Background on Chemical Reactions

Section 2.4 of the Textbook

Chemical Reactions...• change substances into different substances by

breaking and forming bonds• are sometimes easily detected (ex: if there is a

change in temperature, color, or state of matter)

Chemical Equations Show What is Happening During a Reaction

• The equation shows the reactants or products– Reactants: substances changed during a chemical

reaction– Products: substances made during a chemical

reaction (found on the side the arrow points to)

products productsreactants reactants

Energy is Needed for a Chemical Reaction to Occur

• Activation energy is the energy that must be absorbed for a reaction to start

ENZYMES

How the confectioners make the runny yolk-like inside to chocolate eggs?

The answer is : use of an enzyme

What are enzymes?

• Enzymes are proteins that increase the rate of chemical reactions without being changed in the reaction; also known as organic or biochemical catalysts.

• Catalyst: speeds up a chemical reaction without itself being changed by the reaction.

Enzyme Structure

• Enzymes are globular proteins. They have a complex tertiary and quaternary structure in which polypeptides are folded around each other to form a roughly spherical or globular shape.

• The overall 3D shape of an enzyme molecule is very important: if it is altered, the enzyme cannot bind to its substrate and so cannot function. Denature

• Enzyme shape is maintained by hydrogen bonds and ionic forces.

Enzyme Function

Many of the complex chemicals that living organisms need cannot be made in a single reaction. Instead a series of simpler reactions occur, one after another, forming a metabolic pathway. A single pathway may have many steps in which each chemical is converted to the next. A specific enzyme controls each reaction.

Enzymes control cell metabolism by regulating how and when reactions occur.

Enzyme Substrate ComplexThe chemical which an enzyme works on is called

its substrate.

An enzyme combines with its substrate to form a short-lived enzyme/substrate complex. Once a reaction has occurred, the complex breaks up into products and enzyme.

E + S ES EP E + P

The enzyme remains unchanged at the end of reaction and is can interact again with more substrate and be reused.

Vocab Check

1. Substrate: Substance that the enzyme acts upon.

2. Product: Formed from the substrate at the end of the reaction.

3. Enzyme: Biological catalyst. Speeds up the chemical reaction without being changed by the reaction.

Enzyme Substrate Complex

How Do Enzymes Work?

• Lower the activation energy, allowing reactions to occur more quickly.

Enzyme Specificity

• Two models that may explain how enzymes work are:– 1) The lock and key hypothesis

– 2) The induced fit hypothesis

Lock and Key Hypothesis• Enzyme has a particular

shape into which the substrate or substrates fit exactly. This is often referred to as the ‘lock and key’ hypothesis where the substrate is imagined being like a key whose shape is complementary to the enzyme or lock. The site where the substrate bonds in the enzyme is known as the active site and it has a specific shape.

Vocabulary Check

1. Substrate: substance that the enzyme acts upon.

2. Product: formed from the substrate at the end of the reaction.

3. Enzyme: biological catalyst. Increase the rate of the reaction without being changed by the reaction.

4. Active site: area on the enzyme that binds it to the substrate.

Lock and Key Fit

Induced Fit Hypothesis– The active site in many enzymes is not exactly the same

shape as the substrate, but molds itself around the substrate as the enzyme substrate complex is formed.

– Only when the substrate binds to the enzyme is the active site the correct shape to catalyze the reaction. As the reaction’s products form they fit the active site less well and fall away from it. Without the substrate, the enzyme reverts to its ‘relaxed’ state, until the next substrate comes along.

Naming Enzymes

• Is it an enzyme?• Look for the ending. Many, but not all,

enzymes end in –ase.– EXAMPLES: • Lipase• Amylase• Protease• Polymerases• Ligases• Transferases

What factors affect enyzme activity?

• The factors that affect enzyme activity also affect cell functions and ultimately the organism. Enzymes are proteins and their function is therefore affected by:

Temperature

pH

Substrate concentration

Enzyme concentration

Cofactors

Inhibitors

Temperature• For a non-enzymatic chemical reaction,

the general rule is: the higher the temperature, the faster the reaction. This same rule holds true for a reaction catalyzed by an enzyme, but only up to about 40-450C. (104-113 deg.F)

• Above this temperature, enzyme molecules begin to vibrate so violently that the delicate bonds that maintain tertiary and quaternary structure are broken, irreversibly changing the shape of the molecule. When this happens, the enzyme can no longer function and it is said to be denatured.

pH Like other proteins, enzymes are

stable over a limited range of pH. Outside this range, at the extremes of pH, enzymes are denatured. Free hydrogen ions (H+) or hydroxyl ions (OH-) affect the changes on amino acid residues, distorting the 3D shape and causing an irreversible change in the proteins tertiary structure.

Enzymes are particularly sensitive to changes in pH because of the great

sensitivity of their active site. A slight change in pH can denature the enzyme and make it unusable.

•

Substrate Concentration

• The rate of an enzyme-controlled reaction increases as the substrate concentration increases, until the enzyme is working at full capacity (turnover rate). At this point, the enzyme molecules reach their turnover number and assuming that all other conditions such as temperature are ideal, the only way to increase the speed of the reaction even more is to add more enzyme.

Enzyme Concentration

• In any reaction catalyzed by an enzyme, the number of enzyme molecules present is very much smaller than the number of substrate molecules. When an abundant supply of substrate is available, the rate of reaction is limited by the number of enzyme molecules present. In this situation, increasing the enzyme concentration increases the rate of reaction.

Cofactors

• Some enzymes cannot work on their own, they need a molecule called a cofactor in order to work properly. Cofactors modify the enzyme complex so that it has the chemical properties necessary to catalyze a reaction.

• One common type is known as coenzymes: Relatively small organic molecules are not permanently attached to the enzyme molecule.

• Also known as Coenzymes. Vitamins are examples of Coenzymes.

Inhibitors

Inhibitors slow down or stop enzyme reaction. Usually, enzyme inhibition is a natural process, a means of switching enzymes on or off when necessary.

Inhibition can be reversible and the enzyme returns to full activity once the inhibitor is removed. Drugs and poisons can inhibit particular enzymes, this type of inhibition is often non-reversible.

Competitive Inhibitors

• Compete with normal substrate molecules to occupy the active site. A competitive inhibitor fits into the active site of the enzyme preventing the real substrate from gaining access. The inhibitor cannot be converted to the products of the reaction and so the overall rate of reaction is slowed down.

• Type of reversible inhibitor

• Fig: Competitive inhibitors bind reversibly to the enzyme, preventing the binding of substrate. On the other hand, binding of substrate prevents binding of the inhibitor. Substrate and inhibitor compete for the enzyme.

Non-Competitive Inhibitors

• Non-competitive inhibitors bind to the enzyme away from the active site but change the overall shape of the molecule(enzyme), modifying the active site so that it can no longer turn substrate molecules into product. Non-competitive inhibition has this name because there is no competition for the active site.

Non-competitive Inhibitor

Irreversible Inhibitors

• Irreversible inhibitors bind permanently to the enzyme, rendering it useless. For example, cyanide is an irreversible inhibitor.

Review of Important Enzyme Properties

• Enzymes have several important properties:– Enzymes are specific: each enzyme usually catalyses only one

reaction.

– Enzymes combine with their substrates to form temporary enzyme-substrate complex.

– Enzymes are not altered or used up by the reactions they catalyze, so can be used again and again.

– Enzymes are sensitive to temperature and pH.

– Many enzymes need cofactors in order to function.

– Enzyme function may be slowed down or stopped by inhibitors.