If you are wanting to use <alt>-<space> as the key for the excellent Ulauncher with Ubuntu, there are a couple of tweaks that you need to do:
1. Ensure that CompizConfig Settings Manager is installed:
apt-get install compizconfig-settings-manager
2. Start the CompizConfig Settings Manager
Go to General, General options, Key bindings and disable Window Menu which is assigned <alt>-<space> by default.