ENZYMES

IB Topic 3.6 page 74

What is an Enzyme?

An enzyme is a catalytic PROTEIN– It is effective in small amounts– It is unchanged by the reaction

So it’s reusable – It speeds up the rate of reaching

equilibrium in a reversible reaction– The shape is very specific – It cannot force a reaction

The enzyme active site

The starting substance is called the substrate– The substrate is converted to the product

The enzyme works by binding to the substrate– The binding point is called the active site– The active site is located on the enzyme

As the enzyme and substrate bind, the substrate is raised to a transition state (EA)...

– Bonds break– Reactants Products

Enzymes influence the rate of reactions

Reactants will form products at a faster rate than without an enzyme – In other words, enzymes speed up the rate of

reactions

Enzymes lower the energy level needed to start the reaction – Activation energy – Thus, enzymes lower the activation energy

Enzymes lower the activation energy of exothermic reactions

Lock and Key Hypothesis

Due to its precise shape and distinctive chemical properties, each enzyme is specific for a certain substrate or a VERY small group of substrate molecules.

Induced Fit Hypothesis

Some enzymes change shape when combined with a substrate

The active site is then molded into a precise conformation

The bonds of the substrate are stretched to make the reaction easier (less energy needed)

Enzyme changes shape after it connects with the substrate.

Three factors that affect the rate of enzyme-catalyzed reactions

1. Temperature —As temperature increases, molecules are moving faster and are more likely to collide and react.

Different enzymes have different optimum temperatures (ex: bacteria in hot springs, plants of the tundra, enzymes in our bodies)

2. Substrate Concentration

Increasing the amount of substrate will speed up the rate of reaction.

However, there comes a point when there is more substrate than enzyme so adding more substrate molecules will no longer increase the rate of reaction

3. pH — each enzyme has a range of PH in which it functions efficiently

Denaturation

Denaturation is a structural change in a protein that alters its 3-D shape and causes the loss of its biological properties

Denaturation is sometimes permanent and sometimes only temporary

Denaturation may be caused by changes in temperature and pH

Heat—exposure to heat causes atoms to vibrate violently and this disrupts bonds within globular proteins, and causes changes in the chemical characteristics—usually the change is irreversible (ex: raw vs. cooked egg white)

pH—small changes in pH also alter the shape of proteins. However the structure MAY re-form when optimum pH is restored

Homework

#18-21 page 78

Industrial uses of enzymes

Lactose-Free Milk

The enzyme lactase helps digest lactose in milk Many adults do not produce lactase, so drinking milk

causes diarrhea and/or gas Lactose-free milk can be produced using lactase

– Whole-cell preparations may not be appropriate for food– Adding and removing enzymes to each product is

expensive

So immobilized enzymes are used to make the milk

Methods of immobilizing enzymes

Metabolism consists of chains (linear sequences) and cycles of enzyme-catalzyed reactions

Metabolism=anabolic reactions + catabolic reactions

Anabolic reactions—larger molecules built from smaller molecules (ex: protein synthesis)—ENERGY REQUIRING/ ENDOTHERMIC

Catabolic reactions—larger molecules are broken down (ex: digestion)—ENERGY RELEASING/ EXOTHERMIC

Enzymes lower the activation energy of exothermic reactions

Competitive Inhibition

Example: Carbon dioxide should combine with Rubisco during photosynthesis but can be competitively inhibited by oxygen

Non-competitive Inhibition

Example: Nerve gas, Sarin blocks acetyl cholinesterase in synapse transmission

Allosteric Enzymes

Allosteric enzymes have 2 sites. – Active site of the enzyme– Additional site where another substance can lock in– When the other substance is locked in, the active site is

non-functional

End Product Inhibition—a specific type of Allosteric Inhibition

As the end product accumulate, the steps in the product are stopped