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26.1 Nitrogen Fixation
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26.1 Nitrogenfixation
AIM: Tonome ond describethe processby which otmospheric
nitrogen is mode ovoiloble to plontsond onimols.
Focus
Nitrogen enters the biosphere
by nitrogen fixation.
Fewplants can form nitrogen-containing compounds from nitrogen in the
air, no animals can, but certain bacteria can. It is through nitrogen-fixing
bacteria that atmospheric nitrogen enters the biosphere-the domain of
living things. Nitrogen-fixing bacteria are organisms that reduce atnxospheric nitrogen to ammonia, a water-solubleform of nitrogen that can be
usedbyplants and animak.
N2+3H2-+2NHs
In soil and biological fluids, most ammonia is present as ammonium ions.
NHa
Ammonia
+
H*
Proton
--
NHrt
Amrnonium
ion
Plants and animals incorporate ammonia into nitrogen compounds
such as proteins and nucleic acids. The plants and animals die and decay,
aided by other bacteria. Decaying matter returns nitrogen to the soil as
ammonia, nitrite ions (NOz ), or nitrate ions (NO3-). Moreover, some
nitrogen gasis returned to the atmosphere. Theflow of nitrogenbetween the
atmosphere and the Earth and its liuing creatures is the nitrogen cycle,
shornmin Figure26.1.
Atmospheric
nitrogen
Lightning
Animal proteins and
other compounds
ofnitrogen
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Industrial
fixation
#&
ffiw
Fertilizers
&
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ffi
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Biological fixation
(nitrogen-fixing bacteria)
w
Animalwaste
dead plants and animals
@
Decay
(putrefring bacteria)
@
Ammonia
Nitrification
(NH3)
@
Nitrite ions (NO2 )
Figure
25.1
Thenitrogencycle.
@
Nitrate ions (NOt)
Plant
proteins
*@
Denitriflring
bacteria
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CHAPTER
26 Metabolismof NitrogenCompounds
It has been estimatedthat each
year upwards of 10 million tons of
nitrogen is fixed by natural and
industrial processes.Someof this
fixed nitrogen accumulates in soil
and lakes and promotes algae
growth. There are bacteria that
convert fixed nitrogen back to Nz.
Modern agriculture intervenes to a great extent in the nitrogen cycle.
For the past severalyears, the daily amount of atmospheric nitrogen fixed
by industrial processes for the production of fertilizers has actually
exceeded the amount fixed by living organisms in the Earth's forests and
oceans.Besidesbacterial and industrial nitrogen fixation, a smaller amount
of atmospheric nitrogen is flxed by lightning discharges,which produce the
solublenitrogen oxides(NO,NO2,N2O4,N2O5).
26,2Protein turnover
AIM: To exploin the function of cothepsinsond stote their
cellulor locotion.
Focus
Proteins are continuously
hydrolyzed and synthesized by
thebody.
The rate of turnover varies by
protein function. Plasma proteins
have ahalf-life ofabout l0 days;
SOToare hydrolyzed in a l0-day
period. Muscleprotein has a halflife of 180 days, some connective
tissueproteins,as high as 1000
days.
In the human body, dietary proteins are hydrolyzed to their constituent
amino acids in the small intestine as part of digestion. Many of the body's
proteins are continuously hydrolyzed and s)'rrthesizedwithin body cells.
Thecontinuous hydrolysisand synthesisof proteins in the body is calledprotein turnover. The en4rynesresponsible for the intracellular hydrolysis of
proteins are a classof proteasescalled the cathepsins.Incells,the cathepsins are confined to the lysosomes,where protein degradation occurs. We
discussedtranslation, the processof protein sgrthesis from genetic information, in Section20.9.
PRACTICE
EXERCISE
26.I
Why are cathepsins confined to the lysosomesin cells?
26., Tronsominotionreoctions
AIMS: To describesomeusesof the amino ocidsin the omino
ocid pool. To usewords ond equotionsto describe
tronsominationreoctions,indicating the function of
pyridoxol phosphote. Tointerpret the significanceof
increosedlevelsof transominoses
in o person'sblood
serum,
Focus
Amino acids partwith their
arnino groups by transamination reactions.
Amino acids produced by digestion of dietary protein and during protein
turnover in body cells become part of the body's amino acid pool. The
amino acid pool is the total quantity of free amino acids present in tissue
cells,plasma, and other bodyfluids. The amino acids of the amino acid pool
are available for cellular needs, as shown in Figure 26.2.
Many of the reactions of amino acid meiabolism require that amino
acids first lose their alpha amino groups. The most common way for this to
occur is by transamination-the transfer of an amino group from one molecule to another An intermediate of the citric acid cycle, o-ketoglutaric
26.5 TransaminationReactions
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jFY
Urea
ffi+
WI-ffi
+ffi
Figure25.2
Themajorpathways
of aminoacid
metabolism.
acid, is the usual acceptor of the amino goup. The products of the ieaction
are an a-ketoacid and glutamic acid.
oo
iltl
HO
ttl
R-C-C-OH
+ HO-C-CH2CI{2-C-C-OH
oo
iltl
o
tl
=-R-C-C-OH
+ HO-C,
o
tl
CH2CH2CH-C-OH
NH,
An a-amino acid
NHz
a-Ketoglutaric acid
Glutamic acid
An a-keto acid
Tlansamination reactions are catalyzed by enzymes called transaminasesThe transaminaseswere mentionedinTable 19.1as indicators of diseaseor trauma that affects tissues.The principal transaminase of the liver is
glutamic-pyruuic transaminase (GPT), an enzyme that catalyzes the formation of pyruvate from alanine.
HO
ttl
CH3-C-C-OH
+ d-Ketoglutaric acid
GPT
oo
illl
CH3C-C-OH
+ Glutamic acid
I
NH2
Alanine
I
Smrvic acid
Liver diseasecausesan increase of blood serum GPT (SGPT).Elevated levels of the enzyme in serum can be used as a diagnostic test for liver disease.
The principal transaminase of the heari muscle is glutamic-oxaloacetic
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26 Metabolism
CHAPTER
of NitrogenCompounds
transaminase(GOT),which catalyzesthe formation of oxaloacetate,one of
the intermediates of the citric acid cycle, from aspartic acid.
o
o
o
HO-C-CH2CH-C-OH
I
cor
+ a-Ketoglutaricacid .-
ll
oo
ll ll
HO-C-CH2C-C-OH
+ Glutamic acid
NHz
Asparticacid
Oxaloacetic acid
Damaged heart cells die and split open, but some of the GOT moleculesthat
spill into the blood are still active. The levels of serum GOT (SGOT)activity are
a measureof the extent of damageto heart muscle causedby a heart attack.
All knornmtransaminases require p1'ridoxal phosphate, a derivative of
pgidoxol (vitamin Bo),as the cofactor.
'".^/o
?to
"-{-"-.nl'yo''o-$-o-cu,
oo\A.t3
Pyridoxal phosphate
Pyridoxamine phosphate
The aldehyde carbon of pyridoxal phosphate is the acceptor of the amino
group of amino acidsin the first stageof reactionscatalyzedby transaminases.
An cr-ketoacidand pyridoxamine phosphate are the products of the reaction.
Amino acid + Pyridoxalphosphate+
ct-Ketoacid+ Pyridoxaminephosphate
The second stage of transamination consists of the transfer of the amino
group of the pyridoxamine phosphate to cr-ketoglutaratewith regeneration
of the pyridoxal phosphate.
a,-Ketoglutaric
acid + Pyridoxaminephosphate------Glutamicacid + Pyridoxalphosphate
PRACTICE
EXERCISE
26.2
Write an equation for the transamination
tarate as the acceptor.
of t1'rosine with o-ketoglu-
PRACTICE
EXERCISE
26.!
How does an amino acid lose its amino group in transamination?
26.4 Theurea cycle
Focus
Ammonia produced from the
oxidation of glutamic acid is
excreted as urea.
AIMS: To exploin the net resultsof the ureo cycleon o
moleculorond o physiologicscole.To list two cousesof o
negativenitrogen bolonce.
Glutamic acid serves as the depot for receiving amino groups removed
from amino acids by transamination reactions. Since there is a limited
quantity of o-ketoglutarate in cells, it must be regenerated so that
transamination reactions and the citric acid cycle can continue. The cr-