Transcript L21_Protein
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 Transamination • If R2 – = -CH3 a keto acid = pyruvate amino acid = alanine – = -CH2-COOa keto acid = oxaloacetate amino acid = aspartate = -CH2-CH2-COOa keto acid = aketoglutarate amino acid = glutamate 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 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