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membrane-bound enzyme?

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membrane-bound enzyme?

Postby twinklestar1012 » Sun Dec 27, 2009 7:40 am

I read some biology materials saying "protein can serve as membrane-bound enzyme receptor molecule (e.g. in photosynthesis)"

but what is a "membrane-bound enzyme"?
isn't enzyme a kind of protein? how come it can be "membrane-bound"?

can anyone help me? thanks a lot =)
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Postby jwalin » Sun Dec 27, 2009 5:23 pm

enzyme is a protien for sure.
but then membrane bound?
it isn't what you do that matters but it is how you do it
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Postby kolean » Sun Dec 27, 2009 10:03 pm

Lots of words that need to be defined here. First off I shall take on enzyme. Yes, it is a protein (made up of amino acids in a specific configuration) that has a specific function in a reaction that it will help the reaction proceed (catalyst).

Now lets take on "membrane-bound". Membrane-bound means that the enzyme/catalytic protein is embeded in the membrane of the enclosed organelle of the cell it belongs to. I found a really nice example of membrane-bound enzymes, whose function is photosynthesis:
http://en.wikipedia.org/wiki/File:Thyla ... mbrane.png
(Membrane-bound enzymes can also be like the ferrodoxin and the plastocyanin, in which the enzymes are membrane-bound to only one one side of the whole membrane, or even like plastoquinone in which it is inside the membrane).

And last is "receptor". The enzyme can have its "active" site on the outside of the cell that acts like a receptor for a molecular substrate (usually called ligand) to bind to it. This will cause a reaction in the enzyme, so the bound ligand distorts the enzyme in such a way that a new 'active' site happens at the other end of the enzyme (thru the membrane), and this produces a new reaction to the inside of the cell (such as phosphorylation or proton (H+) pumping).
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Postby twinklestar1012 » Mon Dec 28, 2009 2:16 am

Thanks kolean

so... does the whole sentence mean "protein can serve as a kind of receptor molecule with membrane-bound enzyme as its ligand"?
and is a receptor necessary bound to something, say, membrane?
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Postby kolean » Mon Dec 28, 2009 6:49 pm

The whole sentence as you stated in the beginning is correct: a protein can serve as a membrane-bound receptor molecule.

Ligand is the molecule that the protein (membrane-bound receptor) receives in it's ' binding site (or a binding domain)'. Ligands could be a protein molecule, but it can also be other molecules:
http://en.wikipedia.org/wiki/Receptor_proteins

Receptor means it 'receives' something. (Baseball glove)
Ligand is the something. (Baseball)
Membrane-bound means the location: the protein is located in the membrane.
(The membrane could be of the cell/plasma membrane, or the nuclear membrane/nucleus, or the mitochondrion's outside membrane or the inner membrane, the endoplasmic reticulum, or of the any other membrane-enclosed organelles of a cell).

If you read the wikipedia entry on receptor proteins, you will see that a receptor can be bound to a membrane, but it also can just be floating around in the cytoplasm (waiting to bind to its ligand, when it becomes available).
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Postby twinklestar1012 » Tue Dec 29, 2009 4:36 am

Oh I see...
then how come the sentence includes "enzyme"?
doesn' it only mean that "protein can act as a membrane-bound receptor molecule"?
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Re: membrane-bound enzyme?

Postby kolean » Tue Dec 29, 2009 2:41 pm

twinklestar1012 wrote: "protein can act as a membrane-bound receptor molecule"

Yes. Enzyme is a way of 'acting' or 'serving'. It is what the protein can do. It is the protein's function.
So you have 'protein' - composed of amino acids in a specific configuration
'membrane-bound' - its location/where it is/what it is attached to
'receptor' - part of the function, just a specific part of being the enzyme's function.

Remember that proteins can be enzymes, but they are not only enzymes. Proteins can be structural as well as functional.
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Postby twinklestar1012 » Wed Dec 30, 2009 12:00 pm

i can understand the meaning of "protein can serve as membrane-bound receptor molecule (e.g. in photosynthesis)"
but not "protein can serve as membrane-bound enzyme receptor molecule (e.g. in photosynthesis)"
does it actually mean 2 *separate* things:
1. protein can serve as membrane-bound receptor molecule (e.g. in photosynthesis)
2. protein can serve as enzyme

or it means a kind of protein can serve as membrane-bound receptor molecule, at the same time, it is an enzyme. (2 functions for one protein)?

thanks again :)
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Re: membrane-bound enzyme?

Postby jonmoulton » Wed Dec 30, 2009 3:50 pm

Here is another way to read that sentence.

A protein, bound to the membrane, is able to act as a receptor molecule to bind to a mobile enzyme (in the case of photosynthesis, this is probably referring to a membrane-soluble electron carrier such as ferredoxin or flavidoxin).
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Postby twinklestar1012 » Thu Dec 31, 2009 7:58 am

I see~
thanks a lot jonmoulton!
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