Download Nitrogen Anabolism

BC368
Biochemistry of
the Cell II
Nitrogen Anabolism
Ch 22 (22.1 and 22.2)
May 5, 2015
Overview of amino acid anabolism
Biologically useful nitrogen
compounds are generally
scarce in nature.
Most organisms maintain
strict economy in their use of
ammonia, amino acids, and
nucleotides, often salvaging
and reusing them.
Overview of amino acid anabolism
Biologically useful nitrogen
compounds are generally
scarce in nature.
Most organisms maintain
strict economy in their use of
ammonia, amino acids, and
nucleotides, often salvaging
and reusing them.
The nitrogen cycle
maintains a pool of
biologically available nitrogen
in nature.
~Fig 22-1
The nitrogen cycle
Nitrogen Fixation
~1.0 x 1010 kg/yr
~8.0 x 1010 kg/yr
~4% of total
~30% of total
N2 + O2  2 NO  2 NO2
N2 + 3 H2  2 NH3
2NO2 + H2O  HNO3 + HNO2
~1.8 x 1011 kg/yr
~66% of total
N2 + 10 H+  2 NH4+ + H2
Haber-Bosch Cycle
N2 + 3 H2 --> 2 NH3
500oC, 300 ATM
•Ammonia was first made on an
industrial scale in 1913.
•Critical for the German munitions effort.
•Later, principally used to make fertilizer,
allowing more efficient food production.
•Nearly 80% of the nitrogen found in
human tissues originated from the
Haber-Bosch process.
Fritz Haber article
Nitrogen-fixing
bacteria/archaea
N2 + 8 e- + 10 H+ + 16 ATP
2 NH4+ + H2 + 16 ADP + 16 Pi
15-37oC, 0.8 ATM
Azotobacter/Rhizobium/Clostridium
Overview of
Nitrogen Fixation
Reduction of N2 is carried out
by the nitrogenase complex,
which uses ATP to transfer electrons from a donor
(harvested from a redox protein such as ferredoxin).
Nitrogenase Complex
2 enzymes in the
nitrogenase complex:
Nitrogenase Complex
Dinitrogenase reductase (Fe protein; dimer of
two identical subunits; 2 dimers per complex)
Dinitrogenase (MoFe protein; a2b2 tetramer)
Nitrogenase
complex
Green=
Dinitrogenase
Reductase
Blue & Purple=
Dinitrogenase
Note that the
reductase
requires 2 ATP
to pass 1
electron
Nitrogenase
complex
Dimer
Tetramer
Green=
Dinitrogenase
Reductase
Blue & Purple=
Dinitrogenase
Dimer
Nitrogenase Complex
2NH4+ +
H2
Dinitrogenase
Reductase
Dinitrogenase
8
N2 + 10H+
Electrons are passed through redox centers to N2 and
H+ (8 electrons total).
Assimilation of NH3
into amino acids
Once ammonia has been formed via nitrogen fixation,
the nitrogen can be incorporated into either glutamate or
glutamine for further use:
Glu is the source of amino
groups for synthesis of most
amino acids
Gln is the source of amino
groups for synthesis of most
other nitrogen-containing
molecules (e.g., nucleotides)
Assimilation of NH3
into amino acids
Formation of Glu: reductive amination of α-KG via
glutamate dehydrogenase
Formation of Gln: glutamine synthetase reaction
Allosteric
control of
Figglutamine
18-1
synthetase
Complex
control with
many
allosteric
regulators
(end products
of Gln
metabolism)
Fig 18-1
In general,
humans can’t
synthesize amino
acids that require
more than 5 steps.
This one is arginine.
Overview of
amino acid
Fig
18-1
biosynthesis
Carbon skeletons are
made from intermediates
of glycolysis, TCA, or
pentose phosphate shunt
Amine groups are
from Glu (or Gln)
Amino acid biosynthesis
Six biosynthetic families (bold = essential):
Essential
Nonessential
Amino acid biosynthesis
His
Pentose
Tetrose + triose (PEP)
Tyr/Trp/Phe
(aromatics)
(pentose phosphate shunt)
Glucose
(glycolysis)
3PG
Pyruvate
Ser
1.
2.
Cys/Gly
Ala/Val/Leu/Ile
α-KG
Glu
Gln/Pro
(urea cycle)
Ornithine
3. OA
Asp
Lys/Asn/Met/Thr
Arg
Regulation
Typically, first enzyme
in a pathway is
allosterically inhibited by
the end product
(allosteric feedback
inhibition).
Regulation
Interlocking regulatory
mechanisms ensure that
amino acids are
synthesized in the
correct proportions for
protein synthesis.
E. coli regulation of
various amino acids
derived from Asp.
Anabolic Window
The rate of skeletal muscle
protein synthesis is regulated
by food intake and exercise.
•
•
Ingestion of 15-20 g
protein elevates muscle
protein synthesis rates for
up to 5 hours
Exercise stimulates
skeletal muscle protein
synthesis for up to 48
hours
Dreyer et al. (2008) Am J Physiol
Endocrinol Metab. 294(2): E392–
E400.
Anabolic Window
Ingestion of protein within an hour of exercise
greatly increases the rate of skeletal muscle protein
synthesis.
Beverage
containing
essential
amino acids
and
carbohydrate
3:1 ratio of carbs to protein also enhances
recovery of glycogen stores.
 CHO-Pro: carbohydrate-protein (80 g CHO, 28 g Pro, 6 g fat)
 LCHO: carbohydrate only (equal grams: 80 g CHO, 6 g fat)
 HCHO: carbohydrate only (equal calories: 108 g CHO, 6 g fat)