Lecture 23: Nitrogen Metabolism Flashcards
proteasomal complex
releases oligopeptides
degrade these into individual amino acids
urea cycle
uses nitrogen from NH4+ and amino acid aspartate
generate urea
excrete to maintain daily nitrogen balance
what do we use the carbon skeleton of amino acids for?
energy converting rxns
what do we do with nitrogen after taking apart amino acids
remove it to avoid ammonia toxicity
what purpose does nitrogen fixation and assimilation serve?
FIXATION: in bacteria
- main way atmospheric nitrogen is made into ammnia and nitrogen oxides
ASSIMILATION: incorportate ammonium into amino acids (glutamate and glutamine)
Key enzymes
Bacterial Nitrogenase complex: redox rxns and ATP hydrolysis to make NH2 2NH3
Glutamine Synthetase: in all organisms NH4+ into glutamate to make glutamine
Glutamate synthetase: in bacteria and plants remake glutamate so we can continue glutamine synth
Glutamate dehydrogenase: all organisms. interconverts glutamate, NH4+ and alpha ketogluterate
Three processes that fix N2 into the biosphere
Rhizobium bacterium
Atmospheric Lightening
Haber Process
Haber process
important for agriculture
been around a long time
1 part nitrogen, 1 part H2 gas combined, some converted to ammonia gas
not very effcient, so recycle so that not used parts can be used
has to be very high temp and pressure
Nitrogen fixation in bacteria
nitrogenase protein required: 2 components
1 component binds to ATP and obtains electrons to reduce triple bond to double bond
(2nd component) nitrogenase component takes it from nitrogen to ammonia
how many ATP to convert 1 nitrogen to 2 ammonia???
16
2 ATP for every electron used by the nitrogenase protein
why so much ATP?
the bond NRG of N2 is 930 kJ/mol
this is not easy to split!
Dimine has two fates
can become hydrazine or go straight to NH3
Nitrogen assimilation in plants: HIGH NH4+ levels in soil
plants use glutamate dehydrogenase rxn to directly incoperate NH4+ into glutamate using ketogluterate
standard free energy charge is +30… so how do we get it to work??? (for nitrogen assimilation via glutamate dehydrogenase)
need high concs of NH4 in soil!!!!
(high substrate concs)
otherwise, we release reactants from glutamates (if levels not hight)
Nitrogen assimilation in plants: LOW NH4+ levels in soil
we need 2 enzymes/reactions:
glutamine synthetase
glutamate synthase: makes more glutamate that we can use for glutamine synth…provides more substrate to drive reaction forward
Net reaction of nitrogen assimilation: LOW NH4+ in soil
1) glutamate+NH4+ +ATP=glutamine
2) glutamine+alphaketoglutamate+NADPH= 2 glutamate
use ATP AND NADPH
net rxn: alphaketogluterate+ NH4++ ATP + NADPH=glutamate
whats the difference between low and high (besides needed 2 vs 1 enzymes)
low use ATP and NADPH
high uses just NADPH
assimaltion of nitrogen into amino acids done by…
aminotransferase enzymes
aspartate aminotransferase
uses PLP (vitamin B6) as a coenzyme to help with amino group transfer nitrogen temp attached at PLP
glutamate combined with the aminotransferase
PLP attaches to nitrogen temporarily
remove nitrogen from glutamate=alphaketogluarate
nitrogen transferred to OAA to from aspartate
What pathway provides the OAA needed for the aspartate aminotransferase reaction reaction?
citrate cycle
amonia cycle
NH4 in soil comes from decomposition, soil bacteria, fertilizers
converted to NO2- and NO3- by soil bacteria (nitrite and nitrate)
plant roots absorb this and make it NH4+
Positive nitrogen balance
young children and pregnant women
they accumulate nitrogen to support protein synth
negative nitrogen balance
seen in starvation, elevated rates of protein break down
Digestion of dietary proteins
proteases secreted: they secrete inactive pepsinogen
pepsinogen becomes pepsin, which breaks down the dietary protein (stomach)
more proteases in small intestine continue to break down
end result: amino acids and small peptides
What do proteases do
break down proteins into amino acids and peptides
why do we first have pepsinogen: which is inactive?
so its active only in stomach (low pH)
so that it doesnt break down proteins in cells
where are proteases made? How are they activated
pancreas
when they reach low pH of stomach, another protease cleaves them and activates them
degradation of cellular proteins.
why?
to regulate enzyme function
if you degrade the enzyme… its gone and you can’t use it
how to break down proteins in cells
in lysosomes
proteosome complexes
left with amino acids at the end
breakdown of proteins in cells: lysosomes
phospholipid bilayer
low pH inside where enzymes that break things down are
breakdown of proteins in cells: proteosome complexes
ID proteins that have been modified with protein ubiquiton (ubiquitinated proteins) (tag for degredation)
unfold proteins and break them into small peptides
What do we do with amino acids?
use whole thing again to make more proteins
split it up
- carbon skeleton to make pyruvate, OAA, acetyl CoA for citrate cycle
- nitrogen group: toxic, use it to make biomoelcs or excrete it
2 types of amino acids
glucogenic
ketogenic
glutogenic amino acids
form citrate intermediates or pyruvate when broken down
ketogenic amino acids
make acetyl Co-A or acetoacetyl CoA when broken down
Urea Cycle
to make soluble and non toxic urea from ammonium ions, bicarbonate, aspartate
ammonium ion either comes down from breakdown of amino acids or from
bicarb coms from citrate cycle
aspartate comes from glutamate AMINO ACIDS
what is accomplished by the urea cycle
removal of excess nitrogen from body
synthed in liver
exported to kidneys
leaves through baldder
regulated enzymes of urea cycle?
carbamoyl phosphate synthase 1 (first step in pathway)
where does reaction happen?
first step happens in mitochondrial matrix
exportation into cytosol
urea cycle
review slide 22
aspartate-arginiosuccinate shunt
link between urea and citrate cycles
provides substrates so they can continue
Where does amino acid-derived nitrogen enter the urea cycle?
at the glutamate and NH4 (see slide 23)
review slide 23
review slide 23
Urea Cycle disorders: Arginosuccinase deficiency
you can’t convert argininosuccinate
so you’re on low protein diet
except you give them lots of arginine!!! because they can’t make it
would suplementing with ornithine also work?
in theory yea, it still helps us to make citrulline; which is the goal
but we don’t do it b/c its not as practical: arginine is really availible and is easily taken up
