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USMLE > Biochemistry First Aid > Flashcards

Biochemistry First Aid Flashcards

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USMLE® Step 1 flashcards
1
Q

Steps in de novo purine synthesis

A
  1. Start with sugar and phosphate (PRPP) 2. Add base
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2
Q

Steps in de novo pyrimidine synthesis

A
  1. Make temporary base (orotic acid) 2. Add sugar and phosphate (PRPP) 3. Modify base
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3
Q

What 2 metabolic pathways is carbamoyl phosphate involved in?

A
  1. De novo pyrimidine synthesis 2. Urea cycle
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4
Q

What is Ornithine transcarbamoylase deficiency (OTC)?

A

OTC is a key enzyme in the urea cycle. Deficiency leads to an accumulation of carbamoyl phosphate, which is then converted to orotic acid. There hyperammonemia associated with OTC’s increased orotic acid; as opposed to no hyperammonemia in orotic acidura.

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5
Q

What are the various antineoplastic and abx drugs that function by interfering with nucleotide synthesis?

A
  1. Hydroxyurea 2. 6-MP 3. 5-FU 4. MTX 5. Trimethoprim
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6
Q

What does Hydroxyurea inhibit?

A

Inhibits ribonucleotide reductase. This causes an increase in UDP.

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7
Q

What does 6-MP block?

A

Blocks de novo purine synthesis

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8
Q

What does 5-FU inhibit?

A

Inhibits thymidylate synthase (causes a decrease in TMP)

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9
Q

What does MTX inhibit?

A

Dihydrofolate reductase (causes a decrease in dTMP)

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10
Q

What does trimethoprim inhibit?

A

Inhibits bacterial dihydrofolate reductase (decrease in dTMP)

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11
Q

What is orotic aciduria? Findings? Treatment?

A

What is it: Autosomal recessive. Inability to convert orotic acid to UMP due to defect in either orotic acid phosphoribosyltransferase or orotidine 5’-phosphate decarboxylase.

Findings: Increase orotic acid in urine, megaloblastic anemia (does not improve with administration of folic acid or B12), failure to thrive, no hyperammonemia.

Treatment: Oral uridine

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12
Q

What happens when you have ADA deficiency? What disease do you get?

A

Excess ATP and dATP imbalances nucleotide pool via feedback inhibition of ribonucleotide reductase which leads to prevetion of DNA synthesis, thus decrease in lymphocyte count. One of the major causes of SCID

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13
Q

What is SCID?

A

Severe combined immunodeficiency disease. Happens to kids. 1st disease to be treated by experimental human gene therapy

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14
Q

What is Lesch-Nyhan Syndrome?

A

X-linked recessive. Defective purine salvage owing to absence of HGPRT, which converts hypoxanthine to IMP and guanine to GMP. Results in excess uric acid production and de novo purine synthesis.

Findings: retardation, self-mutilation, aggression, gout, hyperuricemia, choreoathetosis (involuntary movements in combo of chorea) “

He’s Got Purine Recovery Trouble”

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15
Q

What single stranded DNA repair system is mutated in Xeroderma Pigmentosum?

A

The nucleotide excision repair system [specific endonucleases release the oligonucleotide-containing damaged bases; DNA polymerase and ligase fill and reseal the gap, respectively]

In Xeroderma pigmentosum, this process is not working which prevents the repair of thymine dimers due to UV light.

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16
Q

What single stranded DNA repair system is mutated in HNPCC?

A

The mismatch repair system is mutated.

Mismatch repair system function: unmethylated, newly synthesized, mismatched nucleotides are removed, and the gap is filled and resealed.

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17
Q

What double strand DNA repair system is mutated in ataxia telangiectasia?

A

Nonhomologous end joining.

Function: brings together 2 ends of DNA fragments. No requirement for homology.

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18
Q

mRNA stop codons:

A

UGA, UAA, UAG

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19
Q

Which RNA polymerase does alpha-amanitin inhibit?

A

RNA polymerase II. Causes liver failure if ingested. Found in death cap mushrooms

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20
Q

Mechanism of tetracyclines?

A

Binds to 30S, preventing attachment of aminoacyl-tRNA

21
Q

Mechanism of action for Aminoglycosides

A

Binds to 30S. Inhibits formation of the initiation complex and cause misreading of mRNA

22
Q

Mechanism of action for Chloramphenicol

A

Inhibit 50S peptidyltransferase

Clindamycin and chloramphenicol block peptide bond formation

23
Q

Mechanism of action for Macrolides

A

Blocks translocation

24
Q

Role of Rough Endoplasmic Reticulum (RER)?

A

Site of synthesis of secretory (exported) proteins and of N-linked oligosaccharide addition to many proteins.

Nissl bodies (RER in neurons): synthesize enzymes (ChAT - choline acetyltransferase, makes Ach) and peptide neurotransmitters.

Free ribosomes: unattached to any membrane; site of synthesis of cytosolic and organellar proteins.

25
Q

Role of SER?

A

Site of steroid synthesis and detoxification

26
Q

Which cells are rich in RER?

A

Mucus-secreting goblet cells of the small intestine and antibody-secreting plasma cells

27
Q

What cells are rich SER?

A

Liver hepatocytes and steroid hormone producing cells of the adrenal cortex

28
Q

What is I-cell disease?

A

Inclusion cell disease: inherited lysosomal storage disease. Failure of addition of mannose-6-phosphate to lysosome proteins, thus enzymes are secreted outside the cell isntead of being targeted to the lysosome.

Findings: Coarse facial features, clouded corneas, restricted joint movement, and high plasma levels of lysosomal enzymes.

Often fatal in childhood.

29
Q

What drugs act on microtubules?

A
  1. Mebendazole/thiabendazole (antihelminthic)
  2. Griseofulvin (antifungal)
  3. Vincristine/vinblastine (anti-cancer)
  4. Paclitaxel (anti-breast cancer)
  5. Colchicine (anti-gout)
30
Q

What is Chediak-Higashi syndrome?

A

Microtubule polymerization defect resulting in decrease fusion of phagosomes and lysosomes.

Findings: recurrent pyogenic infections, partial albinism, and peripheral neuropathy.

31
Q

What is Kartagener’s syndrome?

A

Autosomal recessive microtubular defect –> Immotile ciila due to dynein arm defect.

Findings: Male and female infertility (sperm immobile), bronchiectasis, and recurrent sinusitis (bacteria and particiles not pushed out). Associated with situs inversus.

32
Q

What are the different immunohistochemical stains for intermediate filaments (different cell types)

A

Stain Cell Type

  1. Vimentin Connective Tissue
  2. Desmin Muscle
  3. Cytokeratin Epithelial cells
  4. GFAP NeuroGlia
  5. Neurofilaments Neurons
33
Q

Type I Collagen:

A

Bone, Skin, Tendon, dentin, fascia, cornea, late wound repair

Type I: BONE. Defective in osteogenesis imperfecta

34
Q

Type II Collagen:

A

Cartilage (including hyaline), vitreous body, nucleus pulposus

Type II: carTWOlage

35
Q

Type III:

A

Reticulin: skin, blood vessels, uterus, fetal tissue, granulation tissue

Type III: Defective in Ehlers-Danlos (ThreE D)

36
Q

Type IV:

A

Basement membrane and basal lamina

Type IV: Under the floor (basement membrane)

Defective in Alport syndrome

37
Q

Steps to collagen synthesis:

A

Inside fibroblasts

  1. Synthesis (RER)
  2. Hydroxylation (ER)
  3. Glycosylation (ER)
  4. Exocytosis

Outside fibroblasts

  1. Proteolytic processing
  2. Cross-linking
38
Q

What is Osteogensis imperfecta?

A

Genetic bone disorder (brittle bone disease). May be confused with child abuse.

Most common form is autosomal dominant with abnormal Type I collagen, causing:

  • Multiple fractures with minimal trauma; may occur during the birth process
  • Blue sclerae due to translucency of the CT over the choroid
  • Hearing loss (abnormal middle ear bones), Dental imperfections due to lack of dentin

Type II is fatal in utero or in the neonatal stage.

39
Q

What is Ehlers-Danlos syndrome?

A

Type III collagen most frequently affected. Faulty collagent synthesis causes:

1. Hyperextensible skin

2. Tendency to bleed (easy bruising)

3. Hypermobile joints

6 types. Inheritance and severity vary. Can be AD or AR. May be associated with joint dislocation, berry aneurysms, organ rupture.

40
Q

What is Alport Syndrome?

A

Abnormal Type IV collagent. X-linked recessive (MC)

Characterized by: Progressive hereditary nephiritis and deafness. May be associated with ocular disturbances.

Type IV collagent is an important structural component of the BM of the kidney, eyes, and ears.

41
Q

What is Prader-Willi Syndrome?

A

Individual has normally inactivated maternal allele. Paternal allele should be active but is deleted.

Features: mental retardation, hyperphagia, obesity, hypogonadism, hypotonia

42
Q

What is Angelman’s syndrome?

A

Individual has normall inactivated paternal allele. Maternal allele should be active but is deleted.

Features: mental retardation, seizures, ataxia, inappropriate laughter (“happy puppet”)

43
Q

What is hypophosphatemic rickets?

A

X-linked dominant disease resulting in increased phosphate wasting at proximal tubule. Results in rickets-like presentation.

X-linked disease: either male or female offspring of the affected mother may be affected, while all female offspring of the affected father are diseased

44
Q

X-linked dominant

A

X-linked disease: either male or female offspring of the affected mother may be affected, while all female offspring of the affected father are diseased

45
Q

Mitochondrial inheritance

A

Transmitted only through mother. All offspring of affected females may show signs of disease. Often due to failures in oxidative phosphorylation

46
Q

What are the 22q11 deletion syndromes?

A

DiGeorge Syndrome (thymic, parathyroid, and cardiac defects)

Velocardiofacial syndrome (palate, facial, and cardiac defects)

47
Q

What are the symptoms/presentation of 22q11 deletion syndromes?

A

Cleft palate, Abnormal facies, Thymic aplasia –> T-cell deficiency, Cardiac defects, Hypocalcemia 2ndary to parathyroid aplasia, due to microdeletion at chromosome 22q11

“CATCH-22”

Due to abberant development of 3rd and 4th branchiel pouches

48
Q
A

USMLE (2 decks)

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