You denature any enzyme above a certain temperature. Beta amylase is denatured staring around 144°F, but alpha amylase hits its peak activity around 158°F. So if you start at 158°F, you will have eliminated the beta activity, it will not kick back in as the mash cools. This will also reduce the ability of the alpha to convert starches, because it requires beta to "snip" some of the bonds it cannot before it can fully convert starches. So you are basically creating just the less fermentable sugars, but not very efficiently, leaving some unconverted starches. To get full advantage of the enzymes in your mash, you either need to START at a lower temperature and then raise it (step mash,) or pick a compromise temperature around 153°F for the full duration.