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Denatured enzymes refer to enzymes that have lost their functional shape due to changes in environmental conditions, such as temperature, pH, or the presence of certain chemicals. This loss of structure typically results in a disruption of the weak bonds and interactions that maintain the enzyme's specific three-dimensional conformation. Since the enzyme's activity is closely tied to its structure, any alteration that affects its shape can lead to a decrease or complete loss of its catalytic ability.

In contrast, increases in enzyme activity would not pertain to denaturation, as those conditions would suggest the enzyme is functioning optimally. Additionally, while some enzymes can be activated by temperature changes, denaturation describes the irreversible process of losing shape rather than activation. Lastly, substrate modification pertains to changes in the substrate itself rather than the enzyme's conformation, which is independent of the definition of denaturation. Thus, the correct understanding of denatured enzymes is rooted in the loss of their functional shape, which impairs their biological activity.