Lecture 11: An Introduction to Enzymes

Why are enzymes important?

What do they do?

Critical for metabolism and digestions, they control reactions, for example speeding them up, switching them on and off

A specific enzyme can control each individual reaction, overall controlling complex metabolism

What does the presence of an enzyme catalyst affect?

The activation energy required for the reaction to proceed

What are the molecules that are acted on by enzymes called?

Substrates

What is the relationship between enzymes and activation energy?

Enzymes catalyse nearly all the chemical reactions taking place in the cells

They increase the rate of reaction by lowering the energy of activation

Offer an alternative pathway

What type of molecule are enzymes (for the purpose of this course)

Proteins

Are enzymes specific to one compound?

Some of them are (i.e. urease for urea)

Some are stereoselective (catalyse the isomers of the same compound)

Some catalyse reactions of specific types of compounds or bonds, (i.e. trypsin catalyses hydrolysis of peptide bonds formed by the carboxyl groups of Lys and Arg.

How do you name an enzyme?

Replace the end of the name of either the reaction or reacting compound with -ase

Eg// sucrase catalyses the reaction of sucrose and oxidase catalyses an oxidation reaction

Some of them have common names (eg peptin)

How do enzymes reduce energy requirements (4)?

It lowers the activation energy barrier by:

1. Orienting substrates correctly

2. Straining substrate bonds (make the bonds weaker eg easier to break)

3. Proving a favourable microenvironment (i.e. more acidic or basic)

4. Covalently bonding to the substrate

What is the active site of an enzyme?

A regional within the enzyme that aligns with the substrate

It contains amino acid R groups that bind the substrate

The specific combination of these means that only certain substrates can bind

Do enzymes change delta g and the equilibrium constant?

No, they just lower the activation energy by proving an alternative pathway to gain / lose the same amount of energy

What are the two methods of kinetic measurement for rates of reaction?

1. The decreasing concentration of the substrate

2. The increasing concentration of the products

The amount of change in a defined time interval

What is happening when a maximum rate of reaction is reached?

Because the concentration of an enzyme is usually much lower than the substrate, when all the enzyme is bound to substrate (it is at saturation) then the maximum rate will be reached

What can the Michaelis-Menton model tell you

Whether the substrate and the enzyme have a high affinity or a low affinity

What are variables that can influence an enzyme reaction (8)?

1. Time

2. pH

3. Temp

4. Substrate concentration

5. Enzyme concentration

6. Coenzymes

7. Activators

8. Inhibitors

How does temp affect enzyme activity?

Enzymes all have an optimal pH (for most of the ones that exist in our body it is 37.5, for some it can be higher or lower) they are most active at that temp and then show little activity at low temps and denature at high temps

How does pH affect enzyme activity?

Enzymes are most active at their ideal pH because they contain R groups of amino acids with proper charges at optimum pH.

pH acts as a switch in your body, for example pepsin is turned on in the stomach because the pH is 2, and then turned off in the small intestines where the lab is 7-8.

What are enzyme cofactors?

Non protein enzyme helpers

Can be inorganic such as a metal ion or organic

Can be grouped into 3 types;

1. Activators

2. Co-enzymes

3. Prosthetic groups

What are co-enzymes (an enzyme co-factor)

Non protein organic molecules

Help transfer chemicals form the active site of one enzyme to the active site of another enzyme.

Some are permanently bound (if they are permanently bound then they are a prosthetic group)

What are prosthetic groups (enzyme co-factor)?

Basically built in co-enzymes (non protein organic molecules, but that are attached to the enzyme molecule)

The transfer atomic or chemical groups from the active site of the enzyme to some other substance

***Sometimes prosthetic groups can be removed during a reaction, in which case they experimentally need to be added back

What are the three types of enzyme inhibitors?

Competitive

Noncompetitive

Irreversible