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Substrate Definition

A substrate is a molecule acted upon by an enzyme. A substrate is loaded into the active site of the enzyme, or the place that allows weak bonds to be formed between the two molecules. An enzyme substrate complex is formed, and the forces exerted on the substrate by the enzyme cause it to react, and become the product of the intended reaction. The bonds that form between the substrate and enzyme cause the conformational change, or shape change, in the enzyme. The resulting shape change is what applies pressure to the substrate, either forcing molecules together or tearing them apart.

Almost every molecule in our bodies are substrate molecules at some point. Because of the large amount of energy and time required for most reactions to take place, each reaction needs a specific enzyme to help it along. An enzyme does this by lowering the energy required for a reaction to take place between substrate molecules, or within one molecule. Once the reaction has taken place, the substrate is now chemically different, and is called the product. However, many chemicals produced by our bodies are formulated by many smaller steps, known as intermediates, each having its own enzyme. The products of one reaction become the substrate of the next reaction, until the final product is reached. It is in this way that all the materials in our body take shape.

Nutrients, collected by an organism, are digested in the gut. Here, enzymes recognize various forms of food as the substrate they act on, and work to break them apart. Once broken down, these products are carried to various cells around the body. Now called substrates again, new enzymes work on these substances to combine them into bigger molecules and incorporate them into the body. Whether or not a substance is considered a substrate is dependent on which reaction it is headed to, and which reaction it came from. After a substrate becomes a product, it can instantly become a substrate again if a different enzyme can act on it. Because enzymes are specific and decrease reaction time, we can produce many chemicals that would be completely impossible without intermediate steps, and enzymes doing most of the work.

Examples of Substrate


Lactose is a sugar produced in milk. Mammals typically produce milk for their offspring. It contains a blend of fats, proteins, and growth hormones to get a young mammal to gain a lot of weight in a short amount of time. Humans, interestingly, are the only animals that drink another species milk in a non-predatory way. While some big predators will surely consume the milk of a mammal they just killed, only humans purposefully drink the milk provided by cows. Not surprisingly, many people have lactose intolerance, or an inability to process the sugar lactose.

Lactase, the enzyme needed to act on lactose as a substrate, is produced by humans when they are babies to deal with the lactose in breastmilk. Once weaned from breastmilk, the substrate lactose is no longer present for the enzyme to work on. The lactose, besides being a substrate for lactase, also acts on your DNA. It is thought that in the presence of lactose that DNA produces more lactase. Once weaned, the body produces little to no lactase, causing lactose intolerance. However, most people continue to drink cow’s milk almost immediately, or concurrently, with being weaned from breast milk. In this way, you are continuously able to process lactose, which might not be a good thing. Recent studies have shown that the growth hormones, cholesterol, and animal proteins in cow’s milk may not be good for your health, as a health adult mammal. Although, it does make sense that adults shouldn’t drink baby formula.

ACE Inhibitors as Substrate Blockers

If you know of anyone currently taking ACE inhibitors, you probably know that the pills are helping keep them alive, but you have no idea how. ACE stands for angiotensin converting enzyme. This enzyme is responsible for producing a molecule known as angiotensin II, which causes muscles around blood vessels to contract. These small muscles put pressure on the blood. Normally, the help keep the blood flowing with healthy amounts of pressure. If too much angiotensin II is created by the body, or if the blood vessels are clogged, more pressure can cause vessels to burst or become completely sealed off. Both are life-threatening conditions.

Luckily, ACE inhibitors were created to be substrate “mimics”. The ACE inhibitors are about the same size and shape as angiotensin I, the substrate for the angiotensin converting enzyme. Instead of binding to the substrate, the enzyme binds to the inhibitor instead. Unlike the substrate, the inhibitor cannot undergo a chemical reaction, and becomes stuck to the enzyme. By regulating the amount of ACE inhibitor given to a person, the effectiveness of all their angiotensin converting enzymes can be effected, and a lower level of angiotensin II will be seen in the blood and tissues. Without this chemical, the muscles around blood vessels relax, and blood pressure is lowered. A lower blood pressure prevents many of the dangerous conditions that can arise from high blood pressure.

  • Enzyme Substrate Complex – A large pseudomolecule formed when substrate enters the active site on an enzyme.
  • Conformational Change – A structural shift in an enzyme due to the formation of the enzyme substrate complex.
  • Intermediate – A molecule that serves no function, but exists as a part of a pathway to another molecule.
  • Active Site – The place on an enzyme that the substrate can weakly bind to, causing a conformational change in the enzyme.


1. A particular molecule binds to the active site of an enzyme. An enzyme substrate complex is formed, but no change happens in the molecule and it stays bound to the enzyme. What type of molecule is this, to the enzyme?
A. Substrate
B. Inhibitor
C. Intermediate

Answer to Question #1
B is correct. This molecule is an inhibitor. It acts in the same way as a substrate molecule, binding to the active site. Unlike the substrate molecule, the inhibitor cannot undergo a reaction. As such, the enzyme stays bound to it until other forces allow the two molecules to separate.

2. Each substrate has a particular enzyme associated with it. Sucrase for sucrose, maltase for maltose, etc. Why is this the case?
A. Natural Selection
B. Enzyme specificity
C. Active site bonding

Answer to Question #2
A is correct. The process of natural selection selects for the genes which produce the greatest advantage to the population. Each gene produces a protein, many of which are enzymes. Mutations in the gene allow for different variations of the enzyme to be tested. In a way, each organism is a completely unique combination of different enzymes being tested. Organisms with more beneficial enzymes will reproduce more, while those with deficient enzymes will reproduce less. In this way, enzymes become extremely adapted to the substrate they are intended to act on.

3. Proteins are broken down by enzymes called protease. They are synthesized, or created from individual amino acids, from synthetase enzymes. Why can’t a synthetase enzyme break proteins, or a protease create proteins?
A. They can
B. Nature like complication
C. Enzyme Specificity

Answer to Question #3
C is correct. In all enzymes, there is only one direction: from substrate to product. Once the product is created, the enzyme to longer recognizes the molecule, because it is specific to the substrate it is intended to act up. If enzymes could work both ways, nothing would ever get done because as soon as an enzyme had finished converting a substrate to a product, it would do the same reaction in reverse. Organisms need enzymes to be specific to ensure the right amounts of products in the right places.

Cite This Article

MLAAPAChicago Editors. "Substrate." Biology Dictionary,, 04 Dec. 2016, Editors. (2016, December 04). Substrate. Retrieved from Editors. "Substrate." Biology Dictionary., December 04, 2016.

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