Biochemistry Flashcards
A newborn presents with severe acidosis, vomiting, hypotonia, and neurologic deficits. Laboratory analysis reveals elevated levels of lactate and alanine. These observations suggest a deficiency of which of the following enzymes?
Pyruvate dehydrogenase
Pyruvate dehydrogenase (PDH) catalyzes the irreversible conversion of pyruvate to acetyl-CoA. If PDH is absent, pyruvate will be used in other pathways instead. Pyruvate will be converted to alanine via alanine aminotransferase and to lactate via lactate dehydrogenase.
Glutamate dehydrogenase is involved in oxidative deamination, releasing ammonium ion for urea synthesis. Deficiency of this enzyme would not cause the symptoms described.
Phenylethanolamine N-methyltransferase is an enzyme involved in the synthesis of epinephrine.

Several members of a family have an autosomal recessive disease characterized by intellectual deterioration, weakness, ataxia, seizures, and death at a young age. Deficiency of cytochrome C oxidase activity. Which of the following subcellular organelles is defective in afected members of this family?
Mitochondria
This is leighs disease, a rare condition also known as subacute necrotizing encephalomyelopathy. The underlying problem is a defective form of cytochomre C oxidase, an electron transport chain component, in the mitochondria of muscle and brain. Clinically, the features described in the quesiton stem are seen. Pathologically, there is a symmetric necrosis that affects central areas of the nervous system rom the thalamus to the spinal cord. No effective treatment exists at this time.
17 y/o missed last menstrual period. She has sickle-cell trait and mild asthma but otherwise healthy. bf also has sickle-cell trait. Determined to contiue pregnancy if pregnant. Main cocern is whether her child will have sickle-cell anemia. What is the chance of baby being sickle-celled if both parents have it.
25%
Sickle cell anemia arises due to the mutation of the beta-globin gene. Because both of the beta-globin genes have to be abnormal to produce the disease, it is an example of autosomal recessive. When boht parents are carriers o the abnormal gene the risk of having the disease with two abnormal genes is 25%. The probability of having a child who is a carrier one abnormal gene and one sickle is 50%. Child normal with no abnormal gene is 25%
prokaryotic operon codes for two enzymes and one regulatory protein. The operon is expressed only in the presence of a particular sugar. Initial mapping of the operon has differentiated the nontranscribed sequences from the transcribed sequences. Deletion of 10 nucleotides from one of the nontranscribed sequences results in transcription of the operon in both the presence and absence of the sugar. Which of the following sequences was most likely affected?
Operator
The description of the operon expression pattern identifies it as an iducible operon. After the deletion, the operon is expressed constitutively. This pattern can occur only if the deletion removes a negative regulator, i.e. a repressor protein, or the DNA sequence to which it binds, called an operator. Since the questions stipulates a nontranscribed DNA sequence the operator must have been affected.

Two codon deletion
The RT-PCR product from the patients reticulocytes is 6 nucleotides shorter than the RT-PCR product from normal reticulocytes. This sex base pair deletion would be consistent with removing two codons from the mRNA .
36 y/o greek man with viral pneumonia hs a self-limiting episode of hemolysis. Over the next week, he has an increased rate of reticulocytosis. Which of the following compounds serves as a precursor to heme in the reticulocytes?
Succinyl-CoA
The porphyrin ring of heme is derived rom the citric acid cycle intermediate succinyl-CoA
And the amino acid glycine. The initial synthetic step, which is rate-limiting, is catalyzed by aminolevulinic acid synthase (ALA synthase).
Rest dont play a role in the heme.
22 y/o woman presents with a fusiform swelling of the achilles tendon, which, when biopsied, shows cholesterol-laden macrophages (foam cells) dispersed among the collagen fibers. She had been troubled with joint pains for several years. Both her mother and father developed arthritis associated with production of xanthomas, but their first symptoms occured in middle age. She was referred to a nutritionist and treated with an enzyme inhibitor. Which of the following is most likely elevated in the blood of this woman?
Low density lipoproteins (LDL)
Deposition of xanthomas around the achilles tendon is characteristic of familial type IIa hyperlipidemia, and autosomal dominant deficiency of the LDL receptor. The resultant reduced rate of LDL clearance leads to elevated LDL levels and hypercholesterolemia. For some poorly understood reason, arthritic pain in various joints, often prior to any appearance of xanthomas,, is a characteristic of familial type IIa hyperlipidemia manifestation. Although achilles tendonitis with associated xanthomas is common, other tendons may be involed. Symptoms in the heterozygous patient usually do not occur until the third or fouth decase, but homozygotes are affected much earlier, oten in childhood.
The patient was most likely treated with an HMG CoA reductase inhibitor such as simvastatin. Her condition carries a strong risk for cardiovascular disease. Although xanthomas are common in other familial hyperlipemias, such as type III and hepatic lipase deficiency, these are not associated with achilles tendonitis.
23 y/o 8th month of pregnancy. First child born at home and breast fed had prolonged diarrhea and died form an intracranial hemorrhage at 1 month of age. Resident gives her a free prescription for a vitamin and advises her to take one 20mg tablet each day to make this not happen. Also informs infant should receive an injection of this vitamin soon after birth. The vitamin prescribed is required as a coenzyme by which of the following enzymes?
y-Glutamyl carboxylase
Late onset vitamin K deficiency bleeding can occur between 1 and 6 months after birth. Risk factors include exclusive breast feeding because human milk is low in vitamin K. In hospital settings infants are given injections of vitamin K after birth, but the irst child of this patient was born at home. Additonal factors contributing to itamin K deficiency in infants include poor transer of vitamin K from maternal to fetal blood and sterile intestines (in this case exacerbated by prolonged diarrhea). In addition, hepatic storage of vitamin K is about 1 month. This is the time when late onset vitamin K deficiency may manifest in infants.
Vitamin K is an important coenzyme for y-glutamyl carboxylase, an enzyme that catalyses a post translational modification of a group of calcium-binding proteins. Important examples include factors II, VII, IX, and X, as well as proteins C and S. Vitamin K deficiencies in infants may manifest as gastrointestinal bleeding, skin hemorrhages, and intracranial hemorrhage.
26 y/o pregnant woman complains of persistent dry, ulcerated skin over her knees and elbows. Examination of her eyes reveals small, gray plaques on the conjunctiva. Which of hte following is the most likely diagnosis?
Retinol deficiency
This patient has vitamin A (retinol) deficiency, which typically occurs in people with increased requirements for the nutrient, e.g. pregnant wome and patients with chronic diseases. Night blindness is usually the earliest manifestation of this deficiency and may be followed by retinal degeneration and blindness in utreated patients. Drying of the bulbar conjunctiva (xerosis_) and bitot spots (small, gray plaques on the conjunctiva) may also occur. Keratomalacia (corneal necrosis) is a serious potential complication of this deficiency.
pregnant mother of 6 y/o boy son with glucose-6-phosphate dehydrogenase deficiency is very wories that her female fetus will have the disease. The fatehr and mother are clinically normal. Which of the following is true about her baby?

The baby has a 50% chance of being a carrier.
Glucose-6-phosphate dehydrogenase disease is an X-linked condition. the father has a normal X chromosome since he does not have the clinical disease. The mother has one normal X chromosome and one defective X chromosome, since her 6 y/o son has the disease and she does not. Thereore, the baby has 50% chance of having one normal and one abnormal X chromosome, making her a carrier.
Glucose-6-phosphate dehydrogenase deficiency is a defect in the pentose phosphate pathway of glucose metabolism, leading to defective production of reduced NADPH. Reduced NADPH is used in many biochemical pathways and is specifically used to regenerate the reduced from of glutathione that protects the body against oxidant drugs.

Genetics researcher is trying to identify a ptoential gene from a gene signature/motif that encodes a seven-helix transmembrane domain. Which of the following is an example of a glycosylated, integral membrane protein with seven transmembrane segments.
Beta-Adrenergic receptor for epinephrine
This question requires that you recognize the family of receptors tha tinteract with G proteins to initiate a signal transduction cascade. These receptors are all glycosylated integral membrane proteins that have seven transmembrane segments. Beta-adrenergic receptors for epinephrine are an example.
A previously normal child begins deteriorating developmentally at about 6 months of age. She is seen by an opthalmologist because she no longer responds to visual stimuli. A cherry red spot on the macula is noted on opthalmologic examination. The enzyme that is deficient in this child normally carries otu which of the following functions?
Degradation of glycolipids in the brain
Tay-Sachs disease is a severe autosomal recessive disease characterized clinically by mental retardation, blindness, muscular weakness, and death by 3 years of age.
The chery-red spot is a classic clue for Tay-sachs disease. The disease is prevalent in Ashkenazi (eastern european) Jews. hexosaminidase A normally functions to hydrolyze a bond between N-acetylglucosamine and galactose in the polar head of the ganglioside GM2
an N-acetylneuraminic acid-containing glycolipid found in high concentration in the brain without hexosaminidase A activity, this ganglioside accumulates, leading to degenerative CNS changes.
30 y/o vegetarian presents to his physician complaining of diminished sensation in his lower extremities. he has not eaten meat for the past 15 years. A complete blood count reveals hypersegmented neurophils and elevated corpuscular volume. Which of the following findings would be expected on urine analysis?
Methylmalonic aciduria
This patient has vitamin B12 deficiency (a USMLE fav). Absence of vitamin B12 as a cofactor in the conversion of methylmalonyl-CoA to succinyl-CoA results in the accumulation of methylmalonyl-CoA, which is subsequently excreted in the urine. Methylmalonyl-CoA mutase deficiency can also cause this type of aciduria.
methylmalonyl-CoA —–(Vitamin B12)–> Succinyl CoA
Methylmalonyl-CoA mutase as well in place of B12
12 y/o boy with genetic disease. Mother is a carrier of the mutated gene, but his father is not clinically affected and is not a carrier. The man has four siblings, a sister and a brother who are not clinically affected and are not carriers, a sister who is a carrier but is not clinically affected, and a brother hwo is clinically affected. This inheritance pattern is consistent with which of the following diseases.
Alpha 1 - antitrypsin deficiency
cystic fibrosis
duchenne muscular dystrophy
phenylketonuria
Tay-Sachs disease
Duchenne-type muscular dystrophy is the only disease listed that is X-linked, the other diseases are autosomal recessive. Remembering the genetics of myriad diseases is very problematic, but here are some imperfect rules of thumb that can help you out if you encouunter an unfamiliar disease on an examination. Deficiencies of most enzymes are recessive (either autosomal or x-linked), since each person has at least two copies (from each of the parents) of each enzyme, and one working gene is usually enough (although careful evaluation of herozygotes often shows mildly altered physiology).
In contrast, alterations in structural proteins are often autosomal dominant, since having any amount of abnormal protein in structures such as basement mebranes or in collagen tends to perturb their function. The number of common x-linked diseases (almost all recessive) is small and can be memorized: Duchenne muscular dystrophy, hemophilia A and B, chronic granulomatous disease, glucose-6-phosphate dehydrogenase deficiency, agamaglobulinemia, wiskott-Aldrich syndrome, diabetes insipidus, Lesch-Nyhan syndrome, fragile X-syndrome, and color blindness.
A newborn vomits after each feeding of milk-based formula and does not gain weight. Biochemical testing reveals a severe deficiency of galactose-1-phosphate uridyltransferase, consistent with homozygosity. If this condition goes untreated, which of the following is the likely outcome for this patient?
Death in infancy
Galactosemia occurs in two very different clinical forms. Deficiency of galactokinase produces very mild disease, the only significant complication is cataract formation. In contrast, homozygous deficiency of galactose-1-phosphate uridyltransferase produces severe disease, culminating in death in infancy.
In addition to galactosemia and galactosuria, these patients have impaired renal tubular resorption leading to aminoaciduria, gastrointestinal symptoms, hepatosplenomegaly, cataracts, bleeding diatheses, hypoglycemia, and mental retardation. Pathologically, the CNS shows neuronal loss and gliosis, and the liver shows fatty change and progressing to cirrhosis.
Following the ingestion of glyburide a type 2 diabetic patient begins to experience anxiety, diaphoresis, and hunger. The patient subsequently ingests a health food bar containing glucose. The glycolytic degradation of the ingested glucose commences with the action of which of the following enzymes?
Hexokinase
the process of glycolysis is defined as the sequence of reactions that converts glucose into pyruvate with the concomitant production of ATP.
Glycolysis begins when glucose is converted by hexokinase to glucose-6-phosphate. When this compound interacts with the enzyme phosphoglucose isomerase, fructose-6-phosphate is formed. Fructose -6-phosphate is then converted by phosphofructokinase to form fructose 1,6-biphosphate, which is subsequently converted to glyceraldehyde-3-phosphate by aldolase.
After a number of enzymatic reactions, phosphoenolpyruvate is formed. Phosphoenolpyruvate is converted to pyruvate by pyruvate kinase and the glycolytic pathway is then completed.

A 24 year old woman with phenylketonuria (PKU) gives birth to first child. Although there is no history of PKU in the father’s family, the couple could not afford genetic testing of the father or consistent prenatal care. At birth, the child is small, microcephalic, and has elevated blood phenylalanine. What is the most likely explanation for this neonate’s symptoms?
- Definitions*
- phenylketonuria - an inherited disease due to faulty metabolism of phenylalanine, characterized by phenylketones in the urine and usually first noted by signs of mental retardation in infancy*
- phenylalanine - a crystalline, water-soluble, essential amino acid*
- microcephalic - having a head with a small brain case*
The child is affected at birth, indicating in the utero exposure to high levels of maternally derived phenylalanine. Women with PKU must strictly adhere to their special diet during pregnancy to avoid adversely affecting the fetus.
utero - in the uterus; unborn
The father is a carrier of a reciprocal translocation between 7p and 12q. Although this has no described phenotypic effect on the father, it is likely to increase the rate of spontanious pregnancy loss as described in this question. The most recently aborted fetus has a partial but extensive trisiomy 7 and a small partial monosomy 12. All autosomal trisomies except 21, 18 and 13 are imcompatible with life, as are autosomal monosomies. The cause of the fetal karyotype was adjacent segregation during spermatogenesis in the father.
reciprocal translocation - an exchange of segments between two nonhomologous chromosomes
karyotype - the chromosomes of a cell, usually displayed as a systematized arrangement of chromosome pairs in descending order of size
spermatogenesis - the origin and development of spermatozoa.
A 30 year old man has been fasting for religious reasons for several days. His blood glucose level is now about 60% of its normal value, but he does not feel lightheaded because his brain as reduced its need for serum glucose by using which of the following substances as an alternate energy source?
Ketone bodies, which include acetoacetate, beta-hydoxybutyrate, and acetone, are produced by the liver in the fasting state by beta-oxidation of the fatty acids. They are then released in to the blood stream, where they can be used as alternative energy sources for other organs, such as muscle, kidney, and the brain. The brain specifically still requires a small amount of circulating glucasde to function, but the amount required is reduced when ketone bodies are available.
Ketone bodies - any of three compounds, acetoacetic acid, beta-hydroxybutyric acid, or acetone, that are intermediate in the metabolism of fatty acids
A lethal mutation occurs in a bacterium, rendering it incapable of replicating its chromosome. Because of this mutation, DNA synthesis produces many short fragments of DNA that have RNA sequences at their 5’ ends. The mutation is most likely in a gene encoding

DNA polymerase I in bacteria digests the RNA primers from the 5’ ends of the short Okazaki fragments and extends the adjacet fragment in to the digested area
- Definitions:*
- DNA polymerase I (or Pol I) is an enzyme that participates in the process of prokaryotic DNA replication*
- prokaryotic - a cellular organism that has no nuclear membrane and no organelles in the cytoplasm except ribosomes*
A 2 year old boy has a past medical history significant for mental retardation, hepatosplenomegaly, foam cells in the bone marrow, and neurologic deficits. The boy dies by the age of 3. Which of the following enzymes was most likely deficient in this child?
Hepatosplenomegaly - Enlargement of the liver and the spleen.
The patient has characteristics of Niemann-Pick disease, an autosomal-recessive condition caused by a deficiency of the enzyme sphingomyelinase. This condition is associated with mental retardation, hepatosplenomegaly, foam cells in the bone marrow, cherry red spots on the maculae in 40% of cases and neurolocgic deficits, Death by 3 years of age is common
- Definitions:*
- autosomal - adjective Genetics. occurring on or transmitted by a chromosome other than one of the sex chromosomes:*
A 58 year old woman is admitted to the hospital with fever, abdominal cramps, and severe watery diarrhea. The symptoms began one day after attending a banquet. She was treated wih ciprofloxacin, but her condition did not improve. PCR amplification and analysis of a gene region in the bacteria isolated from the patient revealed a missense mutation that conferred resistance to ciprofloxacin. The missense mutation is most likely in the gene encoding an enzyme essential for which of the following functions?
Ciprofloxacin, a quinolone derivative, inhibits DNA gyrase (prokaryotic topoisomerase II) essential for DNA replication. A mutation in the gene for DNA gyrase has made the enzyme insensitive to the effects of ciprofloxacin
- quinolone - any of a group of synthetic antibiotics, including ciprofloxacin, that inactivate an enzyme required for the replication of certain microorganisms*
- DNA gyrase - a bacterial enzyme that causes supercoiling of DNA.*
A 62 year old man is prescribed a pharmaceutical agent that inhibits the activity of the enzyme HMG-CoA reductase. This patient most likely has which condition?
Familial hypercholesterolemia. The “statins” are a class of medications that inhibit the activity of the enzyme HMG-CoA reductase. Medications such as pravastatin and simvastatin produce a reversible inhibition of this enzyme, which subesquently leads to a reduction in LDL, total cholesterol, and triglycerides in patients with familial hypercholesterolemia.
Definitions:
Familial hypercholesterolemia - an inherited metabolic disorder caused by a lack or malfunction of receptors for the low-density lipoproteins that activate removal of cholesterol from the blood
hypercholesterolemia - the presence of an excessive amount of cholesterol in the blood.
A 5 year old, mentally retarded boy is brought to the city from a rural community for evaluation. A carefull history reveals mental retardation in a number of other family members, especially the males. Physical examination is remarkable for a long face with large ears, a large jaw, an bilateral enlargement of the testes. This presentation is suggestive of?
Enlarged testes are the most specific phenotypic feature to suggest Fragile X syndrome in an individual who appears to have a hereditary mental retardation. The condition has unusual genetics, as it is related to expansion of a CGG repeat sequence lacated on the X chromosome. The larger the number of repeats, the higher the probability of significant retardation: hence, the retardation tends to become more severe in successive generations, as more CGG repeats accummulate. Sisters of affected males tend to show milder retardation than their brothers.
Definitions:
Fragile X syndrome - a widespread form of mental retardation caused by a faulty gene on the X chromosome.









