Optimum pH for Enzymes

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  • 0:00 Defining pH
  • 3:07 pH and Enzymes
  • 4:12 pH Range and Enzyme Action
  • 5:14 Optimum Enzyme pH
  • 6:14 Lesson Summary
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Lesson Transcript
Instructor: Darla Reed

Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

This lesson reviews what pH is and what different pH levels mean. It also gives a brief definition of enzymes and discusses what is meant by optimum pH and how it relates to enzymes.

Defining pH

Would you rather swim in a pool of lemon juice, water, or oven cleaner? As tempting as lemon juice and oven cleaner might be, I think most people would prefer water. But why? They're all liquids. What makes water more preferable?

One answer is pH, or the measure of acidity or basicity of a solution. The pH of water, lemon juice, and oven cleaner are all different. Let's look first at water. Water (or H2O) can be separated into its component parts: H+ (a hydrogen ion) and OH- (a hydroxide ion). The H+ doesn't like to be alone, so it binds to a water molecule to form H3O+, or a hydronium ion. In order to simplify things, scientists just use H+, even though they are really talking about H3O+.

The concentration, or amount, of H+ ions determines pH. Concentration is measured as molar concentration, or molarity (M). Molarity is simply one mole (6.022x10^23 molecules) of a substance in 1 liter of a solution. So, what is the molar concentration of H+ in water? The answer is 1x10^7 M. Since water breaks down equally into both H+ and OH-, the concentration of OH- is also 1x10^-7 M.

By measuring the concentration of H+ present, scientists can take the negative log base 10 to determine pH. The equation is:

pH formula

So, if we plug in our known H+ for water,, we get: pH = -log(1x10^-7), or 7. The pH of water, therefore, is 7. This is known as neutral pH.

Analyzing lemon juice or oven cleaner in a similar manner is more difficult since they're composed of many different molecules, so scientists have come up with various methods of measuring pH. The measured pH of lemon juice is around 2, while the measured pH of oven cleaner is close to 13. But what do these numbers mean?

If a pH of 7 is neutral, what is a pH of 2? A lower pH value equals higher H+ concentration, and vice versa. Anything that gives H+ ions is considered an acid. Since in pH = 2 there are lots of H+ to give, it's considered acidic, while pH = 13 is thought of as basic, or alkaline.

The calculation of pH uses a log base 10. This means that a change of one number (for example, a pH of 7 to a pH of 6) changes the H+ concentration by a factor of 10. So, there are 10 times more H+ ions in pH=6 liquids than pH = 7 liquids, and there are 100 times more H+ ions in pH = 5 than pH = 7.

pH and Enzymes

Just as you would probably rather swim in a liquid with a neutral pH like water, cells have a pH they prefer, too. The proteins inside cells also have preferential pH environments. One type of protein inside a cell that has a preferred pH is an enzyme. Enzymes are proteins that help speed up chemical reactions; they change substrates into products. They have a 3-D structure known as a conformation and are composed of amino acids that are held together by chemical bonds.

You can think of an enzyme as an armchair composed of wood and held together by nails. You can sit in it any time you want. But what happens if you put the armchair in fire or liquid nitrogen? Can you still sit in it? Odds are the chair would be destroyed, or at the very least, you wouldn't want to sit amidst the fire or liquid nitrogen.

Enzymes also have preferred environments. If an environment is too acidic (low pH) or alkaline (high pH), an enzyme won't function or will become denatured, a state where an enzyme loses its conformation and becomes a rubbish pile of amino acids.

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