Protein Metabolism

1
Protein Metabolism
2
Starvation

• Amino acids released by proteolysis
• Channeled to the liver for gluconeogenesis
• Although not all amino acids can be made into
  glucose
• Some must be made into ketone bodies
• Removal of amine group
• Achieved by transaminases
• Put amino group onto a 2-oxo acid
• Creating a new amino acid
• But limited to a select few
• Amino acid becomes a 2-oxo acid
• A carbon-skeleton
• Destined for gluoconeogensis or ketogenesis

3
Transamination
4
Transamination

• If R2
• -CH3
• a keto acid pyruvate
• amino acid alanine
• -CH2-COO-
• a keto acid oxaloacetate
• amino acid aspartate
• -CH2-CH2-COO-
• a keto acid a-ketoglutarate
• amino acid glutamate

5
Fate of NH2

• Amine groups are channeled into urea
• Synthesised from aspartate and glutamates amine
  groups in the urea cycle
• Urea is non-toxic
• The alternative would be conversion to ammonia,
  which is toxic
• Urea cycle only occurs in the liver

6
Protein Metabolism - Fed State

• Protein intake 1g/kg/day
• Mixture of amino acids
• Essential cannot be made by us
• Non-essential can be made from amination of
  carbon skeletons

7
Protein Quality

• Some food (especially vegetables) are deficient
  in some essential amino acids
• Rice thr lys are low
• Maize lysine is low
• Protein quality is
• Low if some essential amino acids are missing
• High if full mixture of essential amino acids are
  present
• If one amino acid is missing, then proteins
  contain that amino acid cannot be made
• cannot make ½ a protein! Its all or nothing.
• Compromises pool of the other amino acids

8
No Protein Store

• Amino acids from diet are used to make new
  proteins
• very expensive to make a new protein, ? new
  proteins are made only when they are needed
• Excess amino acids have to be degraded
• There is no amino acid storage in our body
• Transamination is the key smashing up reaction
• Slightly different emphasis in fed state than in
  starvation
• C-skeletons burnt in the tissues of origin
  rather than being sent, as amino acids, to liver
• Amine groups put onto pyruvate (forming alanine)
  for transport from peripheral tissues to liver
• Amine groups still made into urea by liver

9
Transamination in Muscle Liver