Activation Energy & Enzymes in Chemical Reactions

Activation Energy and the Activated Complex

Chemical reactions involve the breaking of some bonds in the reactants and the formation of new ones, resulting in specific products. For this to happen, a certain amount of energy, *q*, is required to activate the reactants. This energy is called activation energy and is necessary to initiate the reaction.

Molecules reach a higher-energy activated state or transition state where the bonds are partially broken. Because this reaction takes place, the products obtained are in a lower energy state. The higher the activation energy, the more difficult it will be to reach the activated state, and the reaction rate will be lower. One way to initiate the reaction is by heating the substance; the molecules increase their energy and reach the transition state.

Enzymatic Mechanisms to Increase Efficiency

An enzyme acts on a substrate. Most substrates are found in cells in extremely low concentrations. Enzymes enhance reaction efficiency through several mechanisms:

• Multienzyme Complexes: Several enzymes that catalyze successive steps in a sequence of metabolic reactions bind to form a single multienzyme complex.
• Cellular Compartmentalization: The existence of different cellular compartments allows a group of enzymes and substrates to be concentrated in the same organelle. This significantly increases enzyme efficiency.

Enzyme Regulation and Allosteric Enzymes

The most precise enzyme regulation is achieved by enzymes that possess various modulators of their activity. These enzymes, known as allosteric enzymes, have additional sites that allow the binding of regulatory molecules, which modify enzyme activity. They exist in two conformations:

• Form R: High affinity for the substrate.
• Form T: Low affinity for the substrate.

These enzymes consist of several subunits, therefore, they have a quaternary structure.

Feedback Inhibition

Allosteric enzymes are usually localized in the first step of a sequence of metabolic reactions, or at branching points of these routes. The end product of the pathway often acts as an allosteric inhibitor of the enzyme catalyzing the first reaction. This process is called feedback inhibition or retro-inhibition.

Enzyme Kinetics

The smaller the value of Km (Michaelis constant), the greater the enzymatic activity because less substrate is needed to reach half the maximum reaction rate (Vmax/2). The Km value and the maximum velocity can be used to determine the reaction rate for any substrate concentration.