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Catalytic Enzymes

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Cells, and therefore organisms, cannot function without enzymes. Enzymes are specialized proteins that act as catalysts to get complex chemical reactions started. All chemical reactions require some energy to start. The energy required to get a reaction going is called its activation energy. Many of the simpler chemical reactions require so little energy that they can occur at relatively low temperatures without any outside influence. When heat is added, the additional heat energy is often enough to get many other reactions going. More complex reactions require some other stimulus, or catalyst to begin. Enzymes act as catalysts for chemical reactions by changing one or more of the reactants, called the substrate, in a way that lowers the activation energy enough for the reaction to begin. Some reactions won’t occur without their specific enzymes. With some others, the enzyme isn’t necessary, but makes it easier. Unlike the reactants, enzymes aren’t consumed during the reaction. An enzyme's action can be helped sometimes by co-factors, and is affected by temperate, pH, and concentrations of itself or substrates. Competitive and non-competitive inhibition regulates enzyme activity to suit physiological needs of a cell.

Enzymes can be grouped into various categories, such as oxidases, transferases, hydrolases, isomerases, polymerase, nucleases and ligases. Each enzyme is named after the substrate it acts on, with the suffix 'ase' added to show that it is an enzyme.


There are approximately 75,000 enzymes thought to exist in the human body, and they all catalyze different reactions in which molecules are either synthesized or broken down. Sometimes, these enzymes are helped out by co-factors. These co-factors can be inorganic ions like Fe2+, Mg2+, Mn2+, and Zn2+; or organic molecules known as co-enzymes.

Enzymes function like all other catalysts, and show two properties basic to all catalysts.

  1. They do not change the biochemical reaction even though they increase the speed.
  2. They are not altered permanently by the reaction.

Enzymes are specific to substrates

Unlike chemical catalysts, enzymes are specific in their action. To be able to function, the enzyme must first be associated with one of the reacting biochemicals, or substrates. Each enzyme has an active site that has a characteristic shape that fits with a corresponding shape in the substrate.


Originally, these active sites were believed to require an exact fit with the substrate. Under the Lock and Key model, if the substrate didn't fit the enzyme's active site exactly, no catalysis occurred. In 1958, Daniel Koshland suggested that the fit between enzyme and substrate is very close, but not necessarily exact. In his Induced Fit model, he explained that, while each enzyme is in fact specific to its one substrate, the fit isn't always quite perfect, and the substrate has to modify the enzyme slightly for binding to occur.

Enzymes lower activation energy

Enzyme action speeds up biochemical reactions by lowering the activation energy needed. The molecules which are involved in the biochemical reactions are called the substrates. These substrates needed a high transitional energy to be able to react. When there is only one molecule involved and has to to be transformed, then it attaches to the active site of the enzyme, which provides the place where the reaction occurs. When the biochemical reaction involves more than one substrate, they attach to the active site on the enzyme in the proper orientation and position that hastens the reaction. Sometimes, an enzyme may also get involved in the reaction. It reacts with one of the substrates to form an intermediary product in a reaction that needs much less energy. This intermediary product reacts with the second substrate to give the final product, the releases the enzyme.

 Enzyme activity is regulated by cells

 Temperature and pH are two important factors that influence reactions. As the temperature increases, so does the kinetic energy in the cell, which increases reaction speed. However, if the temperature rises more than the optimal, enzymes can get denatured. In human cells the optimum temperature is around 37 degrees Celsius, or 98.6 degrees Fahrenheit, which is also the average healthy human body temperature. The range of pH in which enzymes functional best is usually narrow. Any deviation in pH causes the bonds in enzymes to break, changing its shape, and therefore effectiveness.

Inhibition of enzymes

To save resources and energy when the cell has a high enough concentration of a synthesized product, inhibition of enzyme activity is necessary. Regulatory chemical substances can inhibit enzyme activity by binding to an active site and preventing substrate binding, in which case they are called competitive inhibitors. If inhibitors are attached to another place on the enzyme, but manage to influence the bonding of substrates, they are called non-competitive.


All cells must carry out a number of chemical reactions in order to function. Many of these reactions require their activation energy to be lowered by enzymes so that they can occur in the temperature and pH range in which living cells can survive. Enzymes are specialized proteins that catalyze reactions by binding to and changing one of the reactants, call a substrate, so that it can react more readily with the other reactant(s). This lowers the amount of energy required to begin the reaction, which is called the activation energy. Enzymes aren't consumed or permanently changed during the reaction, and they don't have to be present for the reaction to occur, so long as the activation energy for the reaction is met. Sometimes, enzymes need a little help to get the job done. There are many molecules ,both organic and inorganic, that can act as co-factors to alter the enzyme or substrate just enough that the enzyme can do its job. When there is no more need for a reaction to occur, enzymatic activity can be inhibited, either by the action of special regulatory chemicals or by the simple lack of substrates.

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