Nitrogen metabolism Flashcards
1
Q
why is nitrogen essential for life
A
- major constituent of RNA - needed for genetic continuity, development and survival
- major constituent of protein - needed for biological structure and function
2
Q
process of nitrgoen in body (basic)
A
- dietary nitrogen
- nitrogen metabolism
- nitrogen excretion
3
Q
nitrogen balance
A
dietary nitrogen intake = nitrogen excretion
in most adults
4
Q
major source of dietary nitrogen
A
protein
5
Q
reservoir pools
A
reservoir pool is needed for any balance input-output system to smooth the effects of high and low input stimulation
- unlike fat which have excess storage, protein stores are small and there is very little in the blood
6
Q
positive nitrogen balance
A
= muscle / weight gain
- anabolic states
- growth in children
- growth in pregnancy
- growth in muscle builders
7
Q
negative nitrogen balance
A
= muscle / weight loss
- catabolic states
- malnutrition
- dieting
- infection
- cell toxicty
8
Q
average adult daily protein intake
A
70g
9
Q
average adult daily nitrogen intake
A
10g
10
Q
how much nitrogen do we need to excrete per day
A
~10g because we ingest ~10g
11
Q
main nitrgoen excretion product =
A
urea
12
Q
how much urea is = to 10g nitrogen
A
22g urea/day needs to be synthesised and excretd to = 10g nitrgoen excretion
13
Q
what is an excellent clinical measure of protein intake
A
the urea production/ excretion rate
it shows nitrogen balance
14
Q
what does amino acid catabolism produce
A
urea, CO2 and H2O
15
Q
digestion and absorption of dietary protein
A
- dietary protein hydrolysed in gut to produce AA
- AA absorbed and released as necessary to maintain reservoir pool
16
Q
what are amino acids
A
- key building blocks of protein
- essential for synthesis of glucose, hormones and rane of intermediary metabolites
- major component of energy metaboism
17
Q
structure of AA
A
- carbon skeleton
- amino group
H
|
H2N -----C-----COOH
|
H
18
Q
Role of carbon skeleton of AA
A
Primary synthetic source of carbon. Catabolised to CO2 and H2O to produce energy
19
Q
why is amino group of AA potentially toxic
A
potentially toxic
catabolism prodices highy toxic ammonium ions NH4+
20
Q
toxic product of AA catabolism
A
NH4+ Ammonium ions
21
Q
detoxification of ammonium ions, NH4+
A
conversion of NH4+ to urea - uses energy
22
Q
why do we metabolise AA if potentially toxic?
A
value of the carbon skeletons in intermediary metabolite synthesis and energy production > energy cost of detoxification
AA are valuable - but not all equal
23
Q
NEAA
A
non essesntial amino acids
24
Q
why are some AA NEAA
A
the carbon skeleton of these AA can be synthesised as part of intermediary metabolism therefore they are not required in the diet
25
examples of NEAA
- alanine
- arginine*
- asparagine
- aspartate
- cysteine
- glutamate
- glutamine
- glycine
- proline
- serine
- tyrosine
26
what is special about arginine
it is a NEAA but when things go wrong it becomes EAA
27
key NEAA and what they are synthesised from
alanaine from pyruvate
aspartate from oxaloacetate
glutamate from glutamine
28
what are EAA
AA that the carbon skeleton cannot be synthsised by the body, as part of intermediary metabolism
- they are essential in the diet
- mainly aromatic because cant synthesise benzine ring
29
what does a deficit of EAA result in
negative nitrogen balance because body must conserve EAA for protein synthesis and intermediary metabolism
- renal excretion of EAA is low and they are actively reabsored then reused and recycled
30
examples of EAA
- histidine
- isoleucine
- leucine
- lysine
- methionine
- phenylalanine
- threonine
- tryptophan
- valine
31
what is special about histidine
it is only essential for growth, so sometimes classes as NEAA
32
process of making AA is called
transamination
| - utilise c-skeleton of AA for synthesis and energy production, need to split the AA and detoxify the NH4+
33
what is transamination
transamination reactions are central to AA/nitrogen metabolism
- transfer of amino group of one AA to an 2-oxo acid, with formation of corresponding AA an 2-oxoacid
- requires pyridoxal phosphate as cofactor
- equilibrium reaction
34
cofact needed for transamination
pyridoxal phosphate = vitamin B6
35
transmationation reaction
R group from amino acid going to oxoacid 1 to from oxoacid 2
R group from oxoacid 1 goes to amino acid 1 to form amino acid 2
swapping of R group
36
enzyme for transamination
transaminase AKA aminotransferase
| - specific for each AA, except threonine and lysine
37
transamination equilibrium
- ensures supply of appropriate NEAA and EAA requird for proteinsynthesis
- ensures the supply of appropriate c-skeletons for intermediary metabolism
e.g glutamate can replenish 2-oxoglutarate for the krebs cycle
38
glutamate and 2-oxoglutarate equilibrium
- 2oxoglutarate is very abundant
- therefore glutamate acts as main AA nitrogen pool via transamination
- glutamate canbe used to maintain concentrations of other AA
- when there are AA in excess, transaminatin with 2-oxoglutarate act as the 1st step of catabolism
- BC glutamate can be deaminate
39
what is the only AA we can deaminate
glutamate can be deamintaed to ornithine AKA urea
40
example of glutamate being deaminated
if leucine is in excess, pass the AA of leucine group to oxoglutarate to make a different oxo-acid and glutamate.
glutama can then be deaminated to urea and the other oxoacid can be catabolised to H2O and CO2
41
B6
pyridoxine
42
inactive form of pyridoxial phosphate
pyridoxine = vitamin B6
43
active form of pyridoxine
pyridoxial phosphate = cofactor for transfamintion
44
Schiff Base formation
- hydroxymethyl group of cofactor at postion 4 is oxidised to an aldehyde
- aldehyde is essential for reaction between pyridoxial phosphate and primary amies
the condensation reaction betweem the aldehyde on the cofactor and the amino group of the AA = Schiff Base formation
= aldimine H-C=N
45
how is the pyridoxial phosphate achored in the active site of aminotransferase?
the highly negative charge of the phosphate on pyridoxial phosphate interacts with the postive chagre of thr arginine within the AS
46
what happens when pyrodixial phosphate is achored in transferase active site
the pyridoxamine phosphate is formed as an intermediate via schiff base formation, but remains locked in AS by arginine
- AA and 2oxo acids with approrpiate AS specificity will equilibriate
47
role of arginine within the aminotransferase
immobilises the cofactor so transaminiation can occur
48
variations of schiff base formation
```
common mechanism in AA metabolism
varies depending on architecture of the enzyme AS
- transamintiation
- deamination
- decarboxylation
- racemisation
- side chain radification
but always an equilibrium reaction
```
49
oxidative deamination
- the first step in consigning the AA nitrogen to conversion to urea for excretion
- irreversible reaction
50
what is the first step in consigning the AA nitrogen to conversion to urea for excretion
oxidative deamination
51
example of oxidative deamination
glutamate + H20 + NADP ---> Oxoglutarate + NADPH + NH4+
enzyme: glutamate dehydrogenase
location: inner mitochondrial membrane of liver mitochondria
52
most common AA dehydrogenase
glutamate dehydrogenase, represents 75% of nitrogen flux
53
where is the glutamate dehydrogenase primarily located
inner mitochondrial memebrane of liver mitochondria
54
what does oxidative deamination result in
- liberation of NH4+ ions
| - net loss of nitrogen from AA reservoir pool
55
what must happen to ammonium ions released by glutamate dehydrogenase
they must be rapidly metabolised to urea cus they're toxic
56
carbamyl phosphate synthetase
enzyme in the liver mitochondria
- prepares ammonium ions to enter urea cycle:
catalyses the condensation of ammonium ions and bicarbonate ions to form carbonyl phosphate
- reqiuires 2 ATP
57
UREA CYCLE
1. condensation reaction to form carbamyl phosphate, using amino group from any AA that tansamiates to glutamate, which was then deaminated (1st input)
2. carbamyl phosphate combines with ornithine to form citrulline
3. aspartate doesnt have to be deaminated, and is the 2nd input of N to the urea cycle
4. arginosuccinates forms from citrulline and aspartate, which then converts to fumarate (released) and arginine
5. arginine is an NEAA, bc synthesised here
6. complettion of cycle releases urea from arginine, to produce ornithine again
58
1st input of nitrogen to urea cycle
condensation reaction to form carbamyl phosphate, using amino group from any AA that tansamiates to glutamate, which was then deaminated
59
2nd input of nitrogen to urea cycle
aspartate doesnt have to be deaminated, and is the 2nd input of N to the urea cycle
60
net reaction of urea cycle
CO2 + 3ATP + aspartate + 2H2O + NH4+
--------->
urea + 2ADP + 2Pi + AMP + PPi + fumarate
61
enzymes of urea cycle
- carbamyl phosphate synthase
- ornithine transcarbamylase
- argininosuccinate synthetase
- argininosuccinate lyase
- arginase
62
summary of urea cycle
2 amino acid nitrogens are detoxified at the expensie of 4 high energy bonds
63
how does liver failure effect urea cycle
reduced abilty to synthesis urea results in hyperammonaemia and consequent morbidty and mortality
64
what happens to urea from urea cycle
it is not further metabolised - it is excreted
- urea is filtered by the kidneys
- 40% is reabsored by passsive diffusion in renal tubules
- renal excretion is not efficient
65
neonatal symptoms from problems with nitrogen metabolism
- children with severe urea cycle disorders wil show symptoms after 24 hours of life
- irritable baby at first, followed by vomiting and increased lethargy
- then seiures, hypotonia, respiratory alkalosis and coma may occur
- sever sympotoms more common with OTC and CPS deficiency
- can also occur with citrullinemia or ASA lyase deficiency
66
ASA lyase
argininosuccinate lyase
67
OTC
ornithine transcarbamylase
68
CPS
carbamyl phosphate synthase
69
What causes the neonatal symptoms from problems with nitrogen metabolism
rising ammonia levels in the blood
| baby will die if untreated
70
methods of adapting to live without a urea cycle
- reduce nitrogen intake
- reduce protein intake
- increase nitrogen excretion
- activate alternative pathways
- minimise protein catabolism
- maximise protein anabolism
71
Why must problems of urea cycle be detected early
will effect CNS leading to mental retardation
72
role of the diet in reducing need for urea cycle
- restriction of NEAA
- supplementation of EAA
- supplementation of EKA
73
role of pharmacology in reducing the need for urea cycle
sodium benzoate
sodium phenylacetate
sodim phenylbutyrate
74
OTC deficiency: clinical and biochemical symptoms
clinical:
- failure to excrete nitrogen
- encephalopathy
- coma
- death
Biochemical:
- hyperammonaemia
- arginine becomes EAA
75
treatmetn of OTC deficiency
- reduce protein intake
- supplement EAA
- using transamination
- supplement EOA
76
how is argininge deficiency treated in OTC deficiency
dont need to directly supplement arginine
| - supplement citrulline which means bypass OTC part of urea cycle, and rest can occur as normal to produce arginine
77
why does supplementation of EOA help OTC deficiecny
it activates alternative nitrogen excretion pathway
78
what EOA can be supplemented to help OTC deficiency
- sodium benzoate
| - sodium phenylbutyrate
79
example of supplementing EOA to help OTC deficiency:: sodium benzoate
- sodium benzoate is activated by coenzyme A
- conjugates with glycine in the liver and kidney
= hippuric acid
- hippuric acid is efficinetly excreted by the kidney
= 1 amino nitrogen removed
80
example of supplementing EOA to help OTC deficiency:: sodium phenylbutyrate
- sodium phenylbutyrate activated by Coenzyme A
- metabolised to phenylacetate which conjuagtes with gluatine in liver and kidney
= phenylacetylglutamine
- efficiently excreted by the kidney
= 2 amino nitrogens removed
81
how are hippuric acid and phylyacetylglutamine excreted by the kidnyes
efficiently byt filtration and tubular secretion
82
which EOA removes 2 amino nitrogens
sodium phenylbutyrate by excretion of phenylacetlyglutamine
83
which EOA removes 1 amino nitrogen
sodium benzoate by excretion of hippuric acid
84
ASA synthase deficiency: clinical and biochemical symptoms
same as OTC deficiency
clinical:
- failure to excrete nitrogen
- encephalopathy
- coma
- death
Biochemical:
- hyperammonaemia
- arginine becomes EAA
85
treatment of ASA synthase deficiency
supplement with arginine
86
can you supplement ASA synthase deficiency with citrulline
no, becuase the problem in the urea cycle is downstream of citrulline and so it would have no therapuetic effect
87
ASA lyase deficiency: clinical and biochemical symptoms
same as OTC deficiency if acute
clinical:
- failure to excrete nitrogen
- encephalopathy
- coma
- death
Biochemical:
- hyperammonaemia
- arginine becomes EAA
chronic ASA lyase deficiency causes much less severe symptoms
- friable hair from arginine deficiency is more obvious than hyperammonaemia
88
why is chronic ASA lyase deficiency less severe than acute ASA lyase deficiency
ASA is actively secreted by the kidney which removes 2XN
| excretion of ASA is more efficient that urea secretion!
89
treatment of ASA lysase deficiency
- supplement with arginine
| - activate alternative pathways ONLY during rare acute decompensation episodes
90
how is it possible to live without properly functioning urea cycle
- reduce protein intake
- activate alternative exretion pathways
- supplement with argining or citrulline
91
metabolism of phenylalanine
EAA that is primarily metabolsied to tyrosine
92
Phenylalanine hydroxylase deficiency causes
phenylketonuria (PKU)
93
what is phenylketonuria
Phenylalanine hydroxylase deficiency
- autosomal recessive disorder
- incidence 1:10,000
- heterozygous 1:50
[phenylalanine] increases
[phenylalanine metabolite] increases
but cannot be metabolised and so tyrosine becomes an EAA
94
clinical presentation of Phenylalanine hydroxylase deficiency
severe mental retardatio
95
pathogenesis of Phenylalanine hydroxylase deficiency
increased phenylalanine inhibits tyrosine metabolism which reinforces the tyrosine deficiency
96
why is tyrosine deficieny a problem
tyrosine is used to produce L-DOPA which is esentail in first two yar of life for neurodevelopment
- untreated = irreversible severe mental retardation, IQ<50
97
Treatment of Phenylalanine hydroxylase deficiency
needs early diagnosis - newborn dried blood spot screening
- normalise [phenylalanie] by restriction it inthe diet and supplementing EAA and vitamins.
- diet for life that is started from bith and continued forever
- non compliance in teens shown to increase [phenylalanine] which reduces pigmentation of hair and eyes
- pigmentation returns when they follow their diet again
98
pruine nucleotides
adenine (A)
guanine (G)
- in DNA and RNA
99
pyrimidine nucleotides
cyctosine (C) in DNA and RNA
thyrosine (T) in DNA
uracil (U) in RNA
100
nitrokgen intake from nucleotides
- signicicantly less than from protein
- v poor dataon dietary intake of nucleotides
- very low in vegans
- represents ~27% of dietary nitrogen intake
- end product of metabolism for excretion is uric acid
- uric acid is relatively insoluble
101
sources of nucleotides
some from diet, poor data
| or synthesised engdogenously - very thermodynamially costly!!!
102
endogenous synthesis of purines
glutamine glycine and asparate via inosine
103
endogenous synthesis of pyrimidines
AA via carybamyl phosphate, synthesised in cytosol
104
nucleotide metabolism
very little is metabolsied, it is mostly dietary nitrogen
105
end product of purine metabolism
insouble uric acid, via hypoxanthine/xanthine
106
end product of pyrimidine metabolism
soluble beta-ureidopropionic acid
107
what happens to most neucleotides when metabolsied
they are salvaged, becuase they are energetically expensive to make so they ae recycled instead
108
enzyme for salvage of purines
HGPRT
109
enzyme for salvage of pyrimidines
UTPRT
110
primary clincal problem with purine metabolism
gout
111
what is gout
uric acid precipitation
112
causes of gout
increased dietary purines
increased purine synthesis
= reduced HGPRT activity
113
treatment of gout
reduce high nucleotide foods
| allopurinol; inhibts conversion of hypoxanthine to uric acid
114
What is no HGPRT activity a sign of
Lesch-Nyhan Syndrom
| caused by mutations in the HGPRT gene
115
what is the role of HGPRT1
enzyme responsible for purine recycling to ensure all cells have plentiful supply of building blocks for RNA and DNA
116
what does HGPR1 mutations causes
deficiency of abscence = purines broken down but not recycled causes abnormally high levels of uric acid
117
what else is deficiency of HGPR1 associated with
abnormally low dopamine in the brain
causes movement problems and behavioural problems
118
Acute intermittent porphyria (AIP) is caused by a deficiency in which enzyme
porphobilinogen deaminase
119
Which vitamin is the source of the prosthetic group of the aminotransferases (transaminases)
pyridoxine (vitamin B6)
120
what is NOT an important intermediate in the excretion of urea
uric acid
