The optimal temperature for the action of lactate...

The optimal temperature for the action of lactate dehydrogenase is $36^{\circ} \mathrm{C}$. It is irreversibly inactivated at $85^{\circ} \mathrm{C},$ but a yeast containing this enzyme can survive for months at $-10^{\circ} \mathrm{C}$. Explain how this can happen.

Key Concepts

Cryoprotection Mechanisms

Many cells have developed cryoprotective strategies to survive in sub-freezing temperatures. These mechanisms include the accumulation of cryoprotectants that stabilize proteins and cellular membranes, thus averting mechanical and chemical damage from ice formation. As a result, enzymes and other cellular components remain intact during periods of cold stress, ensuring cell survival over extended periods.

Enzyme Kinetics and Temperature Optimum

Enzymes have an optimal temperature at which their catalytic activity is maximized because their protein structure, including the active site, is ideally configured to efficiently facilitate the conversion of substrates to products. Deviating from this optimal temperature can reduce the enzyme's activity either by slowing down the kinetic processes involved or by destabilizing its structure.

Protein Denaturation and Irreversible Inactivation

Proteins depend on their correct three-dimensional structure for proper function. At high temperatures, the non-covalent interactions that maintain this structure can be disrupted, leading to denaturation. Once an enzyme like lactate dehydrogenase is denatured, it loses its functionality irreversibly, even though it may be perfectly fine at lower temperatures.

Cellular Dormancy and Metabolic Regulation

Organisms such as yeast can enter a state of reduced metabolic activity or dormancy under cold conditions. This dramatic slowdown in metabolic processes allows them to conserve resources and protect vital enzymes from degradation. Even though the enzyme's optimal activity is at higher temperatures, the enzyme remains stable at low temperatures, ensuring that it can resume normal function once conditions improve.

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