Enzyme Substrate Complex

Enzyme substrate complex definition

The enzyme-substrate complex is a temporary molecule that occurs when an enzyme binds perfectly with a substrate.

Some important definitions to know regarding the enzyme-substrate complexes are:

Proteins are organic compounds with many valuable and vital roles within our bodies.

Other vital roles of proteins include:

• building and repairing the tissues in our bodies
• defending our Immune System by making Antibodies
• providing energy when carbohydrate and lipid levels are low in our bodies
• Muscle Contraction with proteins like actin and myosin
• keeping the shape of our cells and bodies (ex) collagen in our skin

Enzymes work by lowering the activation energy of chemical reactions. In biology, the activation energy can be considered the minimum energy required to activate molecules so that the reaction can begin or occur.

Enzymes lower activation energies by binding to substrates in a way where the chemical bonds break and form more easily.

Enzyme substrate complex formation

As previously mentioned, the enzyme-substrate complex formation happens when an enzyme and substrate combine. We can compare the enzyme and substrate interaction as jigsaw puzzle pieces fit together.

When we talk about the enzyme-substrate complex model, we can speak of two "fits."

• Lock and Key model:
  • This model occurs when the enzyme's active site fits like a lock to the substrate, which works like a key.
  • Think of opening the door together into your house. In this case, your house key is the substrate, and the door's lock represents the enzyme. If the substrate or house key fits perfectly, then the door opens, or in the case of the enzyme, it can activate and function.
• Induced Fit model:
  • This model occurs when the substrate binds, causing a change in shape in the enzyme's active site, and can be referred to as the hand-in-glove model.
  • This is because the first finger is usually hard to insert into a glove, but once we do and the glove has adequately aligned, then it's easy to put on the glove. We will expand on this in the section "Enzyme substrate complex diagram."

Figure 1: Lock and Key model. Wikibooks, Waikwanlai (Public Domain).

Enzyme substrate complex diagram

The induced fit model is more widely accepted for the enzyme-substrate complex. This type of enzyme-substrate complex diagram is considered better because scientists believe it can better explain how catalysis happens. This is because the induced fit model introduces a more dynamic interaction between enzyme and substrate than the Lock and Model figure does.

Figure 2: The induced fit model diagram. Wikimedia, TimVickers (Public Domain).

1. The substrate enters the active site of the enzyme.
2. The enzyme/substrate complex is created. Since this is the induced model we are referring to, the enzyme changes shape slightly as the substrate binds. Depending on the chemical reaction and properties of the amino acids, some reactions might occur better in an environment with water, without, acidic, etc.
3. Then, the products are created and released by the enzyme.
4. After the product's release, the enzyme changes to its original shape allowing it to be ready for the next substrate.

Enzyme substrate complex example

Enzymes can be regulated where their activity can be reduced or enhanced by different kinds of molecules.

• Competitive inhibition occurs when a molecule competes with the substrate for the enzyme's active site directly by binding to it and preventing the substrate from doing so.

• Noncompetitive inhibition occurs when a molecule binds to a site other than the active site, which we call the allosteric site. However, this molecule still prevents the substrate from binding to the enzyme's active site.

• Most allosterically regulated enzymes have more than one protein subunit.

This signifies that when allosteric inhibitors bind to one protein subunit at an allosteric site, all the other active sites on the protein subunits change shape slightly so that the substrates bind less efficiently. Less efficiency means that the rate of reaction is lowered.

• Allosteric activators also exist, and they work the same as inhibitors, except they increase the affinity of the enzyme's active sites for its substrates.

Figure 3: Enzyme reaction and inhibition. Wikimedia, Srhat (Public Domain).

Enzyme-substrate complexes generally have three parts: enzymes, substrate, and product. Depending on the reaction being performed, there can be more than one substrate or product.

Featured below are some common enzyme-substrate complex examples.

To help you better understand what's happening in the table featured above, we will go over how the lactase enzyme-substrate complex works.

The lactase enzyme substrate:

• The lactase enzyme breaks down lactose, our substrate, into glucose and galactose products. Breaking down lactose is crucial because it helps us digest dairy products. When humans don't make enough lactase enzymes, they are lactose intolerant and have trouble digesting dairy products. Lactose is also called milk sugar.