Do Protestants bow towards the altar? Apparently Anglicans do, as I have seen members of the British Royal Family do it many times, but for Calvinists like the Dutch Royal Family, I suppose that would be unusual. Calvinist churches don't even have an "altar" strictly speaking and, in any case, the altar is not the center of Reformed Church services nor is church seating organized to face an altar as in Catholic churches.
For Catholics, the custom would be to "curtsy" to the altar, rather than to bow. Technically it should be deeper than an actual curtsy as one knee should touch the floor, but most people I know just curtsy instead without going as low as touching the floor . Technically also, you should only do it when the consecrated elements are stored behind the altar, but that is normally the case in churches before and immediately after a celebration of the mass for example.
Protestants, I believe, do not store non-consumed consecrated elements after a service, nor do they accept the consumption of elements that were consecrated in private since reformers like Luther emphasized that public consecration was a requirement for the validity of the sacrament.
At least that is my understanding not being particularly proficient in Protestant doctrine. I may be wrong.