Download Lecture Topic: Fatty Acid Synthesis

10-21-11 Nitrogen Metabolism
1. Nitrogen Fixation
2. Amino Acid Biosynthesis
Nitrogen is an essential element found in proteins,
nucleic acids and many other molecules
Biologically available nitrogen is scarce
Nitrogen incorporation begins with fixation
(reduction) of N2 by prokaryotic
microorganisms to form ammonia (NH3)
Nitrogen supply is often the rate-limiting factor in
plant growth
Nitrogen is assimilated by conversion into the amide
group of glutamine, which can then be used for
other carbon-containing compounds (e.g.,
amino acids)
The nitrogen cycle is the complex process by which
nitrogen is transferred throughout the living
world
Amino acid metabolism is an important process
involving nitrogen
Animals can only produce half of the amino acids
required (nonessential amino acids); others
must be obtained from diet (essential amino
acids)
Transamination reactions dominate amino acid
metabolism (aminotransferases or
transaminases)
Nitrogen fixation occurs industrially via the Haber
reaction, accounting for 25% of earth’s yearly
fixed nitrogen production as fertilizer
N2 + 3 H2  2 NH3
500oC
300 atmospheres
Lightning strikes and ultraviolet light produce
another 15% of earth’s fixed nitrogen
Biological nitrogen fixation, the cellular method to
execute this thermodynamically favorable
reaction, produces 60% of earth’s fixed nitrogen
Nitrogen fixation is only possible by a limited
number of species
Among the most prominent nitrogen-fixing species
are free-living bacteria, cyanobacteria and
symbiotic bacteria
Organisms such as Azotobacter vinelandii,
Anabaena azollae and Rhizobium species
Energy requirement is extremely high: 16 ATP to
form two NH3 from one N2
The Nitrogen Fixation Reaction
All species that can fix nitrogen contain the
nitrogenase complex
Consists of two proteins dinitrogenase reductase
and dinitrogenase
Dinitrogenase reductase (Fe Prot.) passes electrons
from NAD(P)H one at a time to dinitrogenase
Uses 4Fe-4S cluster and MgATP-binding site
Dinitrogenase (MoFe protein) catalyzes the reaction
N2 + 8H+ + 8e-  2NH3 + H2
Uses P cluster [8Fe-7S] and MoFe cofactor
prosthetic groups
dinitrogenase
dinitrogenase
reductase
The transfer of electrons from NAD(P)H to ferredoxin
is the first step of nitrogen fixation
Electrons then moved to Fe protein FeS cluster; the
movement of these electrons to the MoFe
protein requires MgATP hydrolysis
A total of eight electrons are required to reduce N2 to
2 NH3
Nitrogen Assimilation
Nitrogen assimilation is the incorporation of
inorganic nitrogen compounds into organic
molecules
Nitrogen assimilation begins in the roots of plants
NH4+ (from soil or root nodules) or NO3(nitrate) is incorporated into amino acids
If nitrate is the nitrogen source, a two-step
reaction is used to first convert it to NH4+
Glutamate dehydrogenase synthesizes glutamate
from NH4+ and a-ketoglutarate
Glutamate
dehydrogenase
Glutamine synthetase catalyzes the ATPdependent reaction of glutamate with
NH4+ to form glutamine
Living organisms differ in their ability to
synthesize amino acids
Many plants and microbes can synthesize all of the
amino acids, while mammals cannot
Reactions of Amino Groups
Once amino acids have entered the cell, their amino
groups are available for synthetic reactions
Usually via transamination or direct incorporation
Transamination - Aminotransferases are responsible
for the reactions are found in cytoplasm and
mitochondria
oxaloacetate and pyruvate are converted to amino
acids by transamination
oxaloacetate + glutamate  aspartate + a-ketoglutarate
pyruvate + glutamate  alanine + a-ketoglutarate
Most aminotransferases use glutamate as the amino
group donor
The glutamate/a-ketoglutarate pair play an important
role in nitrogen metabolism
Transamination reactions require the coenzyme
pyridoxal-5ʹ-phosphate (PLP), which is derived
from pyridoxine (vitamin B6)
PLP accepts an amino group to form cofactor PMP
Direct incorporation of ammonium ions into
organic molecules: Two methods
1) Reductive amination of a-keto acids
2) Formation of the amides of aspartic and glutamic
acid
Glutamate dehydrogenase catalyzes the direct
amination of a-ketoglutarate
Ammonium ions are also incorporated into cell
metabolites by the formation of glutamine, the
amide of glutamate (glutamine synthetase)
Glutamate
dehydrogenase
Glutamate
synthetase
Synthesis of the Amino Acids
Amino acids differ from other biomolecules in that
each member is synthesized in a unique
pathway
On the basis of the similarities in their synthetic
pathways, they can be grouped into six
families
Glutamate, serine, aspartate, pyruvate, the
aromatics and histidine
The amino acids in each family are ultimately
derived from one precursor molecule
Amino acids are made from intermediates
of major pathways
Amino acids can be grouped on the basis of their
metabolic origins, as follows
Pathway origins are indicated in blue
Amino acid precursors of other amino acids in yellow
Essential amino acids in humans in bold
Aspartate family
Aromatic family
Pyruvate family
Histidine family
Glutamate family
Serine family
Serine family members (serine, cysteine and
glycine) are formed from 3-PG
Tetrahydrofolate carries activated one carbon
units
The one carbon group is bonded to N-5 or N-10 or
both and can exist in 3 oxidation states
Tetrahydrofolate is critical for DNA replication
and cell growth
Anti-cancer drugs are often compounds that inhibit
the ability to regenerate tetrahydrofolate and
thus slow cancer cell growth
Tetrahydrofolate is important in development of the
fetal nervous system; deficiency can cause
spina bifida and anencephaly
Tetrahydrofolate is derived
from folic acid (Vit. B9)
S-Adenosylmethionine is the major donor of
methyl groups
SAM is synthesized from methionine and ATP
The activated methyl group on SAM makes it a
strong methyl group donor
After methyl group transfer S-adenosyl homocysteine
is hydrolyzed to adenosine and homocysteine
Methionine is regenerated by transfer of a
methyl group to homocysteine from
N5-methyltetrahydrofolate
methionine
synthase
The activated
methyl cycle
High homocysteine levels correlate with
vascular disease
The most common genetic basis for high
homocysteine levels is mutation of the gene for
cystathionine b-synthetase
High homocysteine levels may:
Damage cells lining blood vessels
Increase growth of vascular smooth muscle
Increase oxidative stress
Vitamin treatments are sometimes effective in
reducing homocyteine levels
Pyridoxal phosphate is needed by cystathionine
b-synthetase
THF and vitamin B12 support the methylation of
homocysteine to methionine
Over expression of cystathionine beta-synthetase leads to decreased
homocysteine levels in the blood and may contribute to Down symdrome