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Introduction to Nitrogen
Metabolism
Only an introduction!
Amino Acids
• Twenty in proteins
– More involved in metabolism
– Loads of different side chains
– So pathways of formation and disposal varied
and complex
• Synthesis and degradation of proteins
– Each under different control
– Hard to generalise
Daily Flux
10 kg pool
Intake 100 g
16 g nitrogen
Loss 100 g
300 g turnover
3% per day
skin 0.5 g nitrogen
urea 16 g nitrogen
faeces 2 g nitrogen
Turnover varies massively between tissues: muscle 2%, intestine 15%
But muscle is greatest bulk of protein.
Principles
• Liver important
– First place amino acids go from intestine
– Conversion of amino acids
• Especially to glucose
– Processing of amine groups
• Urea synthesis
• Degradation enzymes have very high Km
– Not ‘controlled’
– Only affected by [amino acid]
– So excess amino acids degraded
Bits to Process
• Amino group
– Fixed nitrogen is quite precious
• Recycle if possible
– But ammonia is toxic
• So need to convert to non-toxic product
• Carbon skeleton
– Normally a 2-oxo acid
• Oxidized or converted to carbohydrate or fat
Transamination
• Shuffling of amino groups
• Involves alpha-keto (2-oxo) acids
– Main acceptors
• Pyruvate  alanine
• 2-oxo glutarate  glutamate
• Oxaloacetate  aspartate
• These are involved in other pathways
– Glycolysis, Krebs Cycle
Other Key Reactions
• Glutamate dehydrogenase
– Oxidative deamination of glutamate
• Regenerates 2-oxoglutarate
• Releases ammonia
• Glutamine synthesis
– Using glutamate and ammonia
• Glutamine effectively carries two amino groups
– Reaction reversible
Essential Amino Acids
• Can’t be synthesized by us
– Sometimes conditional on age/situation
• Don’t learn them all
– But do know which are most definitely NOT
essential 
Getting a Good Mixture
• Protein quality
– Meat generally very high
• “Standard” is egg
– Pulses low in methionine
– Lysine relatively low in grains
• Protein complementing
• Protein malnutrition
– If you are lacking in one amino acid, you may not be
able to make an entire protein
– All the other amino acids then ‘in excess’
Amino Acid Fluxes
• After feeding
• Portal vein (into liver)
– Mixture reflects protein composition
– 20% branched chain amino acids
• Leucine, isoleucine, valine
• Hepatic vein (out of liver)
– 70% branched chain amino acids
• Veins from muscle
– Branched chain amino acids removed
During Starvation
• Hypoinsulinemia stimulates proteolysis
• Muscle releases all amino acids
– But large amount of alanine & glutamine
• These are preferentially taken up by liver
– Small amount of branched chain amino acids
• Where has the alanine come from
– From pyruvate transamination
• Which needs glycolysis from glucose
• So that’s a bit strange!
Amino acids
blood
Protein synthesis
Can the pyruvate be made
from other amino acid
skeletons?
Cell protein
proteolysis
glycolysis
glucose
pyruvate
Amino acids
2-oxoglutarate
alanine
2-oxoacids
glutamate
oxidation
CO2
Export to
liver
glutamine
Export to
liver
Processing Amino Acids
• During normal turnover
– Many amino acids escape re-synthesis
• Oxidized
• During starvation
– Or other times when there is high proteolysis
• When diet is rich in protein
– Surplus amino acids
• Issue is always…
– How to deal with amine groups
– What to do with carbon skeletons
Dealing with amine groups
• Urea Cycle
– Liver only
• Glutamate is the main substrate
– In mitochondria, oxidative deamination
• Giving 2-oxoglutarate and ammonia
– Ammonia quickly ‘fixed’ into carbamoyl phosphate
– Glutamate also transaminated to aspartate
• Carbamoyl Phopshate and aspartate
– combine to give urea
General Principle
• Carrier is ornithine
• Reacts with carbamoyl phosphate
– Giving citrulline
• Now add –NH2 from aspartate
– Releasing fumarate, a Krebs intermediate
• Split off urea
– Regenerating the ornithine
Urea
• Non-toxic
– Can be present in blood at mM levels
– Cleared by kidneys
• Fish can secrete ammonia
– Very dilute!
– Ammonia very toxic to us
Processing Skeletons
• Many different pathways
– Each amino acid is different
• Some feed into Krebs Cycle
– Anaplerotic
– Can be diverted to gluconeogenesis
• If transamination reactions in the liver
– Or at least gluconeogenic substrates
• For export from muscle
• Some can only be made into acetyl CoA
– Ketogenic or energy production only
Defects in Processing
• Both in urea cycle and skeleton breakdown
– See textbook for full table (18-2)
• Notably phenylalanine
– First step, conversion to tyrosine, sometimes
defective
– Build up of phenylalanine and phenylpyruvate
– Developmental problems
– Screening and dietary therapy
• Avoid aspartame
• Difficult to avoid protein
Amino Acid Sythesis
• Very complex!
• But all pathways linked to
– Glycolysis
– Krebs
– Pentose Phosphate Pathway
• Also other nitrogenous products from amino
acids
– Creatine
– Hormones (adrenalin)
– nucleotides
Only Important Things
• Overview of protein fluxes
• Knowing when amino acids are
metabolised
• What goes in and out of muscle
• Transaminases
• Overview of Urea Cycle