Protein Metabolism

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

Transamination

If R 2 –= -CH 3  keto acid = pyruvate amino acid = alanine –= -CH 2 -COO -  keto acid = oxaloacetate amino acid = aspartate = -CH 2 -CH 2 -COO -  keto acid =  - ketoglutarate amino acid = glutamate

Fate of –NH 2 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

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”

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

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

Transamination in Muscle & Liver