Download NME2.35: amino acid and protein metabolism 13/03/08

NME2.35: AMINO ACID AND PROTEIN METABOLISM
13/03/08
LEARNING OUTCOMES
Explain the basis of the essentiality of some amino acids, the inter-convertibility of amino acids and the relationship
to intermediary metabolism
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There are around 300 known amino acids present in animal, plant and microbial systems
o All endogenous human amino acids are of the levo (L) stereoisomer form
Only 20 amino acids are coded for by DNA for protein synthesis
o 9 of these are essential amino acids and rely principally on dietary intake
o Example of essential amino acids include lysine, methionine and phenylalanine
o Age, diet and diseases such as PKU can affect the ‘essentiality’ of amino acids
Amino acids have a variety of crucial functions:
o Protein synthesis – following translation of mRNA
o Energy production – usually a limited contribution
o Biosynthesis – hormones, transport proteins, immunoglobulins, bile acids, neurotransmitters
Amino acids can be protonated at their amino terminal and deprotonated at the carboxyl terminal
o Plasma concentrations of 2mM allow amino acids to play a (minor) role in acid-base buffering
Describe the disposal of nitrogen in the form of urea and of the derivation of other nitrogenous excretory products
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Nitrogen is not specifically stored in any significant manner within the body
The body maintains nitrogen balance by matching daily excretion of nitrogen to daily intake
o Balanced daily consumption of 300g carbohydrate, 100g fat and protein
o Average daily excretion of nitrogen is 16.5g
Proteins are continuously degraded and synthesised within the body
o Every day between 1-2% of total body protein (300-600g in adult) is recycled
o Freed amino acids can then be used to synthesise other proteins as required
There are 2 main degradation pathways for proteins:
o Ubiquitin-proteasome – enzymes in the cytosol activate ubiquitin which catalyses
degradation via a proteasome site; principally degrades abnormal proteins
o Lysosomal – enzymes in the lysosomes degrade long-lived and plasma proteins
Degradation signals include:
o PEST – sequence of Pro-Glu-Ser-Thr triggers rapid degradation
o N-terminal – stabilising (Gly, Ala) or destabilising (Phe, Try) determine long-/short-lived
AMINO ACID DEGRADATION
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Excess amino acids are degraded through cleavage of the amino terminal
o This occurs in all tissues
o The amino-group (NH3) is incorporated into urea for excretion
o The carbon skeleton is metabolised to pyruvate or acetyl-CoA
Transamination is one way in which the amino terminal of an amino acid can be removed:
o Amino group transferred to an α-ketoacid such as pyruvate, oxaloacetate
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o Requires an aminotransferase (or transaminase) enzyme and vitamin B6
o Reversible process occurring in both the cytosol and the mitochondria
o All amino acids except lysine and threonine can undergo transamination
Oxidative deamination is another process for removing the amino terminal:
o Glutamate is converted to α-ketoglutarate and ammonia by glutamate dehydrogenase
o Reversible process occurring in the mitochondria, primarily in the liver and kidneys
Glutamate plays a key role in amino acid metabolism and nitrogen excretion:
o Involved in amino acid synthesis and degradation
o Acts as a source of ammonia through oxidative deamination
o Serves as an amino group carrier
o Buffers ammonia utilisation (glutamine synthetase in liver / brain reduces [NH3])
 Glutamate + NH3 ↔ Glutamine + H2O
AMMONIA METABOLISM AND THE UREA CYCLE
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Ammonia comes from a number of sources in the body:
o Amino acids – most important source
o Enteric bacterial action
o Muscle (purine nucleotide cycle)
o Oxidative deamination of glutamate
o Amines – from diet, hormones etc.
Ammonia is neurotoxic and its plasma concentration is kept within narrow limits (20-50μM)
o The liver is primarily responsible for ammonia metabolism via the urea cycle
o Alanine and to a lesser extent glutamine serve as nitrogen carriers which move from the
periphery to the liver to reduce circulating free ammonia
Short-term intermediate buffering mechanisms include:
o Formation of glutamate (reverse of oxidative deamination)
o Formation of glutamine (from glutamate) – particularly important in the brain
Metabolism of ammonia is ultimately achieved through the formation of urea
o Urea has the chemical structure H2N-C(=O)-NH2
o It is responsible for 90% of nitrogen excretion – 45g urea per day
o It is water-soluble and an efficient nitrogen carrier (50% weight is nitrogen)
o Its formation utilises both cytosolic and mitochondrial space
The urea cycle consumes 3ATP and uses:
o Ammonia – from oxidative deamination of glutamate – provides -NH2
o Aspartate – a non-essential amino acid; converted to fumarate – provides -NH2
o CO2 – provides -C=0
Short-term control of the urea cycle involves:
o Following a high-protein meal excess amino acids are deaminated
o Transamination results in an increase in [glutamate]
o Glutamate + Acetyl-CoA  N-acetyl-glutamate (NAG)
o NAG is an allosteric activator of CPS1 – first enzyme in the urea cycle
Long-term control is achieved through moderating the transcription of urea cycle enzymes
DISORDERS AND TOXICITY
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Genetic deficiencies of urea cycle enzymes have an overall incidence of roughly 1 in 10,000
Generally they present with hyperammonaemia (>100μM) early in life
o Ammonia is one of the most toxic substances in the body
o Excess ammonia can cause tremors, slurred speech, brain damage, coma and death