The yeast does not die, it de-natures. This means that the substrate will not fit into the active site on the enzyme (in this case, yeast). Essentially the substrate fits into the enzyme like a jigsaw. Once it reaches a certain temperature (also known as the optimum temperature), the substrate is moving at such a speed that it starts to change shape. This then means that the substrate will not fit into the enzyme like a jigsaw (de-natures), but it is still moving. Because it can't fit into the enzyme, no chemical reaction can occur meaning that Carbon Dioxide cannot be released, and the rate of respiration will decrease. The Enzyme will continue to move without the release of Carbon Dioxide but it cannot die. Once it has heated up again, the substrate will try and fit into the active site of the enzyme but no chemical reaction will occur as it is essentially the wrong shape. This process will continue every time the yeast is heated up and then left to cool down meaning it will not die.
Hope that makes sense
Aye