Lecture 12 - Nitrogen Metabolism

Learning Outcomes:

  • Quantify the inputs and outputs that represent daily nitrogen balance

  • Recognise the physiological periods in which amino acid processing is highest and lowest

  • Describe the basic principles of amino acid processing

  • Understand the significance of amino acid degradation enzymes having a high Km

  • Describe the central role of transaminases in the shuffling of amino groups during amino acid processing

  • Summarise the key features of the urea cycle

  • Conceptualise the relationship between the urea cycle and other central metabolic pathways

  • Understand why some amino acids are ketogenic and some are glucogenic

  • Appreciate the reasons why most amino acids need to be in the human diet

  • Predict the effects of purine synthesis inhibitors on cell health

  • Deduce the reasons for uric acid accumulation in cells experiencing chronic energy charge crisis

  • Most nitrogen in our body is wrapped up in out amino acids

    • Many in nucleic acids aswell

Amino Acids:

  • Twenty in proteins

    • More involved in metabolism

    • Lots of different side chains

    • Pathways of formation and disposal are varied and complex

    • Synthesis and degradation of proteins

      • Each under different control

      • hard to generalise

Daily Flux:

  • 70 kg of male has 6kg of protein

  • Most of nitrogen is being lost to urea

  • Turnover = breaking polypeptide chain into amino acids and using amino acids to make another protein

  • Turnover varied massively between tissues: muscles 2%, intestine 15%

  • But muscle is greatest bulk of protein

When do we need to process Amino Acids?

  • During normal protein turnover

    • Released amino acids reassembled into protein

    • But many escape resynthesised

      • Oxidised in tissue or bloodstream

  • During starvation

  • When diet is rick in protein

  • How to deal with amine groups (ammonia is toxic)?

  • What to do with carbon skeletons?

    • Might get oxidised to CO2 for ATP

    • Or go to gluconeogenisis

Principles:

  • Liver is very important

    • First place amino acids go from the intestine (through hepatic portal vein)

    • Processing of amine groups

      • Urea synthesis

    • Conversion of amino acid backbones

      • Gluconeogenisis, lipogenesis, oxidation (we can not stor prroteins, so any excess amino acids MUST be degraded)

    • Not inter-conversion of backbone

  • Degradation enzymes have very high Km – not the rate limiting step

    • Not controlled

    • Only affected by [amino acids]

    • So excess amino acids are degraded

After Feeding:

  • Portal vein (into liver)

    • Amino acid mixture reflects protein composition of food we consumed

    • 3/20 are branched chain amino acids (20%)

  • Hepatic vein (out of liver)

    • 70% branched chain amino acids

      • Liver has preferentially kept the non-branched chain amino acids

  • Arterial circulation

    • Same as what went out of liver (previous point)

  • Vein from muscles

    • Branched chain amino acids removed

    • Muscles uses a lot of branched chain amino acids for energy

During Starvation:

  • Hypoinsulinemia stimulates proteolysis

  • Muscle released all types of amino acids

    • But disproportionately high amounts of alanine and glutamine

    • Branched chain amino acids used as fuel (leucine, isoleucine, and valine)

    • Alanine pick up amine group from branch chain amino acids, and others, and drop it back at the liver into the urea cycle, a pyruvate is left to enter gluconeogenesis during starvation

      • Prevents build up of amine n the muscles

Transamination

  • Shuffling of amino groups

  • Moving amine group to:

    • Pyruvate – alanine

    • 2 α-ketoglutarate – glutamate

    • Oxaloacetate – asparate

  • To form amino acids

  • Amino acids can drop amino acids off at the urea cycle and allow amine group to be detoxified

  • Left over carbon skeletons can be used during starvation to fuel gluconeogenesis or to build fat in non-starvation periods

Urea Cycle:

  • Costs a lot of ATP to detoxify ammonia into urea

  • Similar to Krebs cycle

  • Not efficient

  • Original source of amine groups

  • Ammonia releasing reaction

  • Ammonia fixing reaction

  • Route of the carrier ornithine

Processing Skeletons:

  • Some feed into Krebs Cycle:

    • Anaplerotic – replenish

    • Can be diverted to gluconeogenesis

      • If liver transamination reaction in the liver

  • Some can only be made into acetyl CoA

    • Can only be used for FA synthesis or ketone bodies (ketogenic)

    • Acetyl CoA in the Krebs cycle can reduce our carriers and ultimately give ATP

Amino Acid Synthesis:

  • Very complex

  • Human can’t do most so we rely on bacteria for the amino acids we require

  • We can make:

    • Alanin

    • glutamate

    • Aspartate

    • Asparagine

    • Serine

  • Bacteria is required for the other which will steal from a central pathway in metabolism to produce amino acids

Protein Malnutrition:

  • If you do not have all 20 amino acids, you can not build a single protein

  • All the other amino acids then ‘in excess’

  • When a ribosome translates the mRNA, if theres no tRNA, it will stall, release and breakdown the half made protein

Other Nitrogenous ProductsL

  • Creatine

  • Non-peptide hormones (adrenalin)

  • Nucleotides

    • A single purine rings requires many amino acids, requires a lot of ATP

  • Cells do not want to synthesis too and break down too many as they require a lot of energy.

Inhibitor of Purine Synthesis:n

  • Cancer cell growing rapidly have a high demand for purine and pyrimidine synthesis

    • Needs billions of nucleotides to build one more cell – it will try and salvage most purines

    • Needs first enzyme PRPP to build purine

    • Drugs can inhibit PRPP and block de novo purine synthesis

    • Looks like a purine but the cell can’t use it

    • Doesn’t just kill cancer cells, but any cell trying to replicate – lots or terrible side effects

Purine Degradation

  • Cells dying release lots of purine and lots of energy

  • Puts the in an energy crisis – energy charge is very low

  • As a last resort, it can oxidise and breakdown the AMP

  • Force equilibrium to the right and have one final production of ATP

  • Lots of urate waste products

  • Lots of urate (waste products) (uric acid in blood stream) in chemo patients as cells are dying

  • Urate crystallises out in joints and causes gout

  • Inflammation occurs

  • Allopurinol (drug) replicated intermediates and lowers uric acid concentration

  • Seen in alcoholics as well