[BIOCHEM]LAB_205_AMINO_ACIDS_METABOLISM-OF-CARBON-SKELETON.pdf

(005) AMINO ACIDS METABOLISM OF CARBON SKELETON DR. FAHAD ABDUL RAZAK | 01/...

(005) AMINO ACIDS METABOLISM OF CARBON SKELETON DR. FAHAD ABDUL RAZAK | 01/12/2021 OUTLINE dark skinned infants so the affected areas have lighter I. CASE 1 color. - If we consider microcephalic and hypotonia alone, there A. What is Phenylketonuria? are lot of differential diagnosis. B. Symptoms - Hypopigmentation is also due to certain enzyme C. Mechanism deficiencies like tyrosinase. D. Detection E. Treatment II. CASE 2 A. What is Maple Syrup Urine Disease (MSUD)? B. Clinical manifestations C. Classifications D. Treatment III. TEST YOURSELF IV. REFERENCE I. CASE 1 A 1-year-old boy was brought to your clinic due to concerns regarding his growth and development. Apparently delivered term Figure 1. A patient exhibiting hypotonia or decreased muscle at a rural house in the Philippines and was only brought to rural tone. clinic for vaccinations. No previous tests were done. History revealed difficulty talking, sitting and standing. Urine also noted DISCUSSION: to have distinct odor. Physical Examination, the boy was noted to be microcephalic, hypotonia and areas of hypopigmentation A. WHAT IS PHENYLKETONURIA? (including skin and hair). You collected urine for examination and noted a musty/mousy odor. - Is a disease usually diagnosed in childhood - An AUTOSOMMAL RECESSIVE disease What is the most likely diagnosis in this case? - Phenylketonuria Why is hypopigmentation present in this case? - Melanin synthesis is inhibited due to presence of phenylalanine [a competitive inhibitor of tyrosinase] - Urine having a distinct odor consider it as a separate entity, difficulty in talking, sitting and standing and development delays can include other disease like hypothyroidism (if we exclude urine having distinct odor) - The distinct odor does not specify what type of odor it is. It can be something due to CKD (chronic kidney disease). So even children can have CKD as well. It can also be due to an infection. - Hypotonic or there is no muscular tone which is the muscles do not contract. - Areas of hypopigmentation – specific areas that do not Figure 2. A pattern of inheritance in which an affected individual have pigments in them. It could be just a patch of hair or has one copy of a mutant gene on a pair of autosomal we can find spots if they are dark. It’s easier to find in Page 1 of 9 CMED 1F  (005) AMINO ACIDS METABOLISM OF CARBON SKELETON DR. FAHAD ABDUL RAZAK | 01/12/2021 chromosomes. In contrast, autosomal recessive diseases o Metabolic disorders of phenylalanine require that the individual have two copies of the mutant gene. catabolism include PKU and several hyperphenylalaninemias. Early detection through newborn screening: GUTHRIE TEST o We have two alleles, one that is dominant represented by capital letter and one that is o Guthrie test is basically newborn recessive represented by small letter. Most of screening. We take capillary blood from the time both parents will be heterozygotes for newborn babies usually within or after 24 these alleles. So, one dominant gene and one hours. But ideally for some diseases it recessive gene. If it is being passed on as should be done after 48-72 hours. autosomal recessive that means it is passed o PKU is usually the most common amino on only in the recessive allele, the weaker gene acid that is associated with amino acid or the gene that is not expressed. So, if there metabolism. For Guthrie test we usually is a person who is heterozygote one who will detect the amount of phenylalanine. possess even just one of the normal gene or one of the dominant alleles, they will not express the disease or they will not express the symptoms associated with the disease. o Those who have two of the dominant alleles, they will be unaffected and they will not pass it to their children. But those who are heterozygotes will be able to pass it to their children. Most of the time if their partner is also heterozygote, it will be also at a ¼ ratio. If the partner of one who is a heterozygote meets with the one who only carries the dominant gene or who does not have the predisposition for the disease, the chance that their children will present with the disease will most likely be zero. Because the recessive gene will not be expressed if it is associated with the dominant Figure 3. The Guthrie test, also called the PKU test, is a gene. If it is autosomal dominant it will present diagnostic test for infants for Phenylketonuria a few days after the ¾ of the cases. birth. - Incidence in 1 in 13,500-19,000 live births as such B. SYMPTOMS is the most common genetic disorder for amino acid catabolism - Usually appear at 1 year of age o Key words for PKU are urine having musty - Depressed IQ without treatment odor, hypopigmentation and no hospital check-up. o Hypopigmentation is due to inability to produce melanin due to the decrease in amount of tyrosine in the body that is due to the increase in amount of phenylalanine in PKU. - PKU is an autosomal recessive o PKU is an autosomal recessive metabolic disorder in which the body cannot properly use one of the building blocks of protein called phenylalanine, an essential amino acid that converts tyrosine causing elevation of phenylalanine in the blood. o Phenylalanine is first converted to tyrosine. Subsequent reactions are those of tyrosine Figure 4. Severely retarded institutionalizes brothers with untreated PKU. Page 2 of 9 CMED 1F  (005) AMINO ACIDS METABOLISM OF CARBON SKELETON DR. FAHAD ABDUL RAZAK | 01/12/2021 - Developmental delay - Musty/Mousy urine odor (because of phenylketones) - Seizures and Tremors o Other symptoms will present also with neurological symptoms such as neural seizures and neural tremors. C. MECHANISMS Figure 6. A defect in the Phenylalanine hydroxylase causes Classic Phenylketonuria. ATYPICAL PKU - Defect in Dihydrobiopterin Reductase (type II and III) Figure 5. The conversion of Phenylalanine to Tyrosine. CLASSIC PKU - Defect in gene for Phenylalanine Hydroxylase (type I) - frequency 1 in 10,000 births Figure 7. A defect in Dihydrobiopterin reductase causes Atypical Phenylketonuria. Increase amounts of phenylalanine will shift production of phenylpyruvate (phenylketones) - Catalyzed by phenylalanine transaminase - Produces mousy odor urine - Hyperphenylalaninemia is a plasma phenylalanine level above 120 umol/L (neurotoxic) Page 3 of 9 CMED 1F  (005) AMINO ACIDS METABOLISM OF CARBON SKELETON DR. FAHAD ABDUL RAZAK | 01/12/2021 - Phenylalanine competitively inhibits tyrosinase (HYPOPIGMENTATION) ▪ Inability to produce 1-DOPA and melanin ▪ Leads to hypopigmentation - Tyrosine is usually important for multiple processes. When we are not able to produce tyrosine in the body, it becomes essential which means the body is not able to produce it by itself. Increase in amounts of phenylalanine also inhibit the enzyme associated with the breakdown of tyrosine but that would be our tyrosinase. If we are unable to breakdown our tyrosinase, it will also lead to other symptoms like hypopigmentation since we are not able to produce melanin. TYPES IV AND V - Deficiency in dihydrobiopterin biosynthesis D. DETECTION - DNA probes facilitate prenatal diagnosis of defects in phenylalanine hydroxylase or dihydrobiopterin reductase. - Elevated blood phenylalanine may not be detectable until 3 to 4 days postpartum. False- positives in premature infants may reflect delayed maturation of enzymes of phenylalanine catabolism. - An older and less reliable screening test employs FeCl3 to detect urinary phenylpyruvate. FeCl3 screening for PKU of the urine of newborn infants is compulsory in many countries, but in the United States has been largely supplanted by tandem mass spectrometry. - The blood Phenylalanine concentration in Figure 8. Pathways of phenylalanine metabolism in normal newborns is normally 0.5mg to 1mg per dL (30 individuals and in patients with Phenylketonuria. to 60µmol per L). Cofactors are the ones that help decrease energy of the reaction, so that will eventually lead to decrease in the E. TREATMENT amount of reaction. If we do not have the enzyme that will convert the - Diet low in phenylalanine can prevent the mental phenylalanine to tyrosine that represents to be normal. retardation of PKU. But if we do not have the enzyme the phenylalanine will accumulate and once it accumulates, we will end up having to push it using phenylalanine transaminase. These are what produces the symptoms, especially mousy odor. Since they are phenyl ketones and they are being released in the urine, that’s where the name Phenylketonuria comes from. TYROSINE – now becomes essential amino acid Page 4 of 9 CMED 1F  (005) AMINO ACIDS METABOLISM OF CARBON SKELETON DR. FAHAD ABDUL RAZAK | 01/12/2021 II. CASE 2 A 4-day-old infant was brought into the emergency room due to lethargy. This was accompanied by vomiting and poor feeding. The baby was delivered via NSD with no noted complications. Still awaiting new born screening result. Parents deny history of trauma. Upon examination, the patient is afebrile and his pampers gave off a sweet odor. Alternating muscular hypotonia and hypertonia is observed. Computed tomography of the head reveals cerebral edema. Lumbar puncture results show clear cerebrospinal fluid (CSF), containing 2 WBC per high power field, no RBC, normal protein and no hypoglycorrhachia, and no bacteria in Gram stain smears. The CSF culture is negative. Ketone bodies and keto acids are detected in the urine. Serum analysis reveals elevated levels of branched-chain amino acids. Figure 9. A low protein diet is the main treatment for PKU. What is the most likely diagnosis? Maple Syrup Urine Disease (MSUD) - Supplement Tyrosine and BH4 - Since phenylalanine will be toxic, we need to decrease the amount of phenylalanine and we are able to do this by decreasing the amount of amino acid or phenylalanine received from the diet. - Since Tyrosine is usually essential, we have to supplement it. For patients with PKU, they have special diets usually that include a low protein diet and they usually avoid foods that have complete amino acid profile so it does not include all the amino acids because phenylalanine will be toxic to them. - Low protein diet is given to patients with PKU. Foods that are usually acceptable to them are flat breads that do not include eggs, fruits, vegetables, and juices. Peas, seeds, some grains, and rice are not included however they can be eaten in a restricted or infrequent manner because these contain proteins. Even if the amino acid is not complete, it still contains some phenylalanine. Figure 10. The difference between normal individual and a Beef, pork, chicken, eggs, nuts should also be patient with MSUD. avoided. Differential Diagnosis can include Sepsis for - Eating products that contain phenylalanine will patients that are less than a month old. Not all increase the chance for neurotoxicity and that babies who present sepsis have fever. However, will also provoke neurologic symptoms like delayed for signs of infection there can be poor feeding, IQs, etc. The later the disease is treated or the later vomiting, and lethargy. It can either be Meningitis you start decreasing the diet of phenylalanine, the or General Sepsis. more chance the child will have neurologic deficit or Another illness that has hypotonia and hypertonia decreased IQ. PKU is not actually a death usually older people get it is Tetanus. Patients with sentence, as long as an affected child maintains the tetanus can alternate periods of hypotonia and diet properly, they can live up to adulthood. hypertonia but it depends with the severity. The However, it must be diagnosed during early years worst kind is when there is persistent hypertonic that’s why it’s one of the tests being administered in state when all of the muscles will contract and the newborn screening. Page 5 of 9 CMED 1F  (005) AMINO ACIDS METABOLISM OF CARBON SKELETON DR. FAHAD ABDUL RAZAK | 01/12/2021 extend the neck, back muscles, feet, and hands. BCKD is a multienzyme complex with 6 subunits This significant posture is called Opisthotonus. (Enzymatic subunits (E1α, E1β, E2, E3), BCKD Lumbar Puncture was done for this patient. A kinase, BCKD phosphatase). Phosphorylation of normal CSF fluid is clear, as manifested by the the complex inactivates it and, conversely, it is patient. WBC is normally 0-5 or 0-8 depending on activated by dephosphorylation. your reference. RBC is abnormal in the CSF. The Partial or complete deficiency in BCKD only time you will consider the result with a grain of →accumulation of BCAA and α-ketoacid salt is if the lumbar puncture was traumatic that BCKD facilitates a sequence of reactions in the breakdown of BCAAs - it undergoes multiple means the blood vessels under the skin were hit. metabolisms since BCKD contains multiple Blood in the CSF is also toxic causing brain subunits that associate with these reactions. damage. Hypoglycorrhacia is a low glucose level The severity in the deficiency of BCKD will in the cerebrospinal fluid. The normal value of contribute to the build-up of BCA Ketoacids plus the glucose in the CSF is 45 or 48. BCAAs. The accumulation of these BCAAs and NOTE: Symptoms include sweet odor urine, their ketoacids will contribute to the symptoms of vomiting and poor feeding due to cerebral edema MSUD. that is noted with the disease or during attacks. The one that contributes the most to MSUD is Note also if there are no newborn screening Leucine. Build-up of more than 200 results. μmol/L→MSUD What is the mechanism behind this disorder? - Products include: o Acetyl-CoA Mutation in one or more subunit of the branched o Succinyl-CoA chain α-ketoacid dehydrogenase complex (BCKD) o Acetoacetate What is the pathophysiologic mechanism of his symptoms? Decrease in the amount of BCKDs → Inability to degrade BCAAs leads to accumulation of BCAAs especially leucine → interferes with some of the amino acids in the brain → increased osmolarity in the fluids → cerebral edema DISCUSSION: A. WHAT IS MAPLE SYRUP URINE DISEASE? Autosomal recessive genetic disorder in branched chain amino acid catabolism, the disorder is caused by dysfunctional BCKD unable to degrade BCAAs like leucine, isoleucine and valine Increase in leucine in both plasma and organs may Figure 11. The structure of the three branched chain amino acids cause cerebral edema and neurologic problem mainly affected by BCKD in MSUD. (hypotonia/hypertonia). While elevated isoleucine metabolite is characterizing with urine odor Early screening done in newborn screening test - it’s one of the initial 6 diseases There are different types of MSUD depending on the severity of the deficiency in the BKCD complex. It’s a complex because it facilitates multiple reactions in the degradation of branched chain amino acids. Normal degradation of branched amino acids would usually produce acetyl CoA, succinyl CoA, and acetoacetate. These go back into the TCA cycle or contribute with other processes of energy production. The second step is for this process is catalyzed by the mitochondrial BCKD, which converts an α-keto acid to an acyl-CoA via oxidative decarboxylation. Page 6 of 9 CMED 1F  (005) AMINO ACIDS METABOLISM OF CARBON SKELETON DR. FAHAD ABDUL RAZAK | 01/12/2021 Others: coma, seizures, vomiting and dehydration due to the neurotoxic effects of trans amino acids and keto acids. C. CLASSIFICATIONS OF MSUD -MSUD can be classified into classic, intermediate and intermittent -the classification depends on the amount of activity on the BCKDH enzyme complex Classic ▪ BCKDH activity (0%-2% of normal activity), very low so symptoms occur early in the first few days needing early treatment ▪ Symptoms within few days Figure 6. The structure of the three branched chain amino acids mainly affected by BCKD in MSUD. Intermediate ▪ 3%-30% ▪ normal in the neonatal period ▪ symptoms either in infancy or later (typically between the ages of 5 months and 7 years) Figure 12. The abbreviated branched chain amino acid between the intermediate and intermittent the amount of pathway. enzyme deficiency is very minimal however depending on how early the symptoms would appear B. CLINICAL MANIFESTATION MSUD Intermittent ▪ 5%-20% ▪ only display symptoms during times of physiologic stress (when the patient experiences illness such as fever and sepsis it would stress the systems and thus requiring more byproducts and energy that will contribute to the symptoms) Accumulation of leucine consequences interference with transport of other large neutral amino acids across the blood-brain barrier → reduce the availability of these required amino acids → reduced brain growth and synthesis of neurotransmitters ▪ dopamine, serotonin, norepinephrine, and histamine (cell-to-cell communication) leucine is the main culprit and at very high length it Figure 13. Urine smelling like maple syrup could indicate a rare, becomes neurotoxic. Leucine also reduces the inherited disorder. availability of other amino acids such as glutamine, fumarate and many others (however not so clear) Symptoms appear within first few days of life neurotoxic as it disturbs Progressive if unidentified/treated ▪ brain protein accretion /turnover Lethargy, weight loss, alternating hypotonia and ▪ neurotransmitter synthesis (dopamine, hypertonia, these is due to the decrease in BCAA that catecholamine, histamine) contributes to trans amino acids and ketoacids. Their ▪ cell volume regulation accumulation in the brain is toxic leading to the mention ▪ neuron growth symptoms like lethargy etc. ▪ myelin synthesis Sweet smelling urine, due to build-up of ketones in the urine Page 7 of 9 CMED 1F  (005) AMINO ACIDS METABOLISM OF CARBON SKELETON DR. FAHAD ABDUL RAZAK | 01/12/2021 protein synthesis is disturbed due to elevated amount of leucine because it shifts the conversion of protein to other substrates that is usually not in use for these functions α-ketoisocaproic acid a ketoacid that is associated with the breakdown of ▪ inhibition of the mitochondrial electron transport chain→elevated cerebral lactate level that is toxic to the brain that will lead to encephalopathy, seizures and decrease sensory functions ▪ Acute MSUD encephalopathy D. TREATMENT MSUD Figure 14. Infant formula for patients with MSUD. Dietary restriction for branched chain amino acid, particularly leucine. Figure 15. Some human genetic disorders affecting Amino Acid Catabolism. Elevated blood concentrations of valine and isoleucine seem to be more easily resisted as compared to leucine. In addition, too much diet restriction with protein containing valine and isoleucine instead leucine may cause deficiencies for these amino acids and become III. TEST YOURSELF more problematic for the brain. Very risky and should be started as early as possible to 1. What is the most common disease associated with amino avoid taking up foods that will contribute to the buildup acid metabolism? of the amino acids and thus decreasing mortality rate. A. Phenylketonuria Symptoms are similar to sepsis and will have acute B. Maple Syrup Urine Disease flares, treatment involves minimal protein intake. C. Histidinemia D. Hyperxaluria The tables show metabolism disorders that we should be familiarize with, 2. What causes hypopigmentation? A. Decrease in Tyrosine in the body B. Inability to produce melanin C. Increase in Phenylalanine D. All of the above Page 8 of 9 CMED 1F  (005) AMINO ACIDS METABOLISM OF CARBON SKELETON DR. FAHAD ABDUL RAZAK | 01/12/2021 3. At what age does the symptoms for PKU usually appear? A. 3 months IV. REFERENCE B. 6 months C. 9 months D. 12 months 1.Harper's Illustrated Biochemistry, 3th Ed 2015 4. What type of PKU has a deficiency dihydrobiopterin biosynthesis? A. Type I B. Type II C. Type III D. Type IV and V 5. All of the following medical conditions can be caused by defective urea synthesis except which? A. Argininemia B. Homocystinuria C. Arginosuccinic academia D. Carbamoyl phosphate synthethase 1 deficiency 6. What disorder is concerned with mutation in one or more subunit of the branched chain a-ketoacid dehydrogenase complex (BCKD)? A. PKU Type I B. Atypical PKU C. Maple Syrup Urine Disease D. Hyperxaluria 7. Which of the following is not a clinical manifestation of MSUD? A. Sweet-smelling urine B. Lethargy C. Weight gain D. Alternating hypotonia and hypertonia 8. What MSUD classification only display symptoms during times of physiologic stress? A. Intermittent B. Intermediate C. Classic D. None of the above 9. What medical condition exhibits lack of pigmentation and the presence of white hair and pink skin? A. Alkaptonuria B. Albinism C. PKU D. MSUD 10. All of the following are branched chain amino acids except _____. A. Isoleucine B. Leucine C. Valine D. Tyrosine Answers: 1.A 2.D 3.D 4.D 5.B 6.C 7.C 8.A 9.B 10.D Page 9 of 9 CMED 1F