Transcript lect4
BIOC/DENT/PHCY 230 LECTURE 4 Nitrogen Metabolism Many nitrogen containing compounds eg. Amino acids, nucleotides, porphyrins, neurotransmitters There is no dedicated store for nitrogen or nitrogen compounds in humans Nitrogen Balance An individual’s nitrogen balance is dependent on a combination of: 1) Dietary nitrogen intake 2) Physiological state Nitrogen balance status can be: 1) In balance 2) Positive 3) Negative 1) In balance Nitrogen intake = nitrogen excretion Dietary amino acids, nucleotides etc. Urine, faeces, hair and skin loss, perspiration 2) Positive Nitrogen intake > nitrogen excretion Possible causes: Childhood and adolescent growth Pregnancy Body building 3) Negative Nitrogen intake < nitrogen excretion Possible causes: Illness Starvation Post-surgery Amino acids are the major source of dietary N Excess or insufficient dietary amino acid intake leads to the catabolism of amino acids Excess amino acids can be used for energy Insufficient dietary amino acids lead to the catabolism of proteins Insufficient dietary energy leads to the catabolism of proteins For amino acids to be utilised for energy, they must have their a-amino groups removed Deamination of amino acids Deamination generates: a carbon skeleton can be used for anabolic or catabolic reactions a free amino group generally excreted Some amino acids can be directly deaminated Serine, threonine and glutamate can be directly deaminated Glutamate deamination is catalysed by glutamate dehydrogenase (GDH) GDH glutamate + NAD(P)+ + H2O a-ketoglutarate + NH4+ + NAD(P)H Glutamine can be deaminated in a two step process glutaminase glutamine + H2O glutamate + NH3 Glutamate is then deaminated by GDH Glutamine can also be synthesised from glutamate Glutamine synthesis is an energy requiring reaction The reaction is catalysed by glutamine synthetase (GS) GS glutamate + NH4+ + ATP glutamine + ADP + Pi Transamination Those amino acids that can not be directly deaminated have their amino groups transferred to specific substrates These substrates are keto acids found in intermediary metabolism a - ketoglutarate oxaloaceatate pyruvate CAC Addition of amino groups to these keto acids generates amino acids a - ketoglutarate glutamate oxaloacetate aspartate pyruvate alanine Most amino acids are deaminated by donating their a-amino acids to one of these keto acids Thus the deamination of most amino acids leads to the production of either glu, asp, ala or gln. An example transamination glutamate a-keto acid a-KG glutamate aminotransferase a-amino acid Pyridoxal phosphate Derived from vitamin B6 Takes part in all amino transferase reactions Forms a Schiff base intermediate with substrates Role of transamination in metabolism Transamination allows for: 1) the generation of amino acids in short supply 2) the provision of carbon skeletons for energy generation 3) the safe removal of excess amino groups Free ammonia is a by-product of brain metabolism The neurotransmitter GABA is inactivated by deamination glutamate + NH4+ + ATP GS glutamine + ADP + Pi However when ammonia concentrations are high: GDH a-ketoglutarate + NH4+ + NADPH glutamate + NADP+ + H2O Brain requires large amounts of ATP This must be generated via oxidative phosphorylation Therefore the CAC must function efficiently Free ammonia is also produced in muscle Amino groups can be liberated: during normal muscle turnover during starvation during severe muscle activity ATP ADP + Pi 2ADP ATP + AMP AMP IMP + NH4+ AMP deaminase Pyruvate is usually abundant in active muscle Muscle uses pyruvate as an acceptor keto acid alanine aminotransferase glutamate + pyruvate a-ketoglutarate + alanine Thus in muscle most amino groups are shuttled to alanine (via glutamate) Alanine is then exported to the liver where the amino groups can be liberated AMP The take home message Nitrogen balance status depends on the intake and use of N containing compounds Excess N from amino acids must be excreted A series of aminotransferase and deamination reactions shuttle nitrogen to appropriate molecules and tissues Brain and muscle can generate large amounts of excess nitrogen as part of their metabolism The liver is an important tissue for processing excess nitrogen