Q on Resp Burst plz help

[sadaca]

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hey guys, for killing of gram positive diploccoi, which of the following enzymes is going to initiate intracellular killing? It is an NBME Q, I got it wrong even though I know the respiratory Burst steps.

1) NADPH OXIDASE
2) CATLASE
3)SUPEROXIDE DISMUTASE


Plz help, Thank you very much

 

[CherryRedDracul]

Out of those three? Well, catalase neutralizes H2O2 to H2O and O2, so that wouldn't help with intracellular killing at al. Plus, catalase+ bacteria use that enzyme to their advantage so we can definitely eliminate that answer choice.

NADPH oxidase uses NADPH and transfers its electrons to O2 to make O2- (superoxide). Superoxide dismutase transforms superoxide into H2O2. H2O2 is then reacted with Cl- with the help of myeloperoxidase (MPO) to make HOCl (bleach). That's what I remember as the main intracellular killer of bacteria in the O2-dependent system.

If I had my way, I'd go with MPO. Otherwise, I'd go with superoxide dismutase since it would still work in a NADPH oxidase-deficient person by making use of H2O2 produced naturally by other biological reactions.

 

[sadaca]

hey thanks for your help. But MPO was not an option on this question. PPl in other forums said it was NADPH oxidase but I could not find an explanation.

 

[dyspareunia]

NADPH Oxidase because it's the first step in respiratory burst.

Alternatively, NADPH Oxidase because the bacteria might have catalase, thus H2O2 from Superoxide Dismutase is worthless.. maybe there's another reaction for O2- to go down, or maybe O2- on its own can do something.

 

[CherryRedDracul]

hey thanks for your help. But MPO was not an option on this question. PPl in other forums said it was NADPH oxidase but I could not find an explanation.

I was thinking NADPH oxidase first until I read that you knew the respiratory burst steps. So I assumed that you picked NADPH oxidase and got the question wrong which is why I went with SOD. lol

NADPH oxidase would be the true initiator because it creates superoxide which is then converted by dismutase into plenty of H2O2. In other words, NADPH oxidase permits the creation of enough H2O2 so that catalase+ organisms cannot neutralize enough of it and get destroyed by one of the ultimate products of the O2-dependent pathway: HOCl.

EDIT:

To clarify, there are other end-products that are the grandchildren of NADPH oxidase, but if I'm not mistaken, they're not as powerful as HOCl from the MPO pathway. However, in MPO deficiency, you wouldn't see much in terms of symptoms except for an increased risk of Candida infections because the other redundant systems are present. In chronic granulomatous disease, you lose NADPH oxidase, and thus ALL O2-dependent pathways are shut down = disease. Sorry if I wasn't clear earlier.

 

[sadaca]

haha okay great, thanks alot! =)

 

[notbobtrustme]

that's an odd way to test respiratory burst, both because superoxide can kill bacteria and because the question asked "what initiates respiratory burst"

i'd be very surprised if the answer was SOD.

 

[Eric552]

On these types of question, by brain wants to automatically go to the enzyme closest to the end product (SOD). However, I think the key to this question is "initiate" -> NADPH Oxidase.