Define enzyme. Give a note on the factors affecting the rate of enzymic activities in plants.

What are Enzymes?

Enzymes are typically globular proteins that possess a unique three-dimensional structure, a crucial aspect for their catalytic activity. The “active site” within the enzyme molecule binds to the substrate (the molecule upon which the enzyme acts), facilitating the chemical reaction. The enzyme then releases the product, returning to its original state to repeat the cycle. Enzymes exhibit remarkable specificity; a single enzyme often catalyzes only one specific reaction or a set of closely related reactions. They lower the activation energy required for a reaction to occur, thus speeding up the process.

Factors Affecting Enzymic Activities in Plants

Several environmental and biochemical factors influence the rate of enzymic activity in plants. These can be broadly categorized into physical and chemical factors:

Physical Factors

• Temperature: Enzyme activity generally increases with temperature up to an optimum point. Beyond this optimum, the enzyme's structure denatures, leading to a rapid decline in activity. Different enzymes have different optimal temperatures. For example, enzymes involved in cold tolerance in plants like Arctic Poppy function optimally at near-freezing temperatures.
• pH: Each enzyme has an optimal pH range where it functions most effectively. Deviations from this range can alter the enzyme's charge distribution, affecting substrate binding and catalytic activity. Photosynthesis enzymes like RuBisCO have specific pH optima within chloroplasts.
• Light: Some enzymes are activated or inhibited by light. For instance, chlorophyllase, an enzyme involved in chlorophyll degradation, is light-dependent.
• Water Availability: Water acts as a solvent for enzymes and substrates. Water stress can reduce enzyme activity.

Chemical Factors

• Substrate Concentration: As substrate concentration increases, enzyme activity generally increases until a saturation point is reached, where all enzyme molecules are actively engaged.
• Enzyme Concentration: The rate of reaction is directly proportional to enzyme concentration, assuming substrate is not limiting.
• Inhibitors: These substances reduce enzyme activity. They can be:
  • Competitive Inhibitors: Bind to the active site, preventing substrate binding.
  • Non-Competitive Inhibitors: Bind to a different site, altering the enzyme's shape and reducing its efficiency.
  Heavy metal ions like mercury (Hg) often act as enzyme inhibitors.
• Activators: These substances enhance enzyme activity. Magnesium (Mg²⁺) is a common activator for many plant enzymes.
• Cofactors and Coenzymes: Many enzymes require cofactors (inorganic ions like Mg²⁺, Zn²⁺) or coenzymes (organic molecules, often vitamins) to function properly. For example, NAD⁺, derived from niacin, is a crucial coenzyme in redox reactions.

Factor	Effect on Enzyme Activity
Temperature	Increases up to optimum, then decreases due to denaturation.
pH	Optimal range; deviations reduce activity.
Substrate Concentration	Increases until saturation.
Inhibitors	Decreases activity.

Case Study: RuBisCO and Climate Change

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is a crucial enzyme in the Calvin cycle, responsible for carbon fixation in photosynthesis. However, it also reacts with oxygen (photorespiration), reducing photosynthetic efficiency. Rising atmospheric CO₂ levels can partially alleviate this issue by increasing the CO₂/O₂ ratio, favoring carboxylation. However, RuBisCO's efficiency remains a bottleneck for plant productivity, and research focuses on engineering more efficient variants.