@gettheleadout
"Yeah turns out it is. Maybe I'm just forgetting my cell biology, but what happens to active enzymes in a cell? Why are newly synthesized enzymes needed? Do the present enzymes get transported out of the cell/degrade or what?"
This might be too much information for the MCAT, but active enzymes are degraded in a normal cell all the time. Yeah, that might sound weird, but protein turnover is pretty common. A fine example of something that degrades proteins is a proteasome...the protein can not be needed any more (say insulin when there is enough insulin already in the cell) or when it has gone bad (it hit a reactive oxygen species or something weird). If you are really low on energy, the body will begin to metabolize proteins, too (as you can imagine, this is not a good thing). My professor told me we don't know exactly what qualifies a protein's age, but sometimes carbohydrates normally found on the protein have fallen off through a hydrolysis and that can tell the proteasome that it is old and needs to be broken down.
Newly synthesized enzymes are needed in new environmental conditions as one example (more true for bacteria than eukaryotes): say a bacterium has been transported into a very cold environment. To increase the fluidity of the membrane to prevent freezing, it will add more cholesterol to its membrane. It can do this by making more of the enzymes used in cholesterol synthesis. Transcriptional regulation is key here. Sometimes, too, when bacteria become endospore, you've got totally new proteins that are needed that are usually not needed.
Transcription and translation are pretty common processes in cells: we need to break down proteins that are old or we need more because they've been secreted or environmental conditions have changed.
To look at some *super* cool examples, check out the trp operon or iron response elements. It's sweet stuff,
~Ibrahim~
