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USMLE Step 1 General > Biochemistry > Flashcards

Biochemistry Flashcards

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USMLE® Step 1 flashcards
1
Q

What are the stop codons, what do they bind?

A

UAA, UAG, UGA. Bind releasing factor.

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2
Q

Poly-A tail added how and for what on mRNA?

A

AAUAAA = consensus sequence

Add poly-A tail to 3’ end after that so that the mRNA doesn’t get degraded after it exits the nucleus.

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3
Q

Insulin - binds what receptor, what downstream effects in the cell?

A

Insulin-R: tyrosine kinase receptor!

TK -> phosphorylates some substrates -> activation of protein phosphorylase

  1. dephos glycogen synthase - activates - glycogen synthesis
  2. dephos fructose 1,6 bisphosphatase - inactivates - inhibits gluconeogensis.
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4
Q

How can thiamine deficiency be diagnosed?

A

Measuring erythrocyte transketolase activity.
Thiamine involved in enzymes that help utilize glucose.
1. pyruvate dehydrogenase
2. alpha-ketogluturate dehydrogenase
3. transketolase

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5
Q

Which enzyme has 5’ to 3’ exonuclease activity?

A

DNA Poly I - removes RNA primer.

Also has same fxns as DNA Poly III - 5’ to 3’ synthesis and 3’ to 5’ exonuclease.

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6
Q

What are factors that utilize tyrosine kinase receptors with intrinsic enzyme activity vs. tyrosine kinase associated receptors and JAK/STAT pathway?

A

Tyrosine Kinase-R: Insulin, PDGF, FGF, EGF JAK/STAT: Cytokines, Growth Hormone, Prolactin, IL2

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7
Q

How does radiation induce cell death?

A

Causes dsDNA breaks!

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8
Q

How does hyperammonia influence metabolic intermediates in the brain?

A

Glutamate-Glutamine Cycle
1. Decreased Glutamate, Increased Glutamine (Glu + HN4 -> Glutamine in astrocytes). Glutamine cause astrocyte swelling and dysfunction

  1. Decreased alpha-ketoglutarate (alpha-keto + NH4 -> Glutamate in neurons). Depletion causes inhibition of Krebs cycle.
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9
Q

What biochemical processes occur in the cytoplasm?

A

GlycolysisFA synthesis HMP shunt (pentose phosphate shunt) Protein synthesis (RER) steroid synthesis (SER)cholesterol synthesis

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10
Q

What biochemical processes occur in BOTH the cytoplasm and mitochondria?

A

Heme synthesis
Urea cycle
Gluconeogenesis (HUGS take two)

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11
Q

Metabolism of what amino acids may lead to propionic acidemia?

A

Valine
Isoleucine
Threonine
Methionine

Deficiency of proprionyl CoA carboxylase

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12
Q

Gestational diabetes is caused by a decrease in what? If this is genetically predisposed, what enzyme would have decreased activity?

A

Glucokinase - beta cells of the pancreas -> once in glucolysis -> ATP -> close ATP sensitive K channels -> Ca let in -> release of insulin.

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13
Q

What are the sx of phenylketonuria?

A

PKU: deficiency of tyrosine hydroxylase or BH4

Sx: 
intellectual disability
seizures (metabolites of phenylalanine) 
growth retardation 
hypopig skin (no dopa -> melanin) brain nuclei 
eczema
musty odor
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14
Q

What causes accumulation of ceramide trihexoside?

A

Fabry’s disease: X-linked

Deficient in alpha-galactosidase. 
Peripheral Neuropathy
Angiokeratomas 
Renal disease - progressive renal failure
Cardio disease
Hypohidrosis (dec. sweating)
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15
Q

What is pantothenate used for?

A

Oxaloacetate + Acetyl CoA -> Citrate (Citrate synthase)

Fatty Acid synthesis: Acetyl CoA carboxylase It is an essential component of coenzyme A and fatty acid synthase.

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16
Q

What enzyme is defective in xeroderma pigmentosum?

A

Endonuclease. Excise stretches of DNA which contain the pyrimidine dimers formed by UV light.

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17
Q

Deficiency of what vitamin causes peripheral neuropathy, sensory and motor impairments + cardiac failure, edema?

A

B1 = Beri Beri

Wet beri beri = nerve + heart

Dry beri beri = nerve.

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18
Q

What is BH4 used to synthesize? (cofactor)

A

Tyrosine
Dopa
Serotonin
NO

Dihydrobiopterin reductase deficiency (converts BH4 -> BH2) can result in PKU.

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19
Q

What vitamin can present with a long delay btwn insufficient intake and sx?

A

Vitamin B12 - stored in liver.

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20
Q

B6 is used in what reactions?

A

Transamination: Amino acid + alpha-keto acid; transfer of the amino group

Decarboxylation

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21
Q

What does arginase do?

A

Arginine -> Urea + Ornithine Part of Urea cycle.

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22
Q

Metabolism of protein, fat, carbohydrates.

A

4 cal = 1g of protein, carb

9 cal = 1g fat

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23
Q

Riboflavin is needed for what enzyme?

A

succinate dehydrogenase think what needs FAD!

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24
Q

What liver specific enzyme facilitates metabolism of TAG breakdown products?

A

Glycerol kinase. Q1889

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25
Q

What are the two causes of PKU? How do you differentiate it?

A

PKU:
1. dec. phenylalanine hydroxylase

  1. dec. BH4 cofactor - with this, tyr -> DOPA is also decreased, so even if you substitute tyr, will still have dec. dopa -> elevated prolactin.
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26
Q

Vitamin C deficiency causes what?

A

Scurvy

Cause: defective collagen synthesis.

Sx: hemorrhages, subperiosteal hemorrhages, hemarthrosis, gingival swelling, 2nday periodontal infection, anemic, impaired wound healing, weakened immune response, hyperkeratotic papular rashes,

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27
Q

ALL the reactions of the Urea cycle >_>

A

poop!

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28
Q

What are the function of the different apolipoproteins? Apo AI, Apo B48, Apo B100, Apo CII, Apo E

A

ApoE = mediates chylomicron and VLDL remnant uptake

Apo AI = activate LCAT (catalyzes esterification of cholesterol in HDL particles)

Apo CII = lipoprotein lipase cofactor (on vascular endothelial surface - degradation of TG circulating in chylomicrons and VLDLS)

Apo B48 = chylomicron secretion from intestine

Apo B100 = binds LDL-R

29
Q

Deficiency of what vitamin causes pancreatic columnar -> squamous metaplasia?

A

Vitamin A - can be seen in CF due malabsorption of ADEK.

30
Q

Activating mutations of PRPP predispose to what condition?

A

Gout - inc. production of purines.

31
Q

What condition causes urine to turn black when allowed to stand?

A

Alkaptonuria: deficiency of homogentistate oxidase: can’t convert tyrosine -> fumurate.

Accumulation of homogentisic acid, which selectively binds collagen causing “ochronosis” a blue black pigmentation most evident in ears, nose, cheek.

32
Q

Glycogen Storage Diseases:Von Gierke Disease Pompe CoriMcArdle

A

Von Gierke = Glucose 6 phosphatase - inc. glycogen in liver = hepatomegaly - can’t release glucose = hypoglycemia- inc. lactic acidosis

Pompe = Lysosomal alpha 1,4 glucosidase- nl blood sugar, but glycogen accumulates in lysosome
cardiomegaly
liver failure

Cori = debranching enzyme (alpha 1,6)
small dextrin like material acccumulates within hepatic cytosol
same sx as Von Gierke, but no lactic acidosis

McArdle = muscle glycogen phosphorylase
lots of glycogen in muscle, but can’t break it down weakness, fatigue with exercise
muscle cramps, myoglobinuria- predisposed to arrhythmias due to electrolyte abnl.

33
Q

MAP-kinase pathway:

A

Binding of GF ligand -> auto tyrosine phosphrylation -> SOS protein -> activation of RAS with GTP -> RAF -> MAP kinase -> gene transcription

34
Q

Base Excision Repair order of enzymes

A

Glycosylase (removes the altered base)

Endonuclease (5’ stand ends)

Lyase (3’ strand ends) DNA polymerase Ligase

35
Q

Lipoic Acid is a cofactor for what 3 enzymes? What would deficiency result in?

A

Pyruvate Dehydrogenase -> will get lactic acidosis

Alpha Ketogluturate Dehydrogenase

Alpha Ketoacid Dehydrogenase (branched ketoacid dehydrogenase - Isoleucine, Leucine, Valine) -> leads to maple syrup disease

36
Q

Fructose 2,6 Bisphosphate would decrease what?

A

Gluconeogenesis.

37
Q

What enzyme deficiency is associated with hypoglycemia and low ketones after prolonged fasting?

A

Messed up beta-oxidation.Acyl CoA Decarboxylase.

38
Q

Night blindness + Skin dryness + pruritis = indicative of what

A

Vitamin A deficiency Which can result from fat malabsorption, such as with biliary obstruction.

39
Q

Palmitoylation

A

Increases protein hydrophobicity Anchor carboxyl tails to the plasma membrane.

40
Q

Platelet aggregation associated with intracellular signaling mechanism?

A

Platelet Activating Factor:

Gq Phospholipase C -> IP3 -> Ca++ release from ER

41
Q

Neurofibromatosis 1 inheritance, penetrance? How can NF1 arise in children with parents without the disease?

A

Autosomal Dominant
100% penetrance
Variable expressivity

NF1 can arise in children whose parents are unaffected if there is germline mosiacism (novel germline mutation).

42
Q

What oncoprotein is involved in signal transmission from cell surface to nucleus?

A

Ras - GTPase”myc” = transcription factor

43
Q

What percentage of energy per glucose molecule metabolized is produced in RBC vs. CNS neurons?

A

Anaerobic = 2 Aerobic = 385%

44
Q

Wild Mushroom Toxin (Amatoxin) does what?

A

Inhibits RNA Polymerase II
no mRNA
Life-threatening!

45
Q

G6PD is involved in what reaction of the HMP shunt?

A

Glucose 6 P —> 6 phosphogluconate + NADPH

46
Q

What enzyme is responsible for the cataracts developed in galactosemia?

A

Aldose Reductase.

Galactose —> Galactitol.

47
Q

How is lac operon regulated?*

A

No Lactose, Glucose: NO transcription

  1. Repressor protein binds to operator - inhibiting transcription.
  2. GLUCOSE causes DECREASED cAMP. CAP can’t bind upstream of promoter.

Lactose, No Glucose: Transcription.

  1. Lactose binds repressor - it doesn’t bind operator
  2. cAMP-CAP complex binds upstream of promoter - positive regulator.
48
Q

Very LCFA and phytanic acid are oxidized in what part of the cell?

A

Peroxisome!

49
Q

Condition associated with Lipoprotein lipase deficiency or altered apo CII (cofactor for LPL)?

A

Increased chylomicrons
Causes acute pancreatitis!!
Also HSM, eruptive/pruritic xanthomas. But NO increased risk for atherosclerosis.

50
Q

Familial Hypercholesterolemia

A

AD = Absent or defective LDL-R

Premature CAD
Corneal arcus, Tendon Xanthoma, Xanthelasma

51
Q

Familial Dysbetalipoproteinemia

A

ApoE - Inc chylmicron and VLDL remnants

Premature CAD and PVD

Tuboeruptive and Palmar Xanthomas.

52
Q

HyperTAG

A

AD = hepatic overproduction of VLDL Pancreatitis!

53
Q

prokaryotic 16S rRNA used for what?

A

16s rRNA = 30S

  1. Sequence complentary to Shine Dalgarno sequence on all mRNAs - help bind the mRNA
  2. Bind the incoming aminoacyl tRNA
54
Q

Glutamate residue carboxylation of proteins synthesized in liver - need what vitamin?

A

Vitamin K.

55
Q

What must be known for PCR reaction?

A

Nucleotide sequence of the primers flanking the DNA. For PCR, need:

DNA template (doesn’t have to be known)
Two primers - sequence must be known
DNA polymerase
DNA nucleotides

Steps:

  1. Denature
  2. Anneal
  3. Elongation
56
Q

How does cortisol influence catecholamine synthesis?

A

In adrenal medulla, NE —– phenylethanolamine-N-methyltransferase–> Epi PMNT transcription increased by cortisol.

Venous drainage of cortex goes thru medulla, so cortisol has effect and 80% medulla secretion = Epi.

57
Q

Vitamin E deficiency

A

Mech: Antioxidants - protect RBC and cells with high lipid content, such as neurons, from oxidative injury.

Sx: Hemolytic Anemia, Acanthocytosis. Posterior column and spinocerebellar tract demyelination (like B12) Muscle weakness

58
Q

How is glycogen phosphorylase regulated?

A
  1. Glucagon (liver) and Epinephrine (muscle) - Gs -> inc. cAMP -> PKA -> +PO4 on phosphorylase kinase (activate) -> + PO4 on glycogen phosphorylase (activate)-> + PO4 on glycogen synthase (inactivate)
  2. Muscle - Ca++ from contraction -> allosteric regular -> increase activity of glycogen phosphorylase
59
Q

Sorbital Metabolism

A

Glucose –aldose reductase–> Sorbital – sorbital dehydrogenase –> Fructose

Both enzymes: liver, ovaries, seminal vesicles (sperm use fructose as primary energy source)

Both enzymes, but gets overwhelmed in states of longstanding hyperglycemia (DM) : lens - leads to lens opacification, cataracts.

No sorbital dehydrogenase: schwann cells - neuropathy, retina - retinopathy, renal papilla

60
Q

What are the 3 main reactions that require NADPH (and therefore G6PD)?

A
  1. Reduction of glutathione inside RBC
  2. FA synthesis
  3. Cholesterol synthesis.
61
Q

What cells use bisphophoglycerate mutase?

A

Erythrocytes1,3 bisphosphoglycerate -> 2,3BPG sacrifice making ATP in the process. During hypoxemia and chronic anemia.

62
Q

Sx of Alkaptonuria

A

Mech: Deficiency of homogentisate oxidase (AR) - metabolism of tyrosine to fumurate

Sx:
dark connective tissue
brown pigmented sclerae (ochronotic pigment - deposits of homogenistic acid)
urine turns black on prolonged exposure to air
debilitating arthralgias* (homogenistic acid is toxic to cartilage)

63
Q

DNA synthesized in what direction

A

5’ -> 3’ SO READ THE DNA CORRECTLY!!!

64
Q

Some sx of Ehler’s Danlos similar to what vitamin deficiency?

A

Sx:

  1. Hyperextensible skin
  2. Hypermobile joints
  3. Tendency to bleed/Easy Bruising
Different Variants:
Hypermobile type (joint instability)
Classical type (joint and skin) - collagen V 
Vascular type (vascular and organ rupture) - collagen III

Mech:
Kyphoscoliosis type
Lysyl Hydroxylase deficiency - can’t add hydroxy group to lysine and proline residues on procollagen.

Similar to Vit C deficiency (cofactor).

65
Q

Insulin increases what in terms of glycolysis?

A

Fructose 2,6 Bisphosphate Insulin -> dephosphorate the PFK-2/FBPase complex -> more activity of PFK-2 -> fructose 6 phos to fructose 2,6 bisphosphate -> activates PFK1 -> more glycolysis!

66
Q

What synthesizes apoB100 vs. apoB48?

A

apoB48 = synthesized by intestine = mediates chylomicron secretion (cytidine to uridine deamination reaction on the transcribed

apoB100 mRNA resulting in premature stop codon).

apoB100 = synthesized by hepatocytes

67
Q

Enzyme that converts Dopamine -> NE?

A

Dopamine beta-hydroxylase.

68
Q

Restriction Fragment Length Polymorphism (RFLP)s - most common cause of different RFLPs

A

Single nucleotide polymorphisms (SNP) bc restriction enzymes depend on recognition of a specific sequence of base pairs in order to cleave DNA.

69
Q

nuclear protein with repeated leucine residues at every 7th position.

A

leucine zipper!
nucleic acid binding domain.

Others: zinc finger, helix loop helix.

PLEASE READ.

USMLE Step 1 General (7 decks)

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