Which statement about enzymes is true?

A. An enzyme functions to increase the rate of a chemical reaction.

B. Enzymes are proteins that function as catalysts in non living things.

C. Each enzyme can catalyze many different biochemical reactions.

D. Enzymes and substrates fit together like a lock and key.

Diagram showing effect of enzyme on activation energy needed. The substrates are indicated as A+B and product as AB.
Diagram showing the effect of the enzyme on activation energy needed for the reaction. The substrates are indicated as A+B and product as AB.

The correct answer is A. An enzyme functions to increase the rate of a chemical reaction.

Enzymes are protein molecules that function to increase the rate of a chemical reaction by lowering the energy necessary for a reaction to occur. This energy is known as the activation energy and is necessary for a reaction to be initiated.

These proteins are biological catalysts that play a role in many reactions which occur in living organisms. A further feature of these catalysts is that they are specific. In other words, one enzyme generally only binds to and reacts with one particular substrate to bring about a reaction.

An enzyme has specificity and only binds to a particular substrate usually. The binding is called an induced fit and the enzyme changes shape to better fit the substrate.

The fact that these molecules are proteins also influences what factors affect them. Proteins are susceptible and sensitive to changes in temperature and pH and in fact, can be inactivated and destroyed if conditions are not right. This is known as denaturing, and in some cases, this can be reversed.

Regulation of enzymatic activity occurs by substrate availability or by the involvement of other molecules. Some modulation of activity is by covalent bonding involving phosphates (phosphorylation and dephosphorylation) or by allosteric regulation involving other molecules.


Enzymes are large biological molecules that function in increasing the rate of chemical reactions in living organisms. These are proteins which work by lowering the energy, known as activation energy, that is needed to start the reaction.

The protein nature of the enzymes means that they are sensitive to environmental conditions. In fact, each catalyst is specific and works best at a specific pH and temperature.

This explains why different regions of our digestive system have different pH values. Each enzyme that works to digest food acts at a different optimal pH range.

The enzyme pepsin, for example, is secreted from the stomach lining and works best at a very low pH of between 1.5 and 2.2. Thus, the stomach is kept very acidic to ensure that the pepsin can work to digest protein.

In the small intestine, the pH is kept closer to neutral for the enzymes that work there. The trypsin that digests protein and the pancreatic amylase that digests starches act at pH values that are at or near 7.

An enzyme will usually bind with one particular substrate. This is why complex reactions such as those that occur in cellular respiration involve multiple enzymes, with each catalyzing a reaction at a certain step during the process.

Induced fit

An enzyme works by what is called an induced fit model.  This model states that an enzyme changes shape when it binds to a substrate in order to better fit with the substrate.

This conformational change works as a switch at the molecular level to ensure that the correct catalyst is in place and then the reaction can proceed.

The part of the enzyme that binds to the substrate is known as the active site, and it is this specific binding that ensures the correct enzyme attaches to the substrate to form an enzyme-substrate complex.

An incorrect enzyme would quickly be released from the substrate since the fit would not work. Research has been completed on the effect of induced fit on the action of DNA polymerase enzymes, which has shown the importance of this in determining the success of a chemical reaction.

After the reaction has been catalyzed the enzymes are released and recycled to be used again. The enzymatic activity is also regulated by various chemicals.

Enzyme regulation

An enzyme can itself, act as a regulator, and in fact, in a complex metabolic pathway, it is often the first such catalyst that acts to regulate a metabolic pathway. Regulatory enzymes can be allosteric or can be altered by covalent bonding.

Allosteric regulation can entail feedback inhibition in which the amount of the final product formed from a reaction determines if it continues to occur.

In other situations, allosteric enzymes are affected by additional molecules that bind to them. These extra modulator molecules can either activate or inhibit the enzyme and thus the chemical reaction. These molecules can bring about a change in the active site to influence if the catalyst can bind to a substrate.

The phosphorylation and dephosphorylation of enzymes are an example of modification and regulation by covalent bonding. For instance, the tyrosine kinases are activated largely by the addition of phosphates from ATP.

These types of enzyme modulation are common in signal-transduction pathways of the cells. These are the pathways by which much of the cell communication occurs.

Denaturing and enzymes

Proteins are molecules that can become denatured if conditions are not correct. Denaturing is when the chemical bonds of the protein become broken leading to the structure being destroyed. This often occurs when environmental conditions such as pH and temperature are unfavorable.

Since enzymes are proteins they, therefore, can be denatured at temperatures and pH values that are unfavorable. Sometimes if conditions improve the molecule can be renatured and bonds reformed. There are enzymes that can be renatured, for instance, ribonuclease.

When an enzyme is denatured the active site is no longer functional which means that it cannot bind to a substrate to catalyze a reaction.


  1. JM Berg, JL Tymoczko, L Stryer  (2002). Biochemistry, 5th edition. USA: New York, WH Freeman Publishers.
  2. A Blanco, G Blanco (2017). Regulation of enzyme activity. Retrieved from sciencedirect.com.
    KA Johnson (2008). Role of induced fit in enzyme specificity: a molecular forward/reverse switch. Journal of Biological Chemistry.
  3. M Schlamowitz, LU Peterson (1959). Studies on the optimum pH for the action of pepsin on “native” and denatured bovine serum albumin and bovine hemoglobin. Journal of Biological Chemistry.
  4. Editors of Encyclopedia Britannica (2018). Enzymes. Retrieved from Encyclopedia Britannica.


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