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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

Intake 100 g

16 g nitrogen

Loss 100 g

skin 0.5 g nitrogenurea 16 g nitrogenfaeces 2 g nitrogen

10 kg pool

300 g turnover3% per day

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

Cell protein

Amino acids

2-oxoacids

CO2

pyruvate

alanine

glucose 2-oxoglutarate

glutamate

glutamine

proteolysis

Protein synthesis

Export toliver Export to

liver

oxidation

blood

glycolysis

Can the pyruvate be made from other amino acid skeletons?

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