Im a second year so the details of the pathway your referring to are fuzzy for me @ best but I do remember having the same confusion because I think that there is @ least one case in fat metabolism where there are two steps that seem like they could be rate limiting (for reasons that, again, I dont remember) but....
Things can be rate limiting for a few reasons: 1) A key "reactant" for the step is present in the most limited supply compared to other reactants in the system 2) It is the slowest step 3) It is the "committed" step.
I think that FA transport into the mitochondria is rate limiting because it is the "committed step" It's similar to how glucose needs to be phosphorylated so that it doesn't "leak" back out of the cell. Regardless of how slow subsequent steps proceed, if glucose is not present in the cell glycolysis/glycogenisis/whatever its destined for will proceed very inefficiently if @ all. Again, its been a while so Im not sure if we're thinking of the same pathway but I def remember having to look this up and this was my conclusion. Hope that helps.
