Amino Acid Metabolism Lecture Notes PDF

Amino acid metabolism 1. Essential and non-essential amino acids 2. Amino acids as energy sources: Glucogenic and ketogenic amino acids 3. Amino acid degradation (catabolism) 4. Amino acid metabolism and disease Chapter 19-21, Lippincott Biochemistry Dr. Michae...

Amino acid metabolism 1. Essential and non-essential amino acids 2. Amino acids as energy sources: Glucogenic and ketogenic amino acids 3. Amino acid degradation (catabolism) 4. Amino acid metabolism and disease Chapter 19-21, Lippincott Biochemistry Dr. Michael P. Carty School of Biological and Chemical Sciences University of Galway Amino acid structure Carbon skeleton R-group R-group or ‘side chain’ or ‘side chain’ Essential amino acids must be provided in diet Non-essential amino acids can be made in body  Beans are low in methionine  Many cereal crops are very low in lysine ‘Rice and beans’  Examples of importance of amino acid metabolism in medicine. 1. Genetic defects in amino acid metabolism: serious diseases e.g. phenylketonuria (PKU) PKU esp. common in Ireland. 2. Methionine breakdown generates homocysteine: risk factor for heart disease. 3. Amino acid breakdown generates ammonia, and urea: toxicity 4. Nutritional aspects: need for sufficient amino acids in health and disease Amino acid degradation (catabolism)  What are amino acids required for in the body? 1. Majority of amino acids 3. used in protein synthesis Proportion used as precursors of other nitrogen compounds Gluconeogenesis 2. A proportion is (i) completely metabolised for energy or (ii) used to form glucose Energy or ketone bodies  A. Amino acids as source of energy AAs degraded under three different metabolic conditions: (i) Following ingestion of a protein-rich meal, (ii) During degradation (turnover) of cellular proteins, released amino acids are degraded (iii) During starvation, or in diabetes mellitus, cellular proteins [not carbohydrates] are used as fuel Amino acids can not be stored in the body Normally, ~ 15% of energy needs comes from amino acids  Amino acid degradation “Nitrogen-free” fuel Goes to TCA cycle Removed by Transamination Next, oxidative deamination generates Ammonia, which is converted to urea via the Urea Cycle, and excreted.  Transaminase (aminotransferase) enzymes Transfer the amino group from an amino acid to -ketoglutarate Reversible reaction Uses pyridoxal phosphate (Vit B6 derivative) as cofactor: acts as transient carrier of amino group Transamination ‘Carbon generates skeleton’ glutamate is left Transaminases in the clinic Transaminase activity in liver and heart Measurement of transaminase (aminotransferase) activity in serum is used in diagnosis of liver damage (and heart attack) AST (aspartate transaminase) or SGOT (serum glutamate- oxaloacetate transaminase) ALT (alanine transaminase) or SGPT (serum glutamate- pyruvate transaminase) Activity in the serum is evidence of tissue damage - enzymes released Marker for liver toxin exposure Pyridoxal phosphate is an essential cofactor for transaminases Pyridoxal phosphate (Vit B6 derivative) is a cofactor: acts as transient carrier of the amino group during the reaction Reversible reaction How are carbon skeletons of amino acids metabolised further? The ‘carbon skeleton’ of alanine is pyruvate  The carbon skeletons of amino acids form metabolites that feed into the TCA cycle acetoacetate pyruvate Glucose A. OXALOACETATE B. -KETOGLUTARATE C. PYRUVATE D. FUMARATE E. SUCCINYL CoA F. ACETYL-CoA TCA G. ACETOACETATE cycle * *  Classification of amino acids based on ability to give rise to glucose  Glucogenic and Ketogenic amino acids There are two types of amino acids, defined in terms of the metabolic intermediate they are converted to: A. OXALOACETATE B. -KETOGLUTARATE 1. Pyruvate or other TCA cycle intermediates C. PYRUVATE D. FUMARATE E. SUCCINYL CoA Glucogenic amino acids: a. metabolism can lead to net formation of glucose or b. generate energy through the TCA cycle  Glucogenic and Ketogenic amino acids There are two types of amino acids, defined in terms of the metabolic intermediate they are converted to: 2. Acetyl-CoA or acetoacetate F. ACETYL-CoA G. ACETOACETATE Ketogenic amino acids: Lead to the formation of ketone bodies (e.g. acetoacetate), but can not lead to glucose (as there is no net synthesis of glucose from acetyl-CoA). Ketone bodies can be metabolised to generate energy.  Example 1. Glucogenic amino acid: Alanine is converted to pyruvate  Example 2. Glucogenic and ketogenic amino acid: Phenylalanine breakdown generates a glucogenic compound and a ketogenic compound Phenylalanine Fumarate Acetoacetate [glucogenic compound] [ketogenic compound]  Examples of importance of amino acid metabolism in medicine. 1. Genetic defects in amino acid metabolism: serious diseases e.g. phenylketonuria (PKU) PKU esp. common in Ireland. 2. Methionine breakdown generates homocysteine: risk factor for heart disease. 3. Amino acid breakdown generates ammonia, and urea: toxicity 4. Nutritional aspects: need for sufficient amino acids in health and disease Inborn errors of amino acid metabolism 10 per 100,000 =100 per million =500 per 5 million  Inborn errors of amino acid metabolism Genetic defects in amino acid metabolism lead to clinical conditions or to disease. 2 examples in the pathway of phenylalanine and tyrosine metabolism: 1) Phenylketonuria: due to a genetic defect in phenylalanine hydroxylase, the first and rate-limiting enzyme in the breakdown of phenylalanine. 2) Alkaptonuria: due to a defect in homogentisate oxidase No severe clinical symptoms. Dark urine; arthritis. Pheylketonuria (PKU): An Inborn error of metabolism Associated with mental retardation; shortened life expectancy Described in 1934 by Folling. Urine turned green in colour in presence of ferric chloride (reaction with phenylpyruvate) Mutation in gene encoding phenylalanine hydroxylase (PAH) About 1 in 15,000 newborns (relatively common in Ireland) All newborns tested for high concentration of phenylalanine in blood Treat by low phenylalanine diet (no meat, fish, bread) Phenylalanine hydroxylase converts phenylalanine to tyrosine Enzyme defective in Phenylketonuria (PKU)  Inborn errors of amino acid metabolism a. Phenylketonuria b. Alcaptonuria a. b. acetoacetate fumarate  IQ scores decrease with age if PKU is left untreated http://pku.ie/ https://npkua.org/  Metabolism of phenylalanine is altered in PKU patients Normally minor products Palynziq: new drug: phenylalanine ammonia lyase, an enzyme not found in humans. Breaks down Phe Tyrosine becomes an essential amino acid i.e. needed in diet  PKU can be treated using a low phenylalanine diet. IQ scores decrease if low Phe diet is discontinued: Maternal PKU: foetus affected in untreated women.  Inborn errors of amino acid metabolism Alkaptonuria Due to a defect in homogentisate oxidase, an enzyme in the pathway between tyrosine and fumarate No severe clinical symptoms. Dark urine; arthritis  Alkaptonuria: urine turns dark colour, due to oxidation of homogentisic acid. Homogentisic acid (homogentisate) builds up in individuals with alkaptonuria Alkaptonuria: deposition of dark pigment on vertebrae Arthritis develops Inborn errors of amino acid metabolism Genetic defect in methionine metabolism: Homocystinuria Methionine Cysteine Build-up of homocysteine: premature arterial disease; mental retardation; ectopic lentis (displacement of lens of eye); skeletal abnormalities 32  Elevated homocysteine levels are a risk factor for heart disease Elevated levels due to: (a) to homocystinuria, a genetic defect in the enzyme Cystathionine -synthase or (b) deficiency in vitamin B6 (pyridoxine), folate, or vitamin B12, as these are required as cofactors by the enzymes that convert homocysteine to either cysteine or methionine. 33  Elevated plasma homocysteine level: a risk factor for cardiovascular disease Elevated homocysteine levels in about 7% of population Oxidative damage, inflammation, endothelial cell dysfunction Vitamins B6, B12 and folate reduce homocysteine levels Elevated homocysteine in pregnancy: neural tube defects (folate) 34  Homocysteine is derived from methionine S-adenosylmethionine (SAM) Methylated Adenosine products Homocysteine Methionine 35 Homocysteine (HC) is normally either: A. Converted to cysteine (enzyme requires Vitamin B6) or B. Converted back to methionine, if methionine levels are low (enzyme requires Vitamin B12, and tertahydrofolate). If there is a problem with either pathway, HC builds up. 36  Summary Amino acids classified as essential or non-essential. Amino acids can be degraded for energy, or converted to glucose (glucogenic) or to ketone bodies (ketogenic). Carbon skeletons of amino acids are converted to intermediates that feed into TCA cycle. Genetic defects in amino acid metabolism cause important human diseases.