Extending on @ANONGOODNURSE, @SAPPHIRA and @JAMES.
Something you should know about proteins, they have primary, secondary, tertiary and quaternary structures all due to specific folding of the proteins. These foldings are mainly connected via hydrogen bonds to each other along with other bonds (Alberts et al., 2002). On the action of heat, the molecules composing the bonds are vibrated due to higher kinetic energy resulting in the breaking of the bonds which causes unfolding of proteins (Wikipedia denaturation of proteins). This phenomenon is called Denaturation of Proteins which causes the loss of protein function due to loss of shape.
Read this QandA in Reference.com for more information about denaturation of proteins.
Read this for more information about proteins: Molecular Biology of the Cell. 4th edition.
Link this idea to cellular respiration and how Electron Transport Chain and the Krebs Cycle (Citric acid cycle) involve proteins for the completion of the respective processes.