Chapter 17 (Part 1)
Amino Acid Metabolism:
Nitrogen Assimilation
and Amino Acid Biosynthesis
Nitrogen Assimilation
• Nitrogen is required in the synthesis of amino
acids, purine and pyrimidine nucleotides, and a
number of other important biological compounds.
• Organisms need to obtain nitrogen in a usable
form. Nitrogen in the form of ammonia is
assimilated by biological systems
• Nitrogen is originally assimilated from the
environment by microorganisms and plants.
• Animals must obtain biological forms of nitrogen
from their diets.
Nitrogen Fixation
•
•
Most abundant form of nitrogen on Earth is N2
gas (makes up 80% of air)
N2 gas is very stable and inert. 2 N connected
by triple bond (225 kcal/mole required to break
bond).
N2 gas can be converted to biologically accessible
forms in three ways:
1) N2 can be reduced to NO3- by lightning and UV
radiation (15% of fixed nitrogen)
2) N2 can be reduced to NH3 through industrial
processes (25% of total fixed nitrogen) Requires
temperatures of 500oC and 300 atm)
3) N2 can be reduced to NH3 by nitrogen fixing
bacteria (60% 0f total fixed nitrogen)
Nitrogen Cycle
Biological Nitrogen Fixation
• Process performed only by special free living
(cyanobacteria) microorganims.
• Nitrogen fixation can also be performed by
microorganisms (Rhizobium, Bradyrhizobium)
that exist as symbiotes with specific plant
species (Legumes – soybean, alfalfa)
• N2 is converted to NH3 in a reaction
catalyzed by the nitrogenase complex.
Nitrogenase
• consist of two subunits, an Fe-protein (reductase)
and an MoFe-protein (nitrogenase).
• Reductive process: N2 + 8e- + 8H+  2 NH3 + H2
• For every e- used in the process 2 ATP are
consumed, so 16 ATPs are required to convert 1
N2 to NH3
• Enzymes are highly sensitive to O2. Require
anerobic conditions
Nitrate and Nitrite to Ammonia
• NO3- and NO2- must be converted to
NH3 to be assimilated into organisms.
• Process referred to as nitrification
• Requires two enzymes nitrate reductase
and nitrite reductase
Ammonia Assimilation
•
Glutamate
dehydrogenase reductive amination
of a-ketogluturate
to glutamate.
•
Glutamine
synthetase ATPdependent
amidation of
gamma-carboxyl of
glutamate to
glutamine
Glutamate Dehydrogenase
• Impt. in bacteria when ammonia is present at
high concentrations.
• In animal and plant systems enzyme is localized
within the mitochondria. The enzyme is near
equilibrium and runs in the direction toward
glutamate catabolism (anaplerotic rxn to
regenerate a-ketogluturate).
Glutamine Synthetase (GS)
• Glutamine plays important role in Nmetabolism.
• Acts a NH3 donor for purine and pyrimidine
synthesis
• In mammals, glutamine is synthesized in muscle
tissue using GS and transported to other
tissues.
Glutamine back to Glutamate
•Under low NH3 conditions, in conjunction with glutamine
synthetase, bacteria use glutamate synthase to convert
Gln and a-ketogluturate to 2 glutamates.
•The Km of glutamate dehydrogenase for NH3 is high,
the Km of glutamine synthetase is low.
Glutamine Synthetase
• GS in bacteria is regulated in three
ways:
– Feedback inhibition
– Covalent modification (interconverts
between inactive and active forms)
– Regulation of gene expression and
protein synthesis control the amount
of GS in cells
Allosteric
Modulation of
GS Acitivity
in Bacteria
Allosteric Modulators of
Mammalian GS
• Activator – a-ketogluturate
• Inhibitors – glycine, serine, alanine,
carbamyl-phosphate
Regulation of
Bacterial GS
by Covalent
Modification
• Gene GlnA is actively transcribed only if
transcriptional enhancer NRI is in its phosphorylated
form, NRI-P
• NRI is phosphorylated by NRII, a protein kinase
• If NRII is complexed with the activated form of PII
acts as a phosphatase, not a kinase
Glutamate serves as primary N- donor for AA
synthesis through transamination reactions
Transamination rxns involve
Pyridoxol-phosphate and
formation of Schiff Base
Essential vs. Non-Essential AA’s
• Plants and
bacteria can
synthesize all 20
amino acids.
• Animals must
obtain 8 amino
acids (essential
AA’s) from diet
Amino Acid Biosynthesis is
a Diverse Process