When we have 55 bets preflop, we can't really afford to be limping in with garbage, as I would typically expect BTN to raise here as it looks like dead money from UTG and Hero.
That said, he limps behind which is likely a terrible hand that he can't fold for $2.
The flop is a question about game theory more than anything. Do you call the flop with AK and no redraw or just a naked flush draw? If so, then it's fine to call here as well.
Of course betting out in a limped pot into a field of players shows a lot of strength, in that it's either something like a nut flush draw that is hoping for a multiway pot or nuts with some kind of redraw. Given that it was an UTG limp, AKxx hands fit into this range of hands people love to limp, as does Axxx hands where the ace is suited.
I like a call on this flop, especially if BTN comes along for the ride behind us as this gives correct odds for the call (we have 33% equity with top set vs a straight, so throw in an overcall behind and the money already in the pot and it's a good call).
When the turn hit, it also shuts down the action (SB could be going for a checkraise with his nut+flush redraw hand knowing he has nothing to fear) which is great for us as we get a free card. I would not attempt a bluff here 3 ways, we are certainly behind at this point, and I doubt we get any respect.
River is a super easy fold, I imagine SB could well have something like K :diamond:9 :diamond:xx the way he played the hand.
I would never fold this flop, in fact in aggressive games I would be calling this flop with complete air in position given the number of bad turn and river cards that can hit for anyone with just a straight here.