Protein is broken down through proteolysis during starvation, releasing amino acids. The liver uses some amino acids for gluconeogenesis to produce glucose, while others are converted to ketone bodies. Amino acids are transaminated by removing an amine group via transaminases, attaching it to a 2-oxo acid and forming a new amino acid. The amine groups are channeled into urea in the liver to be excreted, avoiding toxicity from ammonia buildup. In the fed state, dietary protein provides amino acids which are used to synthesize new proteins or have their carbon skeletons and amine groups metabolized through similar pathways.

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 glutamate’s 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! It’s 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