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 N
2 by prokaryotic microorganisms to form ammonia (NH 3 ) 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)
Slide 3
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)
Slide 4
Nitrogen fixation occurs industrially via the Haber reaction,
accounting for 25% of earths yearly fixed nitrogen production as
fertilizer N 2 + 3 H 2 2 NH 3 Lightning strikes and ultraviolet
light produce another 15% of earths fixed nitrogen 500 o C 300
atmospheres
Slide 5
Biological nitrogen fixation, the cellular method to execute
this thermodynamically favorable reaction, produces 60% of earths
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 NH 3 from one N 2
Slide 6
Slide 7
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 N 2 + 8H + + 8e
- 2NH 3 + H 2 Uses P cluster [8Fe-7S] and MoFe cofactor prosthetic
groups
Slide 8
dinitrogenase reductase
Slide 9
Slide 10
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 N 2 to 2 NH 3
Slide 11
Nitrogen Assimilation Nitrogen assimilation is the
incorporation of inorganic nitrogen compounds into organic
molecules Nitrogen assimilation begins in the roots of plants NH 4
+ (from soil or root nodules) or NO 3 - (nitrate) is incorporated
into amino acids If nitrate is the nitrogen source, a two-step
reaction is used to first convert it to NH 4 + Glutamate
dehydrogenase synthesizes glutamate from NH 4 + and
-ketoglutarate
Slide 12
Glutamate dehydrogenase
Slide 13
Glutamine synthetase catalyzes the ATP- dependent reaction of
glutamate with NH 4 + to form glutamine
Slide 14
Living organisms differ in their ability to synthesize amino
acids Many plants and microbes can synthesize all of the amino
acids, while mammals cannot
Slide 15
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
+ -ketoglutarate pyruvate + glutamate alanine + -ketoglutarate
Slide 16
Most aminotransferases use glutamate as the amino group donor
The glutamate/ -ketoglutarate pair play an important role in
nitrogen metabolism Transamination reactions require the coenzyme
pyridoxal-5 -phosphate (PLP), which is derived from
pyridoxine(vitamin B 6 ) PLP accepts an amino group to form
cofactor PMP
Slide 17
Direct incorporation of ammonium ions into organic molecules:
Two methods 1) Reductive amination of -keto acids 2) Formation of
the amides of aspartic and glutamic acid Glutamate dehydrogenase
catalyzes the direct amination of -ketoglutarate Ammonium ions are
also incorporated into cell metabolites by the formation of
glutamine, the amide of glutamate (glutamine synthetase)
Slide 18
Glutamate dehydrogenase Glutamate synthetase
Slide 19
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
Slide 20
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
Slide 21
Aspartate family
Slide 22
Aromatic family
Slide 23
Pyruvate family
Slide 24
Histidine family
Slide 25
Glutamate family
Slide 26
Serine family
Slide 27
Serine family members (serine, cysteine and glycine) are formed
from 3-PG
Slide 28
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
Slide 29
Slide 30
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. B 9 )
Slide 31
S-Adenosylmethionine is the major donor of methyl groups SAM is
synthesized from methionine and ATP
Slide 32
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
Slide 33
Methionine is regenerated by transfer of a methyl group to
homocysteine from N 5 -methyltetrahydrofolate methionine synthase
The activated methyl cycle
Slide 34
High homocysteine levels correlate with vascular disease The
most common genetic basis for high homocysteine levels is mutation
of the gene for cystathionine -synthetase
Slide 35
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 -synthetase
THF and vitamin B12 support the methylation of homocysteine to
methionine
Slide 36
Over expression of cystathionine beta-synthetase leads to
decreased homocysteine levels in the blood and may contribute to
Down symdrome