 determine the direction of the reaction.
I. AMINO ACIDS DERIVED ROM GLYCOLYSIS
COO-
I
H3N – C – H
I
CH2OH
Serine
COO-
I
H3N – C – H
I
H
Glycine
+ +
5
H
C
C N
CH2
I
N –
H
H
CH2
N H
10 Tetrahydrofolate
(FH4)
N5-N10-Methylenetetrahydrofolate
(N5-N10-CH2-H4 folate)
Serine hydroxymethyl transferase
PLP
Methylene
5
H
C
C N
CH2
I
N –
H
CH2
N
10 I
H2C
Reactive part of tetrahydrofolate
7
6
9
8
3-Phospho-
glycerate
1. Serine, coming from the glycolytic intermediate 3-phosphoglyerate, is converted reversibly to
glycine via serine hydroxymethyltransferase, with PLP as a coenzyme and tetrahydrofolate (FH4).
2. The reaction involves the transfer of a one-carbon unit from serine with tetrahydrofolate as an
acceptor.
3. FH4 is converted to N5-N10-methylenetetrahydrofolate (methylene, a one-carbon unit is bound to 2 of
the nitrogens of the carrier molecule).
4. Hence FH4 is a carrier of one-carbon units (-CH2) in metabolic pathways; the other carriers are
biotin, a carrier of CO2 in carboxylation reactions and S-adenosylmethionine (SAM), a carrier of
methyl groups (-CH3) in methylation reactions.
B. Glycine – the Major Pathway
I. AMINO ACIDS DERIVED FROM GLYCOLYSIS
B. Glycine
H2C – NH3+
|
COO-
Glycine
PLP
CH3 H
| |
H – C – C – C
| |
OH NH4+
Threonine
O-
O
Minor Pathway
Threonine is degraded into glycine in
the presence of PLP s a cofactor.
I. AMINO ACIDS DERIVED FROM GLYCOLYSIS
C. Cysteine CH2 – OH
|
H – C – NH3+
|
COO-
Serine
SH
|
CH2
|
CH2
|
H – C – NH3+
|
COO-
Homocysteine
-OOC – CH – CH2 – S
| |
+NH3 CH2
|
CH2
| +
H – C – NH3
|
COO-
Cystathionine
-OOC – CH – CH2 – SH
+|
NH3
H2O Cystathione -synthase PLP
H2O
NH4+
PLP
α-Ketobutyrate
Methionine
-Cystathionase
Cysteine
COO-
I
H3N – C – H
I
CH2
I
CH2
I
H3C - S
+
Serine Methio-
nine
Cystathionuria
def
ATP
S-adenosyl
methionine
(SAM; Adomet)
S-adenosyl
homocysteine
(SAH)
CH3
adenosine
Methionine adenosyltransferase
Adenosylhomo- cysteinase
Methylase
1. Methionine (an essential AA) condenses with ATP via
methionine adenosyltransferase to form S-adenosylmethio-
nine (SAM; Adomet); methyl group of SAM is cleaved by
methyltransferase to form S-adenosylhomocysteine (SAH);
Adenosine group of SAH is cleaved by adenosylhomocys
teinase to form homocysteine.
2. Serine condenses with homocysteine (from methionine) to
form cystathionine via cystathione -synthase with PLP as a
co-factor.
I. AMINO ACIDS DERIVED FROM GLYCOLYSIS
C. Cysteine CH2 – OH
|
H – C – NH3+
|
COO-
Serine
SH
|
CH2
|
CH2
|
H – C – NH3+
|
COO-
Homocysteine
-OOC – CH – CH2 – S
| |
+NH3 CH2
|
CH2
| +
H – C – NH3
|
COO-
Cystathionine
-OOC – CH – CH2 – SH
+|
NH3
H2O Cystathione -synthase PLP
H2O
NH4+
PLP
α-Ketobutyrate
Methionine
-Cystathionase
Cysteine
COO-
I
H3N – C – H
I
CH2
I
CH2
I
H3C - S
+
Serine Methio-
nine
Cystathionuria
def
ATP
S-adenosyl
methionine
(SAM; Adomet)
S-adenosyl
homocysteine
(SAH)
CH3
adenosine
Methionine adenosyltransferase
Adenosylhomo- cysteinase
Methylase
3. Cystathione is then cleaved by PLP-dependent -cystathio-
nase into -ketobutyrate (and NH4+) and cysteine.
4. Serine contributes the carbons and nitrogens for cysteine;
methionine provides the sulfur of cysteine via transulfuration
- one of the routes used for methionine catabolism.
5. Because methionine is an essential amino acid, cysteine
synthesis can be sustained only if the dietary intake of
methionine is adequate.
I. AMINO ACIDS DERIVED FROM GLYCOLYSIS
C. Cysteine CH2 – OH
|
H – C – NH3+
|
COO-
Serine
SH
|
CH2
|
CH2
|
H – C – NH3+
|
COO-
Homocysteine
-OOC – CH – CH2 – S
| |
+NH3 CH2
|
CH2
| +
H – C – NH3
|
COO-
Cystathionine
-OOC – CH – CH2 – SH
+|
NH3
H2O Cystathione -synthase PLP
H2O
NH4+
PLP
α-Ketobutyrate
Methionine
-Cystathionase
Cysteine
COO-
I
H3N – C – H
I
CH2
I
CH2
I
H3C - S
+
Serine Methio-
nine
Cystathionuria
def
ATP
S-adenosyl
methionine
(SAM; Adomet)
S-adenosyl
homocysteine
(SAH)
CH3
adenosine
Methionine adenosyltransferase
Adenosylhomo- cysteinase
Methylase
6. A genetic deficiency of cystathione -synthase or
PLP causes cystathionuria or the presence of
cystathione in the urine; a benign disorder with no
clinical abnormalities.
II. AMINO ACIDS RELATED TO TCA INTERMEDIATES
1. Glutamate
A. Amino Acids Related to α-ketoglutarate
COO-
|
C = O
|
CH2
|
CH2
|
COO-
α-ketoglutarate
COO-
|
H3N+ - C – H
|
CH2
|
CH2
|
COO-
Glutamate
NAD(P)+ NAD(P)H
NH4+
Glutamate dehydrogenase
1. Reductive amination of -ketoglutarate to glutamate via glutamate dehydrogenase
is a freely reversible reaction; NAD+ or NADP is a cofactor.
2. The 5-carbons of glutamate came from -ketoglutarate which also came from glucose.
3. Glutamate is used for the synthesis of other amino acids: glutamine, proline,
arginine, ornithine and glutathione (an antioxidant).
Synthesis of other AAs
Glutamine, Proline
Arginine, Ornithine
Glutathione
SH
I
CH2
I
HC – NH3
I
COO-
+
Cysteine
ATP ADP + Pi
-Glutamylcysteine COO-
I
H3N – C – H
I
H
ATP
ADP + Pi Glycine
Glutathione (-Glutamylcysteinylglycine)
GLUTAMATE AS A
PRECURSOR OF
GLUTATHIONE
-carbon or
3rd carbon
Sulfhydryl group of cysteine
SH
I
CH2
I
CH – C – N – CH2
I
COO-
H
O
II
C – N –
I
C3H2
I
C2H2
I
HC1 – NH3
I
COO-
H
+
O
II
COO-
|
H3N+ - C – H
|
CH2
|
CH2
|
COO-
Glutamate
- carboxyl group
- carboxyl group
-glutamylcysteine synthase
Glutathione synthase
1. Glutamate, in the presence of
ATP, condenses with cysteine
to form the dipeptide -gluta-
mylcysteine.
2. Followed by the addition of
glycine, again in the presence
of ATP to form the tripeptide -
glutamylcysteinylglycine; the
letter refers to the 3rd carbon
in the molecule, counting the
one bonded to the amino group
as the 1st carbon (recall that
is the 3rd letter in the Greek
alphabet).
GLUTATHIONE AS AN ANTIOXIDANT
Sulfhydryl
group
2 - Glu – Cys - Gly - Glu – Cys -Gly
I
SH
Glutathione
(GSH; reduced form)
I
S
I
S
I
- Glu – Cys - Gly
Glutathione disulfide
(GSSG; oxidized form)
2 H2O
H2O2
Glutathione peroxidase
2 GSH
(reduced form)
NADPH
+ H+
NADP+
Glutathione reductase
Disulfide bond
2 - Glu – Cys - Gly I
SH
1. The sulfhydryl group of glutathione
(GSH; reduced form) reduces or
scavenges H2O2 (an oxidizing agent)
to H2O, catalyzed by glutathione
peroxidase, forming glutathione
disulfide (GSSH; oxidized form)
consisting of 2 molecules of the
reduced form joined together by a
disulfide bond between the –SH
groups of the 2 cysteine residues.
2. Reduced glutathione (GSH) is
regenerated via glutathione
reductase at the expense of NADPH.
GLUTATHIONE IS INVOLVED IN THE TRANSPORT OF AMINO
ACIDS ACROSS PLASMA MEMBRANES Amino acid
-Glutamyl transpeptidase
-Glu-Cys-Gly Cys-Gly -Glu-Amino acid Outside
Inside Cys-Gly
Cysteine +
Glycine
H2O -Glutamyl
cyclotransferase
ATP + H2O 5-oxopro-
linase
5-Oxoproline
L-Glutamate -Glutamylcysteine
synthase
ADP + Pi
+ 2 H+
ADP + Pi
+ 2 H+
Plasma membrane
-Glu-Amino acid
-Glutamyl- cysteine Glutathione
synthase
Amino
acid
ATP
-Glu-Cys-Gly
(glutathione)
-Glu-Cys-Gly
(glutathione)
1. Transport of amino acids uses the -glutamyl cycle (Meister Cycle).
2. Membrane-bound -glutamyltranspeptidase catalyzes the transpeptidation of -
glutamyl residue from glutathione to the amino acid, forming -glutamyl-amino acid
and cysteinylglycine, the remaining portion of glutathione.
3. The -glutamyl-amino acid (a dipeptide, consisting of 2 amino acids glutamate and the
amino acid to be transported) is transported into the cytoplasm to form 5-oxoproline.
4. Once inside the cell, the cytoplasmic enzyme -glutamylcyclotransferase releases the
amino acid and cyclizes the -glutamyl group to form 5-oxoproline, which is converted
to glutamate via 5-oxoprolinase in the presence of ATP.
GLUTATHIONE IS INVOLVED IN THE TRANSPORT OF AMINO
ACIDS ACROSS PLASMA MEMBRANES
Amino acid
-Glutamyl transpeptidase
-Glu-Cys-Gly Cys-Gly -Glu-Amino acid Outside
Inside Cys-Gly
Cysteine +
Glycine
H2O -Glutamyl
cyclotransferase
ATP + H2O 5-oxopro-
linase
5-Oxoproline
L-Glutamate -Glutamylcysteine
synthase
ADP + Pi
+ 2 H+
ADP + Pi
+ 2 H+
Plasma membrane
-Glu-Amino acid
-Glutamyl- cysteine Glutathione
synthase
Amino
acid
ATP
-Glu-Cys-Gly
(glutathione)
-Glu-Cys-Gly
(glutathione)
5. To complete the -glutamyl cycle, cysteine condenses with glutamate via
-glutamylcysteine synthase, again in the presence of ATP, forming
-glutamylcysteine.
6. -glutamylcysteine then condenses with glycine via glutathione synthase
again in the presence of ATP, forming and resynthesizing glutathione.
II. AMINO ACIDS RELATED TO TCA INTERMEDIATES
2. Glutamine
A. Amino Acids Related to α-ketoglutarate
NH2
|
C = O
|
CH2
|
H – C – NH3+
|
COO-
Glutamine
COO-
|
H3N+ - C – H
|
CH2
|
CH2
|
COO-
Glutamate
NH4+
ATP ADP + Pi
Glutamine synthase
H2O NH4
+
Glutaminase
1. Glutamine coming from transamination of α-ketoglutarate, is aminated to glutamnine
via glutamine synthase by adding free NH4+ (as the amino group donor) to the
carboxyl group (-COOH) of the side chain of glutamate in the presence of ATP.
2. In the presence of glutaminase, glutamine is deaminated to glutamate.
3. Asparagine is the other amino acid that is synthesized from amination of aspartate.
Α-ketoglutarate
II. AMINO ACIDS RELATED TO TCA INTERMEDIATES
3. Proline
A. Amino Acids Related to α-ketoglutarate
H - C – CH2 – CH2 – CH – COO-
II
O
Proline
-COO – CH2 – CH2 – CH – COO-
Glutamate
Glutamate semialdehyde
CH2 H2C
CH – COO- HC +
N
Spontaneous cyclization
NADH +
H+
NAD+
NADH
+ H+
NAD+
ATP
ADP + Pi
NH3
+
NH3
+
Δ1-Pyrroline 5-caboxylate H
+
N
CH2 H2C
CH – COO- H2C
H2
FAD . 2H
FAD
NADPH
+
H+
NADP+
Pyrroline-5-
carboxylate
reductase
1. Glutamate is phosphory-
lated by ATP and then
converted to glutamate
semialdehyde via the
reduction of side chain
carboxyl group (-COOH)
to an aldehyde.
2. Followed by
spontaneous
cyclization of to Δ1-
pyrroline 5-carboxylate.
3. This cyclic compound is
then reduced by NADPH
to proline via pyroline-5-
carboxylate reductase.
II. AMINO ACIDS RELATED TO TCA INTERMEDIATES
4. Arginine
A. Amino Acids Related to α-ketoglutarate
Glutamate
Arginine
+
NH3
|
H – C – CH2 – CH2 – CH – COO-
||
O
Glutamate semialdehyde
+
NH3
|
H3N+ - CH2 – CH2 – CH2 – CH – COO-
Transanimation Ornithine aminotransferase
Ornithine
+
NH NH3
|| |
H2N – C – CH2 – CH2 – CH2 – CH – COO-
Urea
cycle
-Ketoglutarate 1. Glutamate, coming from
-ketoglutarate, is
converted to glutamate
semialdehyde.
2. Followed by transamina-
tion into ornithine (an
intermediate of the urea
cycle) arginine.
3. The amounts of arginine
generated by the urea
cycle is adequate only
for the adult and are
insufficient for growth,
hence during periods of
rapid growth as in
infancy and childhood,
arginine becomes an
essential amino acid.
II. AMINO ACIDS RELATED TO TCA INTERMEDIATES
B. Amino Acids Related to Oxaloacetate
O
||
C – NH2
|
CH2
|
H – C – NH3+
|
COO-
Asparagine
Aspartate & Asparagine
COO-
|
CH2
|
H – C – NH3+
|
COO-
Aspartate Glutamine ATP
AMP + PPi
Glutamate
Asparagine synthetase
NH4+
H2O
Asparaginase
COO-
|
CH2
|
C = O
|
COO-
Oxaloacetate
PLP
Transanimation
1. Oxaloacetate is transami-
nated to aspartate with PLP
as a cofactor in a freely
reversible reaction.
2. Aspartate is then aminated to
asparagine via asparagine
synthetase in the presence of
ATP and glutamine as a
source of nitrogen (or the
amino group donor).
3. Leukemic cells require
asparagine for their growth,
hence asparaginase has been
used as an anti-tumor agent.
It acts by converting
asparagine to aspartate in the
blood, thereby decreasing the
amount of asparagine
available for tumor growth.
III. TYROSINE COMING FROM PHENYLALANINE
Quinonoid dihydrobiopterin (BH2)
- CH2–CH–COO-
NAD+
NADH
+ H+
H2N N H
N H
H
HN
II
O
N
H
H
CH – CH – CH3
I
OH
I
OH
H2N N H
N H
H
N
II
O
N
H
CH – CH – CH3
I
OH
I
OH
+
NH3
I HO -
O2
H2O
- CH2–CH–COO-
+
NH3
I
Phenylalanine
Tyrosine
Tetrahydrobiopterin
(BH4)
Phenylalanine hydroxylase
GTP Biosynthesis
Dihydropteridine reductase
2 1
5 4 3 6
8
9
7
10
3 5
1. Phenylalanine is reduced to tyrosine via phenylalanine hydroxylase (PAH), a mixed-
enzyme oxidase with 2 activities
a. Activity I: Reduction of O2 to H2O and of phenylalanine to tyrosine (molecular O2
donates one atom to H2O and one atom to the product tyrosine).
III. TYROSINE COMING FROM PHENYLALANINE
Quinonoid dihydrobiopterin (BH2)
- CH2–CH–COO-
NAD+
NADH
+ H+
H2N N H
N H
H
HN
II
O
N
H
H
CH – CH – CH3
I
OH
I
OH
H2N N H
N H
H
N
II
O
N
H
CH – CH – CH3
I
OH
I
OH
+
NH3
I HO -
O2
H2O
- CH2–CH–COO-
+
NH3
I
Phenylalanine
Tyrosine
Tetrahydrobiopterin
(BH4)
Phenylalanine hydroxylase
GTP Biosynthesis
Dihydropteridine reductase
2 1
5 4 3 6
8
9
7
10
3 5
b. Activity II: Reduction of the co-factor tetrahydrobiopterin (BH4) to dihydrobiopterin (BH2); the H
atoms on carbons 3 and 5 of BH4 are absent in BH2.
2. BH4 is synthesized in the body from GTP; BH4 is usually used in the hydroxylation of aromatic amino
acids like phenylalanine, tyrosine and tryptophan (with aromatic rings).
3. BH2 is reduced back to BH4 via an NADH-dependent dihydrobiopterin reductase for phenylalanine to
continue forming tyrosine.
III. TYROSINE COMING FROM PHENYLALANINE
Quinonoid dihydrobiopterin (BH2)
- CH2–CH–COO-
NAD+
NADH
+ H+
H2N N H
N H
H
HN
II
O
N
H
H
CH – CH – CH3
I
OH
I
OH
H2N N H
N H
H
N
II
O
N
H
CH – CH – CH3
I
OH
I
OH
+
NH3
I HO -
O2
H2O
- CH2–CH–COO-
+
NH3
I
Phenylalanine
Tyrosine
Tetrahydrobiopterin
(BH4)
Phenylalanine hydroxylase
GTP Biosynthesis
Dihydropteridine reductase
2 1
5 4 3 6
8
9
7
10
3 5
4. Tyrosine is unique because unlike all the other non-essential AAs which are
synthesized from certain intermediates of glycolysis (3-phospholycerate and pyruvate)
and Krebs Cycle, it results from a simple one-step hydroxylation of phenylalanine.
5. Hence the presence of dietary tyrosine decreases the need for phenylalanine.
IV. BIOSYNTHESIS OF HYDROXYPROLINE
AND HYDROXYLYSINE
18O2
Ascorbate
Fe2+
Prolyl hydroxylase (Lysyl hydroxylase)
Pro (Lys)
18OH
Pro (Lys)
α-ketoglutarate [18O] Succinate
COO-
I
CH2
I
CH2
I
C = O
I
COO-
COO-
I
CH2
I
CH2
I
COO-
1. Peptide-bound proline and lysine are hydroxylated by prolyl hydroxylase and lysyl
hydroxylase respectively (found in skin and skeletal muscles, including granulating
wounds), in the presence of molecular O2 (one atom of O2 is incorporated into
proline or lysine; the other atom into succinate), ascorbate , Fe+2 and -ketoglutarate.
2. Hydroxyproline and hydroxylysine are present principally in collagen; collagen is a
glycoprotein in all tissues and organs provides the framework that gives tissues
their form and structure.
3. A deficiency of Vitamin C or hydroxylase results to scurvy.
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