Protein Metabolism PDF

04/11/2023 CHEMISTRY AND BIOCHEMISTRY OF PROTEINS II METABOLISM OF AMINO ACIDS Proteins are the most abundant organic compounds and constitute a major part of the body dry we...

04/11/2023 CHEMISTRY AND BIOCHEMISTRY OF PROTEINS II METABOLISM OF AMINO ACIDS Proteins are the most abundant organic compounds and constitute a major part of the body dry weight (10-12kg in adults). Perform a wide variety of structural and dynamic (enzymes, hormones, clotting factors, receptors) functions. Proteins are nitrogen containing macromolecules consisting of L-α - amino acids as the repeating units or chains connected by peptide bonds. A peptide bond is an amide bond formed between amino acids by the condensation of –NH2 and -COOH, releasing H2O. There are 20 different types of amino acids that constitute proteins, and the sequence of amino acids determines the structure and properties of the resulting protein. Of the 20 amino acids found in proteins, half can be synthesized by the body and half are supplied through diet. The proteins on degradation release individual amino acids. Each amino acid undergoes its own metabolism and performs specific functions. Amino acid metabolism refers to the sum of all chemical reactions in which amino acids are broken down and synthesized for vital processes in the body. Amino acids can be divided into two types: essential and non-essential amino acids. Essential amino acids are amino acids necessary for an organism's survival. Since the organism cannot synthesize these essential amino acids by themselves, they must obtain them from their diets. Generally, animals cannot synthesize nine amino acids (Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine). Non-essential amino acids are amino acids that can be synthesized by the body. The remaining 11 are nonessential or conditionally essential amino acids. Animals have seven conditionally essential amino acids (Arginine, Cysteine, Glutamine, Glycine, Proline, Tyrosine, Serine) that can be synthesized and are usually not required in the diet. However, they are essential components of the diet for specific populations that cannot synthesize them in adequate amounts. There are four “nonessential” amino acids (Alanine, Asparagine, Aspartate, Glutamate) that can be synthesized and, thus, are not required in diet. 1  04/11/2023 Amino acid degradation produces ammonium (NH4+) and a carbon skeleton. The NH4+ is removed either by synthesis of nitrogen-containing compounds such as nucleotides, or excreted in the form of urea. The carbon skeletons of amino acids can be converted into TCA cycle intermediates, which are used either to generate ATP through oxidative phosphorylation or provide the precursors for fatty acid synthesis and gluconeogenesis Some amino acids serve as precursors for the synthesis of many biologically important compounds. Certain amino acids may directly act as neurotransmitters (e.g glycine, aspartate, glutamate). Amino Acid Pool About 100g of free amino acids which represent the amino acid pool of the body. Glutamate and glutamine together constitute about 50%, and essential amino acids about 10% of the body pool (100g). The concentration of intracellular amino acids is always higher than the extracellular amino acids. Amino acids usually enter the cells against a concentration gradient. Sources of Amino Acid Turnover of body intake of dietary protein and the synthesis of non-essential amino acids contribute to the body amino acid pool. Protein turnover: The proteins in the body is in a dynamic state. About 300-400g of protein per day is constantly degraded and synthesized, which represents body protein turnover. 2  04/11/2023 METABOLIC PATHWAYS THAT PRODUCE NONESSENTIAL AMINO ACIDS Animals lack the enzymes to generate the essential amino acids, thus, these amino acids must be obtained from the diet. The structures of the essential amino acids, generally, are more complex than the nonessential amino acids. Essential amino acids require a significantly greater number of enzymatic reactions for synthesis and are found in plants and lower organisms. Tyrosine and arginine are conditionally essential. Tyrosine is derived from the essential amino acid phenylalanine by the enzyme phenylalanine hydroxylase. Endogenous tyrosine production is dependent on dietary phenylalanine, and a significant portion of tyrosine is obtained from diet. A small amount of arginine can be generated from argininosuccinate in the urea cycle. Animals can synthesize the conditionally essential and nonessential amino acids using glycolytic and TCA cycle intermediates. The glycolytic intermediate 3-phosphoglycerate generates serine and glycine. Control of protein turnover The turnover of the protein is influenced by many factors. A small polypeptide called ubiquitin (m.w.8,500) tags with the protein and facilitates degradation. Certain proteins with amino acid sequence proline, glutamine, serine and threonine are rapidly degraded Dietary protein: There is a regular loss of nitrogen from the body due to degradation of amino acids. About 30-50g of protein is lost every day. This amount of protein is supplied through diet to maintain nitrogen balance. There is no storage form of amino acids in the body. Excess intake of amino acids is oxidized to provide energy. 3  04/11/2023 Proteins function as enzymes, hormones, immunoproteins, contractile proteins etc. Many important nitrogenous compounds (porphyrins, purines, pyrimidines, etc) are produced from the amino acids. About 10-15% of body energy requirements are met from the amino acids. The amino acids are converted into carbohydrates and fats. Catabolism of Amino Acids Occur in 4 stages Transamination Oxidative Deamination Ammonia Transport Urea Cycle Transamination The transfer of an amino (-NH2) group from an amino acid to a ketoacid, with the formation of a new amino acid and a new keto acid. Catalysed by a group of enzymes called transaminases (aminotransferases) that uses pyridoxal phosphate (PLP) as a co-factor. The liver, Kidney, Heart, and Brain have adequate amount of these enzymes. Salient Features of Transamination All transaminases require PLP. No free NH3 liberated, only the transfer of amino group. Transamination is reversible. There are multiple transaminase enzymes which vary in substrate specificity. AST and ALT make a significant contribution for transamination. Transamination is important for redistribution of amino groups and production of non- essential amino acids. It diverts excess amino acids towards the energy generation. Amino acids undergo transamination to finally concentrate nitrogen in glutamate. 4  04/11/2023 Glutamate undergoes oxidative deamination to liberate free NH3 for urea synthesis. All amino acids except, lysine, threonine, proline and hydroxyproline participate in transamination. It involves both anabolism and catabolism, since – reversible. AA + α- KG ketoacid + Glutamate Alanine + α- KG Pyruvate + Glutamate Aspartate + α- KG Oxaloacetetae + Glutamate Mechanism of Transmission Step:1 Transfer of amino group from AA1 to the coenzyme PLP to form pyridoxamine phosphate. Amino acid1 is converted to Keto acid2. Step:2 Amino group of pyridoxamine phosphate is then transferred to a keto acid1 to produce a new AA 2 and enzyme with PLP regenerated. 5  04/11/2023 Transamination reaction 6  04/11/2023 Clinical Significance Enzymes, present within cell, released in cellular damage into blood. ↑ AST - Myocardial Infarction. ↑ AST, ALT – Hepatitis, alcoholic cirrhosis. Muscular Dystrophy. Trans-deamination The amino group of most of the amino acids is released by a coupled reaction, transdeamination. Transamination followed by oxidative deamination. Transamination takes place in the cytoplasm. The amino group is transported to liver as glutamic acid, which is finally oxidatively deaminated in the mitochondria of hepatocytes. 7  04/11/2023 Deamination The removal of amino group from the amino acids as NH3 is deamination. Deamination results in the liberation of ammonia for urea synthesis. The carbon skeleton of amino acids is converted to keto acids. Deamination may be either oxidative or non-oxidative Only liver mitochondria contain glutamate dehydrogenase (GDH) which deaminates glutamate to α-ketoglutarate and ammonia. It needs NAD+ as co- enzyme. GDH is an allosteric enzyme. It is activated by ADP and inhibited by GTP. Oxidative deamination is the liberation of free ammonia from the amino group of amino acids coupled with oxidation. Mostly in liver and kidney. Oxidative deamination is to provide NH3 for urea synthesis and α-keto acids for a variety of reactions, including energy generation. Role of Glutamate Dehydrogenase Glutamate is a 'collection centre' for amino groups. Glutamate rapidly undergoes oxidative deamination. Catalysed by GDH to liberate ammonia. It can utilize either NAD+ or NADP+. This conversion occurs through the formation of an α-iminoglutarate Oxidative of glutamate by GDH 8  04/11/2023 Metabolic Significance Reversible Reaction Both Anabolic and Catabolic. Regulation of GDH activity: Zinc containing mitochondrial, allosteric enzyme. Consists of 6 identical subunits. Molecular weight is 56,000. Allosteric regulation GTP and ATP – allosteric inhibitors. GDP and ADP - allosteric activators. ↓ Energy - ↑ oxidation of amino acid. Steroid and thyroid hormones inhibit GDH. Amino Acid Oxidases L-amino acid oxidase and D-amino acid oxidase. Flavoproteins and Cofactors are FMN and FAD. Act on corresponding amino acids to produce α-keto acids and NH3 It occurs in the Liver, kidney, Peroxisomes. Activity of L-amino acid oxidase is low and plays a minor role in amino acid catabolism. L-amino acid oxidase acts on all amino acids, except glycine and dicarboxylic acids. Activity of D-amino oxidase is high than that of L-amino acid oxidase D-amino oxidase degrades D-amino acids in bacterial cell wall. 9  04/11/2023 Amino acid oxidases Fate of D-amino acids D-amino acids are found in plants and microorganisms. They are not present in mammalian proteins. D-amino acids are taken in the diet/bacterial cell wall, absorbed from gut. D-amino acid oxidase converts them to the respective α-keto acids. The α-ketoacids undergo transamination to be converted to L-amino acids which participate in various metabolic pathways. Keto acids may be oxidized to generate energy or serve as precursors for glucose and fat synthesis. 10  04/11/2023 Non-oxidative Deamination This is a direct deamination without oxidation. Amino acid Dehydratases: Serine, threonine and homoserine are the hydroxy amino acids. They undergo non-oxidative deamination catalyzed by PLP-dependent dehydratases Amino acid Desulfhydrases Cysteine and homocysteine undergo deamination coupled with desulfhydration to give keto acids. 11  04/11/2023 UREA CYCLE The urea cycle is the first metabolic pathway to be elucidated. The cycle is known as Krebs–Henseleit urea cycle. Ornithine is the first member of the reaction, it is also called as Ornithine cycle. Urea is synthesized in liver and transported to kidneys for excretion in urine. The two nitrogen atoms of urea are derived from two different sources, one from ammonia and the other directly from the amino group of aspartic acid. Carbon atom is supplied by CO2. Urea is the end product of protein metabolism (amino acid metabolism). Urea accounts for 80-90% of the nitrogen containing substances excreted in urine. Urea synthesis is a five-step cyclic process, with five distinct enzymes. The first two enzymes are present in mitochondria while the rest are localized in cytosol. Glutamine and glutamate generate two nitrogen molecules for the urea cycle. Urea contains two nitrogen molecules. Glutaminase generates NH4 and glutamate. + Subsequently, glutamate can be converted into α-ketoglutarate to liberate another NH4+ by glutamate dehydrogenase. Carbamoyl phosphate synthetase (CPS1) uses NH4+ , HCO3-, and ATP as substrates to generate carbamoyl phosphate, which provides the first source of nitrogen molecules for urea generation. Amino acids, such as alanine, can also be converted into glutamate through aminotransferases. Next, the aspartate aminotransferase converts glutamate into aspartate, which feeds into the urea cycle to provide the second source of nitrogen molecules for urea production 12  04/11/2023 Step 1: Formation of Carbamoyl phosphate Carbamoyl phosphate synthase I (CPS I) of mitochondria catalyses the condensation of NH4+ ions with CO2 to form carbamoyl phosphate. This step consumes two ATP and is irreversible. It is a rate-limiting. CPS I requires N-acetylglutamate (NAG) for its activity. Meanwhile, Carbamoyl phosphate synthase II (CPS II) involved in pyrimidine synthesis and it is present in cytosol. It accepts amino group from glutamine and does not require N-acetylglutamate for its activity. 13  04/11/2023 Carbamoyl phosphate (CPS) CPS I CPS II Mitochondria Cytosol Uses NH3 Uses Glutamine Urea cycle Pyrimidine biosynthesis Activated - N-acetylglutamate Inhibited - CTP Step 2: Citrulline formation The second reaction is also mitochondrial. Citrulline is synthesized from carbamoyl phosphate and ornithine by ornithine transcarbamoylase. Ornithine is regenerated and used in urea cycle. Ornithine and citrulline are basic amino acids (but never found in protein structure due to lack of codons). Citrulline is transported to cytosol by a transporter system. Citrulline is neither present in tissue proteins nor in blood; but it is present in milk. 14  04/11/2023 Step 3: Argininosuccinate formation Citrulline that is exported to the cytosol condenses with aspartate to form argininosuccinate by the enzyme argininosuccinate synthetase. The second amino group of urea is incorporated at this stage. This requires ATP and it is cleaved to AMP and PPi 2 high energy bonds are required which are immediately broken down to inorganic phosphate (Pi). Step 4: Arginine formation The enzyme argininosuccinase or argininosuccinate lyase cleaves argininosuccinate to arginine and fumarate (an intermediate in TCA cycle) Fumarate provides connecting link with TCA cycle or gluconeogenesis. The fumarate is converted to oxaloacetate via fumarase and malate dehydrogenase (MDH) and transaminated to aspartate. Aspartate is regenerated in this reaction. 15  04/11/2023 Step 5: Urea formation Arginase is the 5th and final enzyme that cleaves arginine to yield urea and ornithine. Ornithine is regenerated, enters mitochondria for its reuse in the urea cycle. Arginase is activated by Co2+ and Mn2+ Ornithine and lysine compete with arginine (competitive inhibition). Arginase is mostly found in the liver, while the rest of the enzymes (four) of urea cycle are also present in other tissues. Arginine synthesis may occur to varying degrees in many tissues. But only the liver can ultimately produce urea 16  04/11/2023 The overall reaction may be summarized as: NH3 + CO2 + Aspartate → Urea + fumarate 2ATPs are used in the 1st reaction. Another ATP is converted to AMP + PPi in the 3rd step, which is equivalent to 2 ATPs. The urea cycle consumes 4 high energy phosphate bonds. Fumarate formed in the 4th step may be converted to malate. Malate when oxidised to oxaloacetate produces 1 NADH equivalent to 2.5 ATP. So net energy expenditure is only 1.5 high energy phosphates. The urea cycle and TCA cycle are interlinked and it is called as "urea bicycle“. 17  04/11/2023 Significance of Urea cycle Toxic ammonia is converted into non-toxic urea. Synthesis of semi-essential amino acid-arginine. Ornithine is precursor of Proline, Polyamines. Polyamines include putrescine, spermidine, spermine. Polyamines have diverse roles in cell growth and proliferation Regulation of Urea cycle Carbamoyl phosphate synthase (CPS-I) is the rate limiting enzyme in urea cycle. CPS-I is allosterically activated by N-acetylglutamate (NAG) that is synthesized from glutamate and acetyl CoA by NAG synthase and degraded by a NAG hydrolase. The rate of urea synthesis in liver is correlated with the concentration of N- acetylglutamate. NAG formation and degradation 18  04/11/2023 High concentrations of arginine increase NAG level. The consumption of a protein-rich meal increases the level of NAG in liver, leading to enhanced urea synthesis. CPS-I and GDH are present in mitochondria. They coordinate with each other in the formation of NH3 and its utilization for carbamoyl phosphate synthesis. Urea disposal Urea produced in the liver freely diffuses and is transported in blood to kidneys and excreted. A small amount of urea enters the intestine where it is broken down to CO2 and NH3 by the bacterial enzyme urease. This ammonia is either lost in the feces or absorbed into the blood. Disorders of the Urea cycle The main function of Urea cycle is to remove toxic ammonia from blood as urea. Defects in the metabolism of conversion of ammonia to urea, i.e., Urea cycle leads to Hyperammonaemia or NH3 intoxication. Hyperammonaemia could results from inherited disorders of urea cycle enzymes (familial hyperammonaemia) or acquired disorders (Liver Disease, severe renal disease (acquired hyperammonaemia). Ammonia toxicity can cause: increased levels of ammonia crossing the Blood Brain Barrier, formation of glutamate, increase utilization of α-ketoglutarate, decreased levels of α- Ketoglutarate in Brain. Since α-KG is a key intermediate in TCA cycle, its decreased levels will impairs TCA cycle and dcreased ATP production. 19  04/11/2023 Inherited disorders of Urea Cycle N-Acetylglutamate Synthase Deficiency Autosomal Recessive and a severe neonatal disorder with fatal consequences. Treatment with structural analog N-carbamoyl-L glutamate activates CPS-I. Ornithine Transporter Deficiency (ORNT1 gene) Ornithine is accumulated in Cytoplasm. HHH syndrome – Hyper-ornithinemia, Hyperammonemia, Homocitrillinuria. Symptoms Increased levels of ammonia results in slurring of speech, blurring of the vision, convulsions, nausea, vomiting, neurological deficits, mental retardation, coma and death 20  04/11/2023 Methionine Metabolism Methionine metabolism is necessary for epigenetic regulation and cysteine production. Epigenetics is a powerful mechanism by which genes are modulated without altering the underlying DNA code. The histone proteins that wrap DNA can undergo modifications that make the DNA either accessible or inaccessible to proteins that either activate or repress genes. One modification is methylation catalyzed by histone methyltransferase enzymes that add methyl groups to specific residues on different histones. There are also histone demethylases that remove the methyl groups. Methionine provides the methyl group for many histone methyltransferases, as well as other type of methyltransferases, including those involved in the conversion of norepinephrine to epinephrine. These methyltransferases use S-adenosylmethionine (SAM). SAM is generated by condensation of ATP and methionine catalyzed by methionine adenosyltransferase. The methyl group (CH3) is attached to the methionine sulfur atom in SAM. During the generation of SAM, all the ATP phosphates are lost so that only the adenosine component is attached to methionine SAM is converted into S-adenosylhomocysteine (SAH) upon transferring its methyl group to DNA or proteins. SAH is then cleaved by adenosylhomocyteinase to yield homocysteine and adenosine. Methionine synthase can convert homocysteine back to methionine, a reaction that requires 5-MTHF (5-methyltetrahydrofolate) as the methyl donor. The resulting THF can undergo a series of reactions, known as the folate cycle, to generate 5MTHF. Dietary folate provides THF. Betaine-homocysteine S-methyltransferase can also generate methionine by using homocysteine and betaine as substrates. Homocysteine can generate cysteine by a series of reactions known as trans-sulfuration. Homocysteine condenses with serine to produce cystathionine, which is subsequently cleaved by cystathionine γ-lyase (also called cystathionase) to produce α-ketobutyrate and cysteine. α-Ketobutyrate is converted to propionyl-CoA and then, via a three-step process, to the TCA cycle intermediate succinyl-CoA. The cysteine can undergo a series of reactions with glutamate and glycine to generate the cellular antioxidant glutathione (GSH). The rate- limiting enzyme of GSH synthesis is the generation of γ-glutamyl cysteine by glutamate- cysteine ligase. It is important to note that cysteine found in the blood is typically cystine, an amino acid formed by the oxidation of two cysteine molecules covalently linked by a disulfide bond. Cystine can be transported into cells and readily converted into cysteine by nonenzymatic reduction and provide another source of cysteine for GSH production. The transporter proteins SLC7A11 or xCT transport cystine in exchange for glutamate. 21  04/11/2023 Methionine Metabolism TYROSINE METABOLISM Tyrosine is the precursor for the catecholamines norepinephrine, epinephrine, and dopamine. Tyrosine hydroxylase converts tyrosine to generate dihydroxyphenylalanine (LDOPA), a metabolic precursor to dopamine and dopaquinone. Tyrosine hydroxylase requires the enzyme cofactor tetrahydrobiopterin. L-DOPA is converted to dopamine by the enzyme aromatic amino acid decarboxylase. Dopamine is a potent neurotransmitter that is required for numerous brain functions. Notably, patients with Parkinson’s disease, a debilitating condition marked by motor impairment and tremors, have few normal dopamine-producing cells in the midbrain area called the substantia nigra. Thus, L-DOPA is often prescribed to patients with Parkinson’s disease to elevate their dopamine levels. There is also accumulating evidence that too- high levels of dopamine are observed in schizophrenia. The antipsychotic drugs for treatment work, in part, by limiting dopamine levels. Dopamine is also a precursor to epinephrine and norepinephrine, produced in the adrenal medulla. Catecholamines are associated with the fight-or-flight response and prepare the body to deal with environmental stress. 22  04/11/2023 The effects of catecholamines are associated with the sympathetic nervous system and increase blood pressure, heart rate, and blood glucose levels. Tyrosine is also a precursor to the production of melanin, eumelanin, and pheomelanin by melanocytes to provide skin and hair pigmentation. Eumelanin provide dark pigments, such as brown or black, and pheomelanin give rise to red or yellow pigmentation. The ratio of eumelanin and pheomelanin in melanocytes dictates skin and hair pigmentation. People lacking the gene tyrosinase cannot generate pigments; thus, they exhibit albinism. Given the multiple roles of tyrosine metabolism, the maintenance of tyrosine levels within cells is vital. Tyrosine can be obtained from the diet or generated from phenylalanine. The enzyme phenylalanine hydroxylase converts dietary phenylalanine to tyrosine. Genetic defects in the phenylalanine hydroxylase gene result in the metabolic disease phenylketonuria (PKU),in which phenylalanine accumulates significantly in the blood. PKU patients show neurological and developmental problems because of the high levels of phenylalanine, which generate toxic metabolites, such as phenylpyruvate, phenylacetate, and phenyl lactate. PKU disease is autosomal recessive and one of the more common metabolic genetic disorders. PKU patients are diagnosed shortly after birth as a result of a simple and routine blood test and must be on lifelong strict phenylalanine-limited diet to limit the buildup of phenylalanine and so prevent neurological and developmental complications. Tyrosine metabolism produces neurotransmitters, catecholamines, and melanin. Tyrosine hydroxylase converts tyrosine to generate dihydroxyphenylalanine (DOPA), which is converted to dopamine by the enzyme aromatic amino acid carboxylase. Dopamine generates norepinephrine through dopamine β-hydroxylase. Phenylethanolamine N-methyltransferase converts norepinephrine to epinephrine. Tyrosine is also a precursor for melanins through tyrosine or DOPA oxidation to dopaquinone by the enzyme tyrosinase. Phenylalanine hydroxylase produces tyrosine from phenylalanine. Genetic defects in the phenylalanine hydroxylase gene result in the metabolic disease phenylketonuria (PKU). PKU patients show high levels of phenylalanine that generate toxic metabolites, such as phenylpyruvate, resulting in neurological and developmental problems. 23  04/11/2023 INBORN ERRORS OF METABOLISM Inborn errors of metabolism occur when some enzyme involved in metabolism is abnormal. The abnormality occurs due to a mutation in gene encoding the enzyme The affected enzyme may be absent or deficient  Inborn errors may occur in metabolism of all nutrients including amino acids When an enzyme is absent or deficient, metabolism of the concerned amino acid becomes abnormal. Over 50 inborn errors of metabolism of amino acids have been discovered due to decreased synthesis of products ,accumulation of intermediates ,formation of alternate metabolites. Many disorders result in neurological abnormalities and mental retardation. Early diagnosis and treatment can prevent neurological abnormalities. Generally, the treatment comprises restricted intake or exclusion of the affected amino acid from the diet. Some relatively common inborn errors are: Maple syrup urine disease (MSUD) Cystinuria Phenylketonuria (PKU) Alkaptonuria Albinism PHENYLKETONURIA Phenylketonuria (PKU) is the commonest inborn error of amino acid metabolism. It has an incidence of about 1 in 10,000 live births. It was the first inborn error of amino acid metabolism to be treated successfully by diet manipulation. In PKU, there is a block in the conversion of phenylalanine into tyrosine which is then converted to phenyl pyruvate and excreted in urine. 24  04/11/2023 PKU was first described by Asbjørn Følling, one of the first Norwegian physicians to apply chemical methods to the study of medicine. In 1934, the mother of two intellectually impaired children approached Følling to ascertain whether the strange musty odour of her children’s urine might be related to their intellectual impairment. The urine samples were tested for a number of substances including ketones. When ketones are present, urine usually develops a red-brown colour upon the addition of ferric chloride, but in this instance the urine yielded a dark-green colour. After confirming that the unusual result was not due to any medications and repeating the test every other day for two months, Følling proceeded with a more detailed chemical analysis involving organic extraction and purification of the responsible compound, and determination of its melting point. Følling postulated that the compound was phenylpyruvic acid. Family studies of affected individuals led to the suggestion of an inherited recessive autosomal trait. He published his findings and suggested the name ‘imbecillitas phenylpyruvica’ relating the intellectual impairment to the excreted substance, thereafter renamed ‘phenylketonuria’. The condition is also called PAH deficiency, phenylalanine hydroxylase deficiency, Folling’s disease, PKU. Autosomal recessive disorder caused by mutation in Phenylalanine Hydroxylase (PAH) gene that is located on chromosome 12q23.2. The gene codes for the formation of the enzyme phenylalanine hydroxylase. A carrier does not have symptoms of the disease, but can pass on the defective gene to the progeny. The common symptoms includes mental retardation, microcephaly, hyperactivity, stunted growth, skin rashes, fair skin and blue eyes (phenylalanine cannot transform into melanin), musty odour in breath or urine, seizure, tremor, jerking movement in the arms and legs, behavioral and social problems. Treatment: PKU can be treated with a low-protein diet and dietary supplements. The diet must be strictly followed. If continued there is a high chance of better physical and mental health. Administration of tetrahydrobiopterin is recommended 25  04/11/2023 Biochemistry of PKU: Phe exists as D and L enantiomers, and L-Phe is an essential amino acid required for protein synthesis in humans. As with many other metabolites, Phe concentrations are regulated to a steady state level with dynamic input and runout flux. Persistent disturbance to the flux will eventually result in alteration of the steady state concentrations. Dietary intake of Phe along with endogenous recycling of amino acid stores are the major sources of Phe, whereas, utilisation or runout of Phe occurs via integration into proteins, oxidation to Tyr, or conversion to other metabolite. The conversion of Phe to Tyr occurs by a hydroxylating system consisting of: (1) Phenylalanine Hydroxylase (PAH), (2) the unconjugated pterin cofactor {tetrahydrobiopterin (BH4)}, (3) enzymes which serve to regenerate BH4, namely dihydropteridine reductase and 4α-carbinolamine dehydratase While the para-hydroxylation of Phe is essential for the rupture of the benzene ring, it is not required for further metabolism of the alanine side chain. This alternative pathway of transamination and decarboxylation leads to the formation of metabolites such as phenylpyruvate, phenylactate, and o-hydroxyphenylacetate which are excreted in urine. Conversion of Phe to Tyr has two outcomes. First, it drives the endogenous production of the non-essential amino acid, Tyr. Second, the hydroxylation reaction is the rate limiting step for complete oxidation of Phe to CO2 and H2O and contributes to the pool of glucose and 2-carbon metabolites. 26  04/11/2023 Phenylalanine Hydroxylase (PAH) catalyses the stereospecific hydroxylation of L-Phe, the committed step in the degradation of this amino acid. Phe catabolism and PAH activity is mainly associated with the liver, although minor activity has been demonstrated in rat kidney. In humans, the PAH enzyme exists as a mixture of tetramers and dimers; the monomer is about 50 kDa in size and is comprised of 452 amino acids. PAH requires BH4 as a cofactor, as well as molecular oxygen for its activity. PAH can be divided into a number of functional domains. The regulatory domain contains a serine residue which is thought to be involved in activation by phosphorylation. The catalytic domain contains a motif of 26 or 27 amino acids responsible for cofactor and ferric iron binding. The C-terminal domain is thought to be associated with inter-subunit binding. Structural components of PAH. The catalytic domain of PAH contains a motif of 26 or 27 amino acids which are responsible for ferric iron and cofactor (BH4) binding. 27  04/11/2023 PAH is regulated by a number of possible mechanisms. After a protein meal, it is postulated that the increased Phe in the amino acid pool causes a release of glucagon from the pancreas. Hepatic PAH is subject to control by cAMP-dependent protein kinase and α- andrenergic agent stimulated Ca2+/calmodulin dependent protein kinase phosphorylation–dephosphorylation processes. It has been further reported that these control mechanisms influence BH4 co-factor interaction with PAH. In addition, there is evidence that Phe may also be able to cause a conformational change in PAH, as well as up-regulate cAMP activity. Taken together, these mechanisms enable fine regulation of Phe concentrations by balancing levels sufficient for maintenance of protein biosynthesis while minimising tissue exposure to high concentrations of Phe. Clinical presentations of PKU Untreated PKU is associated with an abnormal phenotype including growth failure, microcephaly, seizures and intellectual impairment caused by the accumulation of toxic by-products of Phe. Moreover, decreased or absent PAH activity can lead to a deficiency of Tyr and its downstream products, including melanin, L-thyroxine and the catecholamine neurotransmitters. 28  04/11/2023 CYSTINURIA Cystinuria is an inherited disease that causes stones made of the amino acid cystine to form in the kidneys, bladder, and ureters. Inherited diseases such as cystinuria are passed down from parents to their progeny. A defect or mutation in the genes, SLC3A1 and SLC7A9 causes cystinuria. These genes encode for the proximal tubule dibasic amino acid transporter which facilitates reabsorption of cysteine from tubular fluid To come down with cystinuria, a person must inherit the defect from both parents. The defect in the gene causes cystine to accumulate inside the kidney leading stones containing the amino acid cystine. There is a defect in proximal renal tubular reabsorption of cystine therefore, urinary excretion of cystine is increased. Being sparingly soluble, cystine deposits in the kidneys and forms cystine stones. This same problem also affects ornithine, lysine, and arginine, but only cystine is clinically significant as it is the only amino acid in this group that will form stones. Cystathionine synthetase is severely deficient in homocystinuria. This impairs the conversion of methionine into cysteine. Cysteine accumulates and is converted into homocysteine. Urinary excretion of homocystine is increased. Genetics of Cystinuria 29  04/11/2023 Symptoms include osteoporosis, dislocation of lenses in the eyes, mental retardation, ischaemic vascular disease, blood in the urine, severe pain in the side or the back, almost always on one side, nausea and vomiting, pain near the groin, pelvis, or abdomen. Treatment: Changes to diet, medications, and surgery are options for treating the stones that form due to cystinuria. Dietary changes: the treatment consists of a low-methionine, high-cysteine diet, pyridoxine supplements may be given to activate the residual cystathionine synthetase, reduction in salt intake to less than 2 grams per day has also been shown to be helpful in preventing stone formation in human. ALKAPTONURIA Alkaptonuria is an inborn error of tyrosine metabolism. It is due to absence of homogentisate oxidase. Alkaptonuria is caused by a mutation on the homogentisate 1,2- dioxygenase (HGD) gene. Homogentisate, an intermediate in catabolism of tyrosine, cannot be metabolised and then is excreted in urine. Freshly voided urine is normal in color, but urine becomes dark on exposure to air due to oxidation of homogentisate by oxygen. It also called black urine disease. This condition is rare, affecting 1 in 250,000 to 1 million people worldwide. 30  04/11/2023 DIAGNOSIS A urine test (urinalysis) is done to test for alkaptonuria. If ferric chloride is added to the urine, it will turn the urine a black color in patients with this condition. TREATMENT There’s no specific treatment for alkaptonuria. Low-protein diet is recommended. Large doses of ascorbic acid, or vitamin C is given to slow down the accumulation of homogentisic acid in the cartilage. Some patients benefit from high-dose vitamin C. 31  04/11/2023 Albinism Albinism (from Latin albus, "white“ also called achromia, achromasia, or achromatosis) is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes. This is due to absence or defect of tyrosinase, a copper-containing enzyme involved in the production of melanin. Albinism results from inheritance of recessive gene alleles and is known to affect all vertebrates, including humans. A person inherits one or more defective genes that cause them to be unable to produce the normal amounts of a pigment called melanin. The genes are located on "autosomal" chromosomes. Autosomes are the chromosomes that contain genes for general body characteristics. Both parents must carry a defective gene to have a child with albinism. When neither parent has albinism but both carry the defective gene, there is a one in four chance that the baby will be born with albinism. Treatment There is no real cure for albinism, but the goal of treatment is to relieve symptoms. Treatment involves protecting the skin and eyes from the sun: Reduce sunburn risk by avoiding the sun, using sunscreen, and covering up completely with clothing when exposed to the sun. Sunscreen should have a high SPF. Sunglasses may relieve light sensitivity. Glasses are often prescribed to correct vision problems and eye position. Eye muscle surgery is sometimes recommended to correct abnormal eye movements. 32  04/11/2023 Characteristics: There is little to no melanin (important pigment) in the eyes, skin, and hair. Vision problems are a result of the low amounts of melanin in albinos. The eyes are usually blue or light brown, but can sometimes appear red. Skin and hair is very pale in color. More likely to sunburn and sensitivity to bright light Types of Albinism Ocular Albinism (OA) affects only the eyes, not the skin or hair. It results from an X-linked chromosomal inheritance and so occurs mostly in boys. Oculocutaneous Albinism (OCA): affects the eyes, hair and skin and includes several different forms. The first form, OCA1 involves the tyrosinase enzyme, which converts tyrosine (an amino acid) into melanin. Melanin is a chemical that colors our skin, eyes and hair. Tyrosinosis Since the first report of abnormal tyrosine metabolism by Medes in 19321 elevated blood tyrosine has been reported in several different biochemical and phenotypical variations. Tyrosinosis is an inherited inability of the body to metabolize tyrosine, para (p) hydroxyphenylpyruvic acid to homogentisic acid because the enzyme parahydroxyphenylpyruvic acid (PHPPA) oxidase is inactive. Disorders of tyrosine metabolism range from the benign condition of neonatal tyrosinemia to a rapidly fatal hereditary tyrosinosis. The common biochemical denominator is increased plasma tyrosine level and tyrosyluria (presence of tyrosine and its derivatives in urine). The dysfunction in tyrosine degradation may be subdivided into four etiological groups: Group I: Tyrosine elevation secondary to severe liver damage, pernicious anemia or vitamin C deficiency. Group II: Immaturity of the enzyme system, parahydroxyphenylpyruvic acid (PHPPA) oxidase, as in neonatal tyrosinemia (benign hypertyrosinemia). This is a well-known condition of transient elevation of serum tyrosine, together with tyrosyluria, occurring occasionally in premature and in some term infants fed a high protein diet. Group III: Disorders described in the literature as tyrosinosis, hereditary tyrosinemia and inborn hepatorenal dysfunction, in which tyrosinemia and tyrosyluria are associated with liver or kidney damage. 33  04/11/2023 Methioninemia, aminoaciduria and glycosuria have been almost constant features of this form of tyrosinosis in the untreated state. In spite of early conjecture it has not been established that tyrosinosis associated with hepatorenal disease is a primary defect of tyrosine metabolism and not a consequence of liver disease. It is probable that decreased ability to metabolize tyrosine and methionine is an independent secondary manifestation of a disease process as yet unidentified. In its most frequently described form hereditary tyrosinosis presents as an acute progressive illness starting in the neonatal period. There are signs of hepatic failure, renal tubular dysfunction and vitamin D- resistant rickets, and in 90% of cases death occurs in infancy. Children surviving the first year of life show severe psychomotor retardation and most of them die of hepatic and renal failure in the first decade. Group IV: More recently identified cases of tyrosinemia and tyrosyluria without hepatorenal disease. These cases may actually be examples of "essential tyrosinemia". a primary genetic defect in tyrosine metabolism. For lack of a better name this condition is usually referred to as "tyrosinosis without hepatorenal disease". In the cases so far reported, this form of tyrosinosis is associated with mental retardation. It runs a non-progressive course and, in spite of high plasma tyrosine levels and tyrosyluria, there is no elevation of the methionine level and both liver and kidney function remain normal. A dysfunction in tyrosine metabolism along the major pathway has been identified in several patients having tyrosinosis with hepatorenal disease, suggesting a block at the level of PHPPA-oxidase, and the deficient activity of liver PHPPA oxidase has been measured. 34