Effect of pH on Enzyme Activity

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  • 0:03 What Are Enzymes?
  • 1:42 What Is pH?
  • 3:12 PH and Enzymes Interaction
  • 4:08 Optimal pH
  • 5:17 Inactive Enzymes
  • 5:59 Lesson Summary
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Lesson Transcript
Instructor: Sarah Phenix
Let's explore the relationship between enzymes and pH. Learn how enzymes function, what pH really means, and how the two interact to result in optimal enzymatic activity.

What Are Enzymes?

Enzymes belong to a class of molecules known as proteins, meaning they have one or more large chains of amino acids joined together by peptide bonds. The job of an enzyme is pretty specific: to catalyze, or speed up, chemical reactions by reducing the amount of energy needed for the reaction to take place. In other words, the reaction could be completed without the addition of the enzyme, but the enzyme essentially 'discounts' the energetic cost of the reaction so that it can happen faster and with less energy.

Another important thing to know about these specialized proteins is that enzymes are not consumed or changed in any permanent way by the reaction. This means that once they disengage from one reaction, they can move right on to another reaction.

An enzyme works by binding with a specific substrate (the molecule that will undergo change) at a place on the enzyme called an active site. The active site works like a lock and key, in that it fits with only one type of substrate, or 'key'. Once the two bond, the newly joined substance is known as an enzyme-substrate complex. This new complex activates an energy change, transforming the substrate and yielding an enzyme-product complex. Now that the enzyme has finished its work, it releases the products and goes on to find a new substrate to bind with.

Another way to think about this interaction is like a chemical equation:

Enzyme + Substrate --> Enzyme-substrate Complex --> Enzyme-products Complex --> Enzyme + Products

where an enzyme plus a substrate creates the enzyme-substrate complex, which in turn creates the enzyme-products complex, which then releases the products and leaves the enzyme ready for a new reaction.

What Is pH?

You've likely heard of pH before, especially if your family has a fish tank or a pool. But have you ever thought to consider what it means? The 'p' stands for the German word, potenz, meaning power; 'H' stands for the hydrogen (chemical symbol H+) that pH measures. So, pH literally means the power (or concentration) of hydrogen atoms in a substance.

Hydrogen atoms affect the acidity of a solution or substance and are balanced out by the presence of hydroxide (OH-) atoms. Thus, substances with a higher concentration of hydrogen atoms than hydroxide molecules have a greater acidic 'power' and are known as acids. On the other hand, those with lower H+ concentrations and higher OH- concentrations have less acidic power and are known as bases.

The pH scale runs from 0 to 14: 7 is a neutral pH, or an even balance of acidic H+ and basic OH- atoms. Values below 7 (6-0) are increasingly acidic, while values above 7 (8-14) are increasingly basic. What's more is that there is a 10-fold difference in concentration between these values, meaning that pH 0 (e.g., hydrochloric acid) is 10 times stronger than pH 1, 100 times stronger than pH 2, and 10,000,000 times stronger than pH 7 (for example, water).

pH and Enzymes Interaction

Now that you know that pH 0 is 10,000,000 times stronger than a pH of 7, I bet you can imagine that the pH of an environment might affect the way in which an enzyme functions. Why is this? Enzymes, being proteins, gain their molecular shape based on the interactions between the charges of the amino acids that make up their long protein chains. Think back to the last time you played with magnets. What happened when you faced two positive sides or two negative sides together? They pushed against each other and resisted touching, right? Well, that same resistance occurs between the charges of atoms that compose molecules, too.

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