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Horton • Moran • Scrimgeour • Perry • Rawn
Principles of Biochemistry
Fourth Edition
Chapter 17
Amino Acid Metabolism
Copyright © 2006 Pearson Prentice Hall, Inc.
Chapter 17 - Amino Acid Metabolism
• Metabolism of the 20 common amino acids is
considered from the origins and fates of their:
(1) Nitrogen atoms
(2) Carbon skeletons
• For mammals:
Essential amino acids must be obtained from diet
Nonessential amino acids - can be synthesized
17.1 The Nitrogen Cycle and Nitrogen Fixation
• Nitrogen is needed for amino acids, nucleotides
• Atmospheric N2 is the ultimate source of
biological nitrogen
• Nitrogen fixation: a few bacteria possess
nitrogenase which can reduce N2 to ammonia
• Nitrogen is recycled in nature through the
nitrogen cycle
Fig 17.1 The Nitrogen cycle
Nitrogen Fixation
• Most green plants and some microorganisms
contain nitrate reductase and nitrite
reductase, enzymes that together catalyze
the reduction of nitrogen oxides to ammonina.
Nitrogenase
• An enzyme present in Rhizobium bacteria that
live in root nodules of leguminous plants
• Some free-living soil and aquatic bacteria also
possess nitrogenase
• Nitrogenase reaction:
N2 + 8 H+ + 8 e- + 16 ATP
2 NH3 + H2 + 16 ATP + 16 Pi
17.2 Assimilation of Ammonia
• Ammonia generated from N2 is assimilated into
low molecular weight metabolites such as
glutamate or glutamine
• At pH 7 ammonium ion predominates (NH4+)
• At enzyme reactive centers unprotonated NH3 is
the nucleophilic reactive species
A. Ammonia Is Incorporated into Glutamate
• Reductive amination of a-ketoglutarate by
glutamate dehydrogenase occurs in plants,
animals and microorganisms
• Glutamine is a nitrogen donor in many biosynthetic
reactions
Fig 17.5 Glutamate
synthase catalyze the
reductive amination of
a-ketoglutarate
• Animals do not have glutamate
synthase.
B. Transamination Reactions
• Transfer of an amino group from an a-amino
acid to an a-keto acid
• In amino acid biosynthesis, the amino group of
glutamate is transferred to various a-keto acids
generating a-amino acids
• In amino acid catabolism, transamination
reactions generate glutamate or aspartate
Fig 17.6 Transfer of an amino group
from an a-amino acid to an a-keto acid
Fig 17.7 Pig (Sus scrofa) cytosolic
aspartate transaminase
(Space-filling model: the coenzyme pyridoxal phosphate)
Fig 17.8 Assimilation of ammonia
into amino acids
a. The glutamate dehydrogenase pathway.
Fig 17.8 Assimilation of ammonia
into amino acids
b. Combined action of glutamine synthetase and
glutamate synthase under conditions of low
NH4+ concentration.
17.3 Synthesis of Amino Acids
• Most bacteria and plants (not mammals)
synthesize all 20 common amino acids
• Nonessential amino acids for mammals are
usually derived from intermediates of glycolysis
or the citric acid cycle (11 of the 20 a.a.)
• Amino acids with the largest energy
requirements are usually essential amino acids
Box 17.3 Essential and Nonessential
Amino Acids in Animals
Fig 17.9 Biosynthesis of Amino Acids
A. Asparate and Asparagine
• Oxaloacetate is the amino-group acceptor in a
transamination reaction that produces asparate.
B. Lysine, Methionine, and Threonine
• Aspartate is the precursor of lysine, methionine,
and threonine.
C. Alanine, Valine, Leucine, and Isoleucine
• Pyruvate is the amino group acceptor in the
synthesis of alanine by a transamination reaction.
C. Alanine, Valine, Leucine, and Isoleucine
• Pyruvate is also a precursor in the synthesis of the
branched-chain amino acids valine, leucine, and isoleucine.
D. Glutamate, Glutamine, Arginine, and Proline
Fig 17.13 Conversion of glutamate to proline and arginine
E. Serine, Glycine, and Cysteine
• Serine, glycine, and cysteine- are derived from
the glycolytic/gluconeogenic intermediate 3phosphoglycerate.
Fig 17.14 Biosynthesis of serine.
E. Serine, Glycine, and Cysteine
• Serine, glycine, and cysteine- are derived from
the glycolytic/gluconeogenic intermediate 3phosphoglycerate.
Fig 17.15 Biosynthesis of glycine.
E. Serine, Glycine, and Cysteine
• Serine, glycine, and cysteine- are derived from
the glycolytic/gluconeogenic intermediate 3phosphoglycerate.
Fig 17.16 Biosynthesis of cysteine from serine in
many bactera and plants.
Fig 17.17 Biosynthesis of cysteine in
mammals
• Animals do not have the normal cysteine
biosynthesis pathway shown in fig 17.16.
F. Phenylalanine, Tyrosine, and Tryptophan
• Chorismate, a derivative of shikimate, is a key branch-point
intermediate in aromatic amino acid synthesis.
• Animals can not synthesize chorismate.
Fig 17.19 Biosynthesis of phenylalanine
and tyrosine from chorismate in E. coli
Indole glycerol phosphate for Trp biosynthesis
• Anthranilate is produced from chorismate
• Anthranilate is then converted into indole
glycerol phosphate for Trp synthesis
Fig 17.21 Reactions catalyzed by
tryptophan synthase
G. Histidine
17.4 Amino Acids as Metabolic Precursors
• The primary role of amino acids is to serve
as substances for protein synthesis.
• Some amino acids are essential
precursors in other biosynthesis pathways.
– Glutamate, glutamine, and asparate
• Required in the urea cycle
• Involved in many transamination
• Purine and pyrimidine biosynthesis
– Serine and glycine (Fig 17.23)
– Arginine (Fig 17.24)
Fig 17.23 Compounds formed from
serine and glycine
Synthesis of Nitric Oxide (NO)
from Arginine
• Nitric oxide (.N=O) is a gas which can diffuse
rapidly into cells, and is a messenger that
activates guanylyl cyclase (GMP synthesis)
• NO relaxes blood vessels, lowers blood
pressure, and is a neurotransmitter in the brain
(high levels of NO during a stroke kill neurons)
• Nitroglycerin is converted to NO and dilates
coronary arteries in treating angina pectoris
Fig 17.24 Conversion of arginine to
nitric oxide and citrulline
Sidenafil citrate is the active
ingredient in Viagra®
17.5 Protein Turnover
• Proteins are continuously synthesized and
degraded (turnover) (half-lives minutes to weeks)
• Lysosomal hydrolysis degrades some proteins
• Some proteins are targeted for degradation by a
covalent attachment (through lysine residues) of
ubiquitin (C terminus)
• Proteasome hydrolyzes ubiquitinated proteins
Fig 17.25 Ubiquitination and
hydrolysis of a protein
• Ubiquination enzymes attach multiple ubiquitins
• Proteasome hydrolyzes uniquinated proteins
17.6 Amino Acid Catabolism
• Amino acids from degraded proteins or from diet
can be used for the biosynthesis of new proteins
• During starvation proteins are degraded to
amino acids to support glucose formation
• First step is often removal of the a-amino group
• Carbon chains are altered for entry into central
pathways of carbon metabolism
Catabolism of the Carbon Chains of
Amino Acids
• After removal of amino groups, carbon chains
of the 20 amino acids can be degraded
• Degradation products:
Citric acid cycle intermediates
Pyruvate
Acetyl CoA or acetoacetate
Catabolism of Carbon Skeletons
• Fig 17.26 (next slide)
• Conversion of the carbon skeletons of
amino acids to:
Pyruvate
Acetoacetate
Acetyl CoA
Citric acid cycle intermediates
Glucogenic vs Ketogenic Amino Acids
• Glucogenic amino acids can supply
gluconeogenesis pathway via pyruvate or citric
acid cycle intermediates
• Ketogenic amino acids can contribute to
synthesis of fatty acids or ketone bodies
• Some amino acids are both glucogenic and
ketogenic
A. Alanine, Asparagine, Aspartate,
Glutamate, and Glutamine
• Reentry into pathways from which carbon
skeletons arose by reverse transamination
Alanine
pyruvate
Aspartate
oxaloacetate
Glutamate
a-ketoglutarate
• Glutamine and asparagine are first hydrolyzed
to glutamate and aspartate
B. Arginine, Histidine, and Proline
Fig. 17.27
C. Glycine and Serine
D. Threonine
• Alternate routes for the degradation of
threonine to glycine
• Figure 17.29 (next slide)
Fig 17.29 Alternate routes for the
degradation of threonine
E. The Branched-Chain Amino Acids
• Leucine, valine and isoleucine are degraded by
related pathways
• The same three enzymes catalyze the first
three steps in all pathways
• A branched-chain amino acid transaminase
catalyzes the first step
Fig 17.30 Catabolism of branched-chain
amino acids
F. Methionine
Fig 17.31
(X represents any
of a number of
methyl-group
acceptors)
F. Methionine
Fig 17.31
(X represents any
of a number of
methyl-group
acceptors)
Fig 17.31 (cont)
Fig 17.31 (cont)
G. Cysteine
Fig 17.32 Conversion of cysteine to pyruvate
H. Phenylalanine, Tryptophan, and Tyrosine
Fig 17.33
Fig 17.34 Conversion of tryptophan to
alanine and acetyl CoA
I. Lysine
Fig 17.35
I. Lysine
Fig 17.35
Fig 17.35 (cont)
17.7 The Urea Cycle Converts
Ammonia into Urea
• Waste nitrogen must be removed (ammonia is
toxic to plants and animals)
• Terrestrial vertebrates synthesize urea (excreted
by the kidneys)
• Birds, reptiles synthesize uric acid
A. Synthesis of Carbamoyl Phosphate
Fig 17.37
• Synthesis of carbamoyl
phosphate (removal of NH3)
• Catalyzed by carbamoyl
phosphate synthetase I
(CPS I)
Fig 17.37 Synthesis of carbamoyl phosphate (removal of
NH3) catalyzed by carbamoyl phosphate synthetase I
(CPS I)
Fig 17.37
(cont)
B. The Reactions of the Urea Cycle
• Urea cycle (Fig 17.38 & 39 next four slides)
Rxn 1 (mitochondria), Rxns 2,3,4 (cytosol)
• Two transport proteins are required:
Citrulline-ornithine exchanger
Glutamate-aspartate exchanger
• Overall reaction for urea synthesis is:
NH3 + HCO3- + Aspartate + 3 ADP
Urea + Fumarate + 2 ADP + 2 Pi + AMP + PPi
The Urea Cycle
C. Ancillary Reactions of the Urea Cycle
• Supply of nitrogen for the urea cycle can be
balanced by supply of NH3 and amino acids
• Glutamate dehydrogenase and aspartate
transaminase catalyze near equilibrium reactions
• Flux through these enzymes depends upon
relative amounts of ammonia and amino acids
• Two cases (next slides): (a) NH3 in excess,
(b) aspartate in excess
Fig 17.40 Balancing the supply of nitrogen for
the urea cycle
NH3 in extreme excess
Aspartate in extreme excess
Glucose-alanine cycle
• Some amino acids are deaminated in muscle
• Exchange of glucose and alanine between
muscle and liver
• Provides an indirect means for muscle to
eliminate nitrogen and replenish its energy
supply
Fig 17.41 Glucose-alanine cycle
17.8 Renal Glutamine Metabolism
Produces Bicarbonate
• Bicarbonate can be lost by buffering H+ in blood
• Bicarbonate can be replenished by glutamine
catabolism in the kidneys
• a-Ketoglutarate (formed from glutamine
oxidation) can be further metabolized to yield
bicarbonate
Glutamine
a-ketoglutarate2- + 2 NH4+
Glutamine → → a-ketoglutarate2- + 2 NH4+
2 a-ketoglutarate2- → → glucose + 4 HCO3-
2C5H10N2O3 + 3O2 + 6H2O → C6H12O6 +
4HCO3- + 4NH4+
Fig 17.42 Loss of bicarbonate as a buffer