While most enzymes are highly specific, catalyzing reactions with a single type of substrate, some enzymes can act on multiple substrates. These enzymes, known as multisubstrate enzymes, have active sites that can accommodate different substrates with similar structures or chemical properties. A classic example is the enzyme lipase, which breaks down various types of lipids. Another example is hexokinase, an enzyme involved in glycolysis, which can phosphorylate different six-carbon sugars, although it has a higher affinity for glucose. The ability of these enzymes to act on multiple substrates is crucial for metabolic flexibility and efficiency, allowing organisms to adapt to changes in nutrient availability and to efficiently utilize different metabolic pathways.

The lock and key model and the induced fit model are two concepts used to describe enzyme-substrate interactions. The lock and key model, proposed by Emil Fischer, suggests that the enzyme's active site is exactly complementary to the shape of the substrate, similar to a key fitting into a lock. This model implies a very rigid and precise interaction. In contrast, the induced fit model, proposed by Daniel Koshland, suggests that the binding of the substrate induces a change in the shape of the enzyme's active site. This model accounts for a more dynamic interaction where the active site conforms to the shape of the substrate upon binding. The induced fit model is more widely accepted as it explains why enzymes are so specific yet able to catalyze reactions rapidly and efficiently. It also accounts for the flexibility of enzyme structures, which is not explained by the lock and key model.

Enzyme concentration plays a vital role in determining the rate of a biochemical reaction. In a given reaction, increasing the concentration of the enzyme typically speeds up the reaction, as more enzyme molecules are available to form enzyme-substrate complexes. However, this increase in reaction rate is only observed up to a certain point. Once all the substrate molecules are bound to enzymes (i.e., when the substrate is saturated), adding more enzyme will not increase the reaction rate further. This plateau occurs because there are no more substrate molecules available for additional enzyme molecules to act upon. Therefore, enzyme concentration is a crucial factor in controlling the speed of metabolic reactions within cells, and in industrial applications where enzymes are used to catalyze specific reactions. Understanding the relationship between enzyme and substrate concentrations is fundamental in biochemistry, especially in enzyme kinetics and metabolic regulation.

Enzyme inhibitors are molecules that decrease or halt the activity of enzymes. They can bind to enzymes in a way that either prevents substrate binding or reduces the enzyme's ability to catalyze reactions. There are two main types of inhibitors: competitive and non-competitive. Competitive inhibitors resemble the substrate and compete for the active site, thereby reducing enzyme activity. An example is methotrexate, which inhibits the enzyme dihydrofolate reductase, used in cancer treatment. Non-competitive inhibitors bind to a different part of the enzyme, causing a change in its shape and function. This binding can either reduce enzyme activity or completely inhibit it. Enzyme inhibitors play a significant role in regulating metabolic pathways, serving as a control mechanism to prevent overproduction or underproduction of specific substances within cells.

Environmental factors such as pH and temperature have a profound impact on enzyme activity and structure. Each enzyme has an optimal pH and temperature range within which it functions most efficiently. Deviations from these optimal conditions can lead to changes in the enzyme's three-dimensional structure, particularly the active site. For instance, extreme pH levels can cause denaturation, wherein the enzyme loses its specific shape, rendering it inactive. Similarly, high temperatures can increase the kinetic energy of enzyme molecules, leading to structural instability and denaturation. Conversely, low temperatures may not denature enzymes but can significantly reduce their activity by decreasing molecular movement, thus slowing down the rate of enzyme-substrate complex formation. Understanding these environmental impacts is crucial for controlling enzymatic reactions in industrial processes and in understanding how organisms adapt to different environmental conditions.