Proteins!!!

[peacefulheart]

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When proteins are transported to cells of the rest of body from small intestine, "The cells immediately create proteins from the amino acids so that the intracellular amino acid concentration remains low."

Why the intracelluar amino acid concentration needs to be low?

thanks .

 

[contraband]

I don't really agree with how they word this explanation. The intracellular AA concentration doesn't need to be low since active transport can be used to absorb AA. However, the cell does use the AA for protein formation and other metabolic needs, and also transports the AA across the basolateral membrane, since the overall goal of the enterocyte absorbing AA is to provide them as building blocks for the rest of the body.

 

[peacefulheart]

thanks a lot for the explanation!

 

[TieuBachHo]

The intracellular AAs remain low because it's a balancing work. I somehow disagree with Contraband on this. The AAs can be actively transported into the cells but that will cost energy. It's not a very efficient way to preserve energy. Proteins can also be broken down into AAs when needed. So it's a way for cells to conserve resources.

 

[Captain Sisko]

Someone might need to back me up on this as it's been a while since I reviewed GI but my understanding is that the active transport mechanism used to import amino acids is really a secondary active transport, really a cotransport with metal ions, I believe sodium, that's powered by the electrochemical gradient. So if they're cotransported in, drop their ion, they could simply diffuse back out if they didn't get made into something too big to leave. That seems like a reasonable explanation for why they're immediately joined to make small polypeptides. Can anyone else weigh in?

 

[image187]

Someone might need to back me up on this as it's been a while since I reviewed GI but my understanding is that the active transport mechanism used to import amino acids is really a secondary active transport, really a cotransport with metal ions, I believe sodium, that's powered by the electrochemical gradient. So if they're cotransported in, drop their ion, they could simply diffuse back out if they didn't get made into something too big to leave. That seems like a reasonable explanation for why they're immediately joined to make small polypeptides. Can anyone else weigh in?

While this sounds good, it doesn't really work like that. The mechanism you have is right though, via a 2ndary active transport where sodiums are pushed out against the gradient and they then ride back into the cell down the gradient bringing with them something else that goes against its own gradient (amino acids in this case). Every little step along the way of each cycle this Na-AA cotansporter goes through causes some change in conformation, and it only works in one direction. The two substrates come in simultaneously. A good clinical example of this to maybe make some sense of it is in digitalis, which selectively binds to the potassium slot of the Na/K pump, but jams the whole thing up so neither Na can go out or K can go in

whatever happens to the AAs once in the cell doesnt really appear to be related to this transporting business

 

[contraband]

The intracellular AAs remain low because it's a balancing work. I somehow disagree with Contraband on this. The AAs can be actively transported into the cells but that will cost energy. It's not a very efficient way to preserve energy. Proteins can also be broken down into AAs when needed. So it's a way for cells to conserve resources.

There's no need for the cell to use "extra" energy for secondary active transport into the cell - remember that all cells, not just neurons use Na-K ATPase to pump sodium out and potassium in.