An ace can be used as a 1 for a five-high straight, as per your example.
Otherwise, it HAS to be used as a high card. Here's an example of THIS.
Supposing the cards on the board are: J-J-77-A; you have K-9 and your opponent has Q-9. You CAN'T declare the ace to be low, which would make you the winner. The ace HAS to count as high, meaning you and your opponent have a tie, & must split the pot.
The only 2 exceptions to this rule are in the game of lowball, in which aces ALWAYS count as one; and in any high-low split game, where you can CHOOSE whether or not your ace is high or low. In high-low split, you can even count your ace as high when determining who has the high hand, and THEN turn around and call that SAME ace as LOW in order to also win the other half of the pot with the lowest hand!