BIOCHEMISTRY USMLE QUESTIONS: BIOCHEMISTRY PDF

B. Isocitrate dehydrogenase:

1. This enzyme is 1/3 regulatory enzymes in the TCA cycles and the RATE LIMITING ENZYME OF THE TCA CYCLE. *conversion of Isocitrate --> AKG the first of the multiple producing steps in the TCA cycle that produces NADH and CO2 (hint its a DEHYDROGENASE) so it can later donate its hydrogen ions.

#15 During an investigational study, colloidal gold labeled antiboides are used to characterize the distribution of macromolecules in subcellular compartments. This technique is used to localize enzymes involved in intermediary metabolism. An antibody to which of the following enzymes will most likely selectively label the area indicated by arrow 1 in the electron micograph shown on the flip side?

A. Carnitine acytransferase II B. Fatty acyl coa synthetase C. glucose 6 phosphte dehydrogenase D. Hexokinase E. Pyruvate Kinase Explained on following notecards.

Glycolysis Question: #13 An investigator is measuring the activity of various enzymes involved in reactions of intermediary metabolism (glycolysis, TCA cycle, etc). The activity of one of the enzymes is greatly DECREASED compared to reference values. The buffer used for the enzyme assay contains sodium CITRATE. The activity of which of the following enzymes will MOST LIKELY be directly affected by the use of CITRATE? A. Fructose-2,6,-bisphophatase B. Iscocitrate dehydrogenase C. Phosphofructokinase 1 D. Pyruvate carboxylase E. 6 Phosphogluconate dehydrogenase

Answer is C. Citrate allosterically inhibits (-) PFK1, the rate limiting enzyme in glycolysis, which inhibits the conversion of: Fructose 6 phospahte--(PFK1)-->fructose 1,6 bisphosphate.

C. glucose 6 phosphate dehydrogenase... D. hexokinase .... E. pyruvate kinase ...

C. glucose 6 phosphate dehydrogenase, the first enzyme in the pentose (5c sugar) phosphate pathway is an irreversible enzyme that converts glucose 6 phosphate into g phosphogluconolatone, D. hexokinase which converts glucose to glucose 6 phosphate and requires 1 ATP but forms 1 NADH and is involved in GLYCOLYSIS and E. Pyruvate kinase vonverts PEP into Pyruvate are all located AWAY from the mitochodniral inside the CYTOPLASM.

A. correct answer Carnitine acytransferase II (CATII) or carnitine palmitoyl transferase II (CPT II).

In the blue top LEFT. The reason being arrow 1 is pointing to the Inner mitochondrial membrane and the 2 enzymes complexes lie on this membrane. 1. carnitine acycarnitine translocase (which moves the acyl carnitine into the mitochondiral matrix via the Inner membrane and 2. Carnitine acyltransferase II (which recycles the carnitine back into the intermembrane space to be used for another fatty acyl coa moleulce to be brought through the inner membrane space into the MM). This recycling is what takes the fatty acyl carnitine ---fatty acyl (coa, gained) to be converted into ACETYL COA by FATTY ACYL COA DEHYDROGENASE CAT1 (CPT1) is the other enzyme but this enzyme converts which helps convert the fatty acyl coa into fatty acyl carnitine is located on the OUTER MITOCHONDRIAL MEMBRANE. Fatty acyl carnitine has to be converted first to allow LONG CHAIN fatty acids into the mitchondrial matrix for B oxidation but SMALL and MEDIUM DONT REQUIRE CARNITINE.

D. phenylalaine hydroxylase deficiency (phenylketonuria)

Mandatory screening once born.

#19 A 3 month old boy has delayed motor development. nine months later, he develops spasticity and writhing movements. Two years later, he exhibits compulsive bitting of fingers and lips and constantly bangs his head. By age 14 he develops arthritis and by age 20, he develops renal failure. which of the following is most likely to be increased in serum in this patient? A. Galactitol B. glucocerebroside C. Monosodium urate D. Orotic acid E. phenylalanine

c. monosodium urate crystals (uric acid accumulation). This patient has the classic case of leschn nyhan syndrome (HGPRT) deficiney. this is an x-linked recessive disorder. H: hyperuricemia (increased uric acid crystals) G: GOUT P: Pissed off, anger, aggression (bitting his fingers) R: retard (mentally slow) T: DysTonia ( spastic movements)

E. 6 phosphogluconate dehydrogenase:

converts 6 phosphogluconate ---> ribulose 5 phosphate in the pentose phospahte pathway (HMP shunt), and In the process NADP+ gets reduced into NADPH (reduced is when you obtain the (H ions, electrons). Having Hs is reducing over time. HELL is below REDUCED. When you donate those you are now oxidize (NADP+, no H+/electrons). NADP+ gets reduced to NADP(H) for biosynthesis in the liver, for generating superoxide to kill bacteria (neutrophils), and for protecting erythrocyte membranes from oxidative damage and hemolysis (RBC destruction, can be caused by a build up of pyruvate).

D. Pyruvate carboxylase:

is a mitochondrial enzyme that converts: Pyruvate --> oxaloacetate regenerating the levels of oxaloacetate and be used in gluconeogenesis! (oxalacetate can then be converted back into PEP via PEP carboxykinase to go through the gluconeogenesis steps. #15 pyruvate carboyxlase requires activation from acetyl coa AND BIOTIN COFACTOR. that donates CO2 groups.

A. Fructose 2,6 bisphosphatase:

is an enzyme that breaks down Fructose 2,6 bisphosphate into BACK into fructose 6 phosphate for gluconeogenesis. Fructose 2,6 bisphosphatase is involved in forming NEW GLUCOSE to raise blood glucose levels vs BREAKING DOWN GLUCOSE. 1. Inhibitor (-) of fructose 2,6 bisphosphate formation which is an activator + of PFK1 and glycolysis but also an inhibitor - of GLUCONEOGENESIS by inhibiting - FRUCTOSE 1,6 BISPHOSPHATASE converting fructose 1,6 phosphate ---> fructose 6 phosphate---into glucose (because in glycolysis we WANT to BREAKDOWN glucose, NOT REFORM IT. 2. Fructose 2, 6 bisphosphatase is activated by cyclic AMP dependent protein kinase (ATP--adenylate cyclase--> cAMP + Protein kinase A + glycogen phosphorylase (glycogen---> NEW GLUCOSE via glycogenolysis).

B. Fatty acyl syntheatse

lies on the outermembrane which converts the free fatty acids + coa + ATP!!! into FATTY ACYL COA.

Arrow 5: is pointing to the mitochondrial matrix where ALL BUT ONE enzyme of the TCA cycle are located. Which enzyme is not located in the mitochondrial matrix? the enzymes of B oxidation, pyruvate dehydrogenase, ketogenesis (liver) and ketogenesis (EHT) are also all located.

succiante dehydrogenase because this enzyme is also known as complex II of the ETC that receives FADH2 by converting succiante ---> fumarate, but LIES ON THE INNER MEMBRANE and not within the MATRIX. NOT succinate thiokinase (that comes before succinate dehydrogenase) that converts succinyl coa into succinate (ASe)