Enzymes are proteins that allow certain chemical reactions to take place much quicker than the reactions would occur on their own. Enzymes function as catalysts, which means that they speed up the rate at which metabolic processes and reactions occur in living organisms.
Usually, the processes or reactions are part of a cycle or pathway, with separate reactions at each step. Each step of a pathway or cycle usually requires a specific enzyme. Without the specific enzyme to catalyze a reaction, the cycle or pathway cannot be completed.
The result of an uncompleted cycle or pathway is the lack of a product of that cycle or pathway. And, without a needed product, a function cannot be performed, which negatively affects the organism.
Catalysts and activation energy
Reactions are not impossible without enzymes. Enzymes do not change during reactions, nor do they change the other contents of the reaction. They just speed up the rate at which all parts of the reaction react.
In a chemical reaction, the reaction is said to be completed when equilibrium is reached. Chemical reactions have forward directions and backward directions, and reactions tend to move in both directions until no more products are created from the reactants, and products are no longer converted back into reactants.
That is the point of equilibrium. The equilibrium constant is written as:
Reactions will occur with the free energy available in the system (system is referring to the area where the reaction is occurring). There is always some energy in the system before a reaction begins, and this free energy is called G. The amount of change in the free energy of a reaction is labeled ΔG (the Greek letter delta, Δ, is used to represent change).
Exergonic reactions give off energy, so they represent a negative change in free energy (-Δ G) — that is, the free energy is given off, so there is a “loss” of free energy. In actuality, the energy is just transferred. Exergonic reactions will continue until equilibrium is reached, because they yield energy.
Endergonic reactions absorb energy into the system, so the free energy in the system increases (+Δ G). This increase appears to be a “gain” in energy, when really it is just another energy transfer. Endergonic reactions kind of quit while they are ahead. Because endergonic reactions take in energy, the reactions peter out so that less energy is taken in. They usually do not reach equilibrium.
There are two theories as to how reactions occur:
In the collision theory, it is thought that reactions occur because molecules collide; the faster they collide, the faster the reaction occurs. The energy level that must be reached for the molecules to collide is called the activation energy. The activation energy is affected by heat, because a higher temperature increases the energy of each molecule.
In the transition state theory, reactants are thought to form bonds and then break bonds until they form products. As this forming and breaking happens, free energy increases until it reaches a transition state (also called activated complex), which is viewed as the midpoint between reactants and products. Reactions proceed faster if there is a higher concentration of activated complex.
If the free energy of activation is high, the transition state is low, and the reaction is slow. The reaction rate is proportional to the concentration of the activated complex. If the activation energy is lower, the reaction occurs faster because more activated complexes can form.
In living organisms, the reactions that need to occur have high activation energies. So, to get reactions to occur, either the temperature must be increased, or the activation energy must be decreased. But the internal temperature of a living thing cannot be raised too high like chemicals in a laboratory can be. Instead, living organisms rely on enzymes to lower the activation energies so that reactions can occur quickly.
Without enzymes, toxic chemicals could build up in the body to dangerous levels, or the energy-producing Krebs cycle would not be able to produce adenosine triphosphate (ATP), which is the main fuel of the body produced from food that is eaten and digested.
Cofactors and coenzymes: Coexisting with enzymes
Enzymes are made mostly of proteins, but they also have some nonprotein components. When these nonprotein components must be included in order for the enzyme to act as a catalyst, then the nonprotein component is called a cofactor. Examples of cofactors are potassium, magnesium, or zinc ions.
A coenzyme is a type of cofactor. Coenzymes are small molecules that can separate from the protein component of the enzyme and react directly in the catalytic reaction. An important function of coenzymes is that they transfer electrons, atoms, or molecules from one enzyme to another.
Vitamins are closely connected to coenzymes. The function of vitamins is that they help to make coenzymes. Niacin, which is one of the B vitamins, helps to make nicotinamide adenine dinucleotide (NAD), which is one of the coenzymes that carries electrons from Krebs cycle through the electron transport chain to produce ATP. Without NAD, very little ATP would be produced, and the organism would be low in energy.