Amino Acid Metabolism CHY2026: General Biochemistry

Overview  Nitrogen enters the body by consuming proteins  It leaves the body as urea and uric acid  Free amino acids are present in the cells, blood and extracellular fluids  Amino acids are needed for the synthesis of proteins and other biomolecules  Excess amino acids cannot be stored  The excess is used as metabolic fuel or excreted

Amino Acid Metabolism  In order for amino acids to be used as fuel, the amino group must first be detached from the structure (transamination)  The carbon skeleton is also metabolised to provide fuel by a process called oxidative deamination

Transamination  The reaction occurs in the cytosol of the hepatocytes (liver cells)  The amino group is transferred from an amino acid to α -ketoglutarate forming an α -keto acid (derived from the amino acid) and glutamate

Transamination  Therefore α -ketoglutarate is the amino group acceptor and the amino acid is the amino group donor  Glutamate is the new amino acid formed in the process  All amino acids participate in transamination except lysine and threonine  Lysine and threonine lose their amino group by deamination. The amino group that was removed is metabolized in the liver forming ammonia

 Transamination basically collects the amino group of the amino acids and convert them to one form i.e. glutamate  The α -ketoglutarate can accept the amino group from other amino acids forming glutamate. Glutamate is transferred from the cytosol to the mitochondria where it undergoes deamination  The glutamate formed can be used as an amino group donor for the synthesis of nonessential amino acid  The enzyme used in the catalysis is found in the cytosol and mitochondria  The enzyme is called aminotransferase or transaminase…the cofactor pyridoxal phosphate is required for enzyme activity  The enzyme is specific for the amino acid, e.g. transamination of alanine – alanine transaminase or alanine aminotransferase

Transamination  Aspartate aminotransferase disobeys the rule of transamination – the enzyme does not catalyze the transfer of amino group to form glutamate  Instead it transfers the amino group from glutamate to oxaloacetate forming aspartate (source of nitrogen for the urea cycle)

Oxidative Deamination  This results in the release of the amino group from glutamate (amino acid formed during transamination)  Glutamate is the only amino acid capable of undergoing rapid deamination  The amino group is released as ammonia  The reaction is catalyzed by glutamate dehydrogenase and occurs in the liver and kidney

Oxidative Deamination  The glutamate dehydrogenase is found only in the mitochondrial matrix  The enzyme is able to use either NAD + or NADP + as coenzymes  The amino group removed – ammonia (NH 3 )  The ammonia is then used in the biosynthesis of amino acids, nucleotides and biological amines  Excess ammonia is toxic to animal tissues (hyperammonemia)  It can cause retardation, mental disorder, coma and even death

Oxidative Deamination  The ammonia must first be converted to a non toxic form before it is transported to the liver  Glutamine and Alanine helps with the transport of ammonia to the liver or kidneys  In the kidneys, ammonia is then excreted directly or converted to urea or uric acid and excreted

Metabolic Products  Amino acids can be classified as glucogenic or ketogenic based on their catabolic product  Those that generate precursors of glucose e.g. pyruvate and TCA cycle intermediates are glucogenic  Those that are able to form precursors for the synthesis of ketone bodies e.g. acetyl CoA and acetoacetyl CoA are ketogenic

Nitrogen Excretion  Amino acid can be excreted in three different forms depending on the life form  In ureoletic animals - 95 % nitrogen is excreted in the urine and 5 % in the faeces Animal typeNitrogen form Ammonotelic e.g. fish, amphibianammonia Ureotelic e.g. humans, sharkurea Uricotelic e.g. reptiles, birdsuric acid

Sources of Ammonia 1. Amino acids 2. Glutamine – reactions occur in the kidneys and intestines 3. Bacterial action in the intestines 4. Amines (obtained from the diet) and monoamines (hormones and neurotransmitters) 5. From the catabolism of purines and pyrimidines

Regulation of Ammonia  The concentration of ammonia can be kept at low concentrations by two main processes 1. Urea formation 2. Glutamine synthesis

Urea Cycle  It was the first cyclic metabolic pathway to be discovered  It begins in the mitochondria of the hepatocytes  It involves the conversion of ammonia into urea (mitochondrial matrix)

Urea Cycle  one of the nitrogen is obtained from free ammonia  the other nitrogen is obtained from the amino acid, aspartate  The carbon comes from carbon dioxide

Urea Cycle  Ornithine is a carrier of the carbon and nitrogen atoms  This molecule is an amino acid, however it is not a precursor of protein The overall reaction for the urea cycle – CO 2 + NH 3 + 3ATP + aspartate urea + fumarate + 2 ADP + AMP + 2 P i + PP i + 3 H 2 O

Defects in Amino Acid Metabolism  Disorders which occur due to a defect in amino acid metabolism includes – (a) Phenylketonuria (PKU) – most common (b) Maple Syrup Urine Disease (MSUD) (c) Albinism (d) Homocystinuria (e) Alkaptonuria

Inborn Errors of Amino Acid Metabolism

Phenylketonuria Phenylketonuria (PKU)  The most common in born error of amino acid metabolism  It is recessive inheritance (obtained from both parents)  This results in an inability of the body to utilize the amino acid phenylalanine phenylalanine hydroxylase  Phenylalanine Tyrosine →→ melanin ↓ tissue proteins

Phenylketonuria Phenylketonuria (PKU)  In the blood [phenylalanine] normal – 1 mg/100 mL PKU – 6 – 80 mg/100 mL  The disorder is more common in Caucasians than in Blacks

Phenylketonuria Phenylketonuria (PKU) – biochemical features  Caused by a deficiency in phenylalanine hydroxylase  This results in an accumulation of phenylalanine, phenylpyruvate, phenyllactate and phenylacetate  The accumulation of these substances gives the urine a musty odour

Phenylketonuria Phenylketonuria (PKU) - Clinical features (a) mental retardation – affects children 1 – 4 years. Adults usually have low IQ (b) hypopigmentation – (light hair colour and skin pigmentation (c) epilepsy (d) hyperactivity

Phenylketonuria Phenylketonuria (PKU) - treatment  Regulating the amount of phenylalanine taken in the diet  Phenylalanine is an essential amino acids  It is therefore needed in the diet as the body is unable to synthesize this amino acid

Maple Syrup Urine Disease (MSUD)  It is a recessive disorder  This results in the accumulation of valine, leucine and isoleucine and their corresponding α -ketoacids  These amino acids are needed for normal growth and development

MSUD – Biochemical features  The defective enzyme is α -keto acid dehydrogenase  This results in an accumulation of α -keto acids derived from valine, leucine and isoleucine

MSUD – Clinical features  Sweet smelling urine/ urine has the smell of maple syrup  mental retardation  Infants usually have seizures, difficult to feed, constantly vomiting and dehydrated

MSUD – Treatment  Restricting the amount of valine, leucine and isoleucine incorporated in the diet