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Biology 12 Mr. Kruger

Enzymes An enzyme is a catalyst (a substance that speeds up a reaction

without being consumed). Enzymes are proteins and are reusable. They work in low concentrations and speed up the reaction rate.

Starch Glucose

Lipids Fatty Acids and Glycerol

Proteins Amino Acids

Enzymes allow reactions to proceed at lower temperatures than they would normally occur.

The reactant(s) that an enzyme acts upon is known as the substrate(s).

Enzymes work by forming a very temporary complex with the substrate. This is called the ENZYME SUBSTRATE COMPLEX

Enzyme Structure Enzymes are large globular proteins with very specific 3-D shapes

(tertiary structure). Enzymes have grooves (or pockets), which may contain chemically

functional groups. These areas are called active sites and this is where the substrate

attaches.


Lock and Key Analogy for enzyme structure & Induced Fit Specific groove shapes and chemical groups in an active site means that

enzymes can only bond with one specific substrate or reactant.

When the substrate and enzyme join together, the shape of the enzyme changes, which makes it more reactive.

This is called INDUCED FIT


Co-enzymes Many enzymes are made up of 2 pieces. The APOENZYME – the protein portion (inactive) The CO-ENZYME – a non-protein portion When these join together, the enzyme becomes active and the substrate

will now ‘fit’ into the active site..

Co-enzymes usually fit into the ALLOSTERIC site, which changes the shape of the active site so the substrate can “fit.”

The co-enzymes are often large molecules that the body cannot make on its own.

Most co-enzymes are VITAMINS which we get from food or supplements.


Anabolism & Catabolism The active site of an enzyme is not an exact perfect fit to the substrate. When the substrate attaches to the enzyme, this causes stress in the

substrate, which will cause: A substrate to break apart (in a hydrolysis reaction). Another word for this is CATABOLISM:breaking big molecules in to smaller

ones. Two substrates to form a bond (in a synthetic reaction). Another word for this is ANABOLISM: putting small molecules together to

make bigger ones.

Anabolism + Catabolism = Metabolism• Definition: Metabolism is the constantly occurring chemical reactions that

take place in a cell.

• These chemical reactions occur in organized sequences from reactants to end products with the help of enzymes.

• This organized sequence of reactions is known as a metabolic pathway.

Enzyme ActionUsually, heat is used to speed up chemical reactions by increasing the number of collisions that occur between reactants.

Excessive heat, however, destroys the tertiary structure of protein (denatures it).

Therefore, heat cannot be used to speed up reactions within living organisms.

REACTANT

Intermediate products









PRODUCT


Enzymes operate by lowering the energy of activation needed for a reaction to occur.

Enzymes act as a CATALYST and are not consumed in a reaction. This means that they can be used over and over again.

Factors affecting Reaction rates


1. Concentration: The amount of enzyme and/or substrate available to react can affect enzyme activity.

a. The reaction speeds up as the [substrates] increases, and it levels out when the enzymes are working at the maximum speed (saturation).

The reaction speeds up as you increase the [enzyme], and slows down as the substrate has all been turned into product.

Enzyme Concentration

Add enzymes

Add substrate


2. Temperature: As temperature rises, the reaction rate will increase as the enzymes and substrates bump into each other more often (kinetic molecular theory).

At a certain point, the rate of these collisions will be at the fastest rate. This is the OPTIMUM TEMPERATURE

However, once you get above the optimum temperature, the enzyme becomes denatured (changes shape) and no longer functions properly.

Most of our enzymes have an optimal temperature of 37oC (body temperature).

3. pH: The 3D shape of an enzyme can be affected by pH. All


enzymes have an optimal pH to work at depending on where they are in the body.

• Saliva pH 7

• Stomach pH 2.5

• Intestines pH 8.5

When the pH is too low, the positive hydrogen ions interact with the negative R groups in the protein and tear them away. This denatures the enzyme by changing its shape.

When the pH is too high, the negative hydroxide ions interact with the positive R groups in the protein and tear them away. This denatures the enzyme by changing its shape

4.Inhibitors: Chemicals that interfere with the enzyme action.

There are two types of INHIBITORS:

a) Competitive Inhibitors

b) Non-Competitive Inhibitors

Allosteric Site


Competitive Inhibitors are chemicals that so closely resemble an enzyme’s normal substrate that it can attach to the enzymes active site. The substrate and inhibitor “COMPETE”.

If the inhibitor occupies the active site of the enzyme, the substrate will not be able to join and no product will form from that enzyme.

If the Inhibitor is removed, the enzyme will become active again.

Many

medications, such as aspirin and antibiotics are inhibitors. The enzyme substilin digests proteins, and is used in

laundry detergent. Rennin, an enzyme extracted from calves, is used in

curdling milk to make cheese. Glucose oxidase detects glucose in the urine (for example

in diabetics).


Non-Competitive Inhibitors are atoms or molecules that attach to an enzyme at an allosteric site and this denatures the enzyme.

Non-competitive inhibitors will sometimes destroy an enzyme by permanently binding to the allosteric site.

An example of this is heavy metals, such as lead in the nervous system.

Another type of non-competitive inhibition is when a metabolic product can feedback on a metabolic pathway to control how much product is made.

The final product can temporarily attach to the allosteric site on the first enzyme.

The enzyme will be denatured and the reaction will stop.

This is an example of NEGATIVE FEEDBACK or FEEDBACK INHIBITION.

When the concentration of the final product gets low again, there will be less inhibition on the enzymes and the metabolic pathway is reactivated.


Feedback Inhibition and Control of Metabolic Rate Thyroxin, the hormone that controls the metabolic rate

of all of the cells in your body, is produced by the thyroid gland in the neck.

If the concentration of thyroxin in your body is high, your metabolic rate will be raised, and if thyroxin levels are low, your metabolic rate will be low.

The thyroid gland is stimulated to release thyroxin by a hormone produced in the pituitary gland called TSH (thyroid stimulating hormone).

But the enzymes in cells of the pituitary that make TSH are inhibited by thyroxin.

Therefore, if thyroxin levels are high, the pituitary stops producing TSH, and if thyroxin levels are low, the pituitary makes the TSH.

Thus, the metabolic rate of cells in your body are maintained by the feedback inhibition of an enzyme

Pituitary gland TSH Thyroid gland Thyroxin Increased cellular metabolic rate

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