Lecture 23: Nitrogen Metabolism

Question 1

proteasomal complex

Answer 1

releases oligopeptides

degrade these into individual amino acids

Question 2

urea cycle

Answer 2

uses nitrogen from NH4+ and amino acid aspartate
generate urea
excrete to maintain daily nitrogen balance

Question 3

what do we use the carbon skeleton of amino acids for?

Answer 3

energy converting rxns

Question 4

what do we do with nitrogen after taking apart amino acids

Answer 4

remove it to avoid ammonia toxicity

Question 5

what purpose does nitrogen fixation and assimilation serve?

Answer 5

FIXATION: in bacteria
- main way atmospheric nitrogen is made into ammnia and nitrogen oxides
ASSIMILATION: incorportate ammonium into amino acids (glutamate and glutamine)

Question 6

Key enzymes

Answer 6

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

Question 7

Three processes that fix N2 into the biosphere

Answer 7

Rhizobium bacterium
Atmospheric Lightening
Haber Process

Question 8

Haber process

Answer 8

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

Question 9

Nitrogen fixation in bacteria

Answer 9

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

Question 10

how many ATP to convert 1 nitrogen to 2 ammonia???

Answer 10

16

2 ATP for every electron used by the nitrogenase protein

Question 11

why so much ATP?

Answer 11

the bond NRG of N2 is 930 kJ/mol

this is not easy to split!

Question 12

Dimine has two fates

Answer 12

can become hydrazine or go straight to NH3

Question 13

Nitrogen assimilation in plants: HIGH NH4+ levels in soil

Answer 13

plants use glutamate dehydrogenase rxn to directly incoperate NH4+ into glutamate using ketogluterate

Question 14

standard free energy charge is +30… so how do we get it to work??? (for nitrogen assimilation via glutamate dehydrogenase)

Answer 14

need high concs of NH4 in soil!!!!
(high substrate concs)

otherwise, we release reactants from glutamates (if levels not hight)

Question 15

Nitrogen assimilation in plants: LOW NH4+ levels in soil

Answer 15

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

Question 16

Net reaction of nitrogen assimilation: LOW NH4+ in soil

Answer 16

1) glutamate+NH4+ +ATP=glutamine
2) glutamine+alphaketoglutamate+NADPH= 2 glutamate

use ATP AND NADPH

net rxn: alphaketogluterate+ NH4++ ATP + NADPH=glutamate

Question 17

whats the difference between low and high (besides needed 2 vs 1 enzymes)

Answer 17

low use ATP and NADPH

high uses just NADPH

Question 18

assimaltion of nitrogen into amino acids done by…

Answer 18

aminotransferase enzymes

Question 19

aspartate aminotransferase

Answer 19

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

Question 20

What pathway provides the OAA needed for the aspartate aminotransferase reaction reaction?

Answer 20

citrate cycle

Question 21

amonia cycle

Answer 21

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+

Question 22

Positive nitrogen balance

Answer 22

young children and pregnant women

they accumulate nitrogen to support protein synth

Question 23

negative nitrogen balance

Answer 23

seen in starvation, elevated rates of protein break down

Question 24

Digestion of dietary proteins

Answer 24

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

Question 25

What do proteases do

Answer 25

break down proteins into amino acids and peptides

Question 26

why do we first have pepsinogen: which is inactive?

Answer 26

so its active only in stomach (low pH)

so that it doesnt break down proteins in cells

Question 27

where are proteases made? How are they activated

Answer 27

pancreas

when they reach low pH of stomach, another protease cleaves them and activates them

Question 28

degradation of cellular proteins.

why?

Answer 28

to regulate enzyme function

if you degrade the enzyme… its gone and you can’t use it

Question 29

how to break down proteins in cells

Answer 29

in lysosomes
proteosome complexes

left with amino acids at the end

Question 30

breakdown of proteins in cells: lysosomes

Answer 30

phospholipid bilayer

low pH inside where enzymes that break things down are

Question 31

breakdown of proteins in cells: proteosome complexes

Answer 31

ID proteins that have been modified with protein ubiquiton (ubiquitinated proteins) (tag for degredation)
unfold proteins and break them into small peptides

Question 32

What do we do with amino acids?

Answer 32

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

Question 33

2 types of amino acids

Answer 33

glucogenic

ketogenic

Question 34

glutogenic amino acids

Answer 34

form citrate intermediates or pyruvate when broken down

Question 35

ketogenic amino acids

Answer 35

make acetyl Co-A or acetoacetyl CoA when broken down

Question 36

Urea Cycle

Answer 36

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

Question 37

what is accomplished by the urea cycle

Answer 37

removal of excess nitrogen from body
synthed in liver
exported to kidneys
leaves through baldder

Question 38

regulated enzymes of urea cycle?

Answer 38

carbamoyl phosphate synthase 1 (first step in pathway)

Question 39

where does reaction happen?

Answer 39

first step happens in mitochondrial matrix

exportation into cytosol

Question 40

urea cycle

Answer 40

review slide 22

Question 41

aspartate-arginiosuccinate shunt

Answer 41

link between urea and citrate cycles

provides substrates so they can continue

Question 42

Where does amino acid-derived nitrogen enter the urea cycle?

Answer 42

at the glutamate and NH4 (see slide 23)

Question 43

review slide 23

Answer 43

review slide 23

Question 44

Urea Cycle disorders: Arginosuccinase deficiency

Answer 44

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

Question 45

would suplementing with ornithine also work?

Answer 45

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