Inhibitors

Many drugs and poisons are inhibitors of enzymes in the nervous system.

Competitive Inhibitor

• resembles shape of substrate closely
• competes with substrate for active site
• binds to active site either reversibly or irreversibly
• prevents substrate from binding to enzyme active site, reducing rate of reaction

Non-competitive Inhibitor

• has a different structure from substrate
• binds to an allosteric site, causing a change in the shape of active site
• substrates can no longer bind to the active site and no enzyme-substrate complex can be formed

Factors affecting enzyme reactions

Substrate concentration

• At low substrate concentration, increasing the substrate concentration will result in a proportional increase in the rate of reaction.
• As there are sufficient active sites, the increase in substrate concentration will increase the frequency of effective collisions between the enzyme and substrate molecules, increasing the rate of formation of enzyme-substrate complex.
• However, at high substrate concentration, increasing the substrate concentration will no longer speed up the reaction.
• All the active sites of the enzyme molecules will be occupied at any given moment.
• Any added substrates have to ‘wait’ until existing enzyme-substrate complexes are dissociated and the active sites become available for binding.
• The rate of reaction can only be increased with the addition of enzymes.

Image taken from: http://alevelnotes.com/content_images/i73_Image3.gif

Enzyme concentration

• At low enzyme concentration, increasing the enzyme concentration will result in a proportional and linear increase in the rate of reaction.
• The increase in enzyme concentration provides more active sites which the substrate molecules can bind to, increasing the rate of effective collision and the formation of enzyme substrate complex.
• However, at high enzyme concentration, increasing the enzyme concentration will no longer have effect on the reaction as all the substrate molecules would already have been converted into their products.
• The rate of reaction can only be increased with the addition of substrates.

Image taken from: http://alevelnotes.com/content_images/i74_Image4.gif

Temperature

• Enzymes work best at their optimum temperature, which is usually 40°C.
• Below the optimum temperature, the rate of reaction increases linearly with the increase in temperature.
• As the temperature increases, kinetic energy of the molecules increase. This increases the rate of effective collisions between enzyme and substrate molecules and the formation of enzyme-substrate complex.
• Beyond the optimum temperature, the rate of reaction decreases even though the frequency of collisions increases.
• Thermal agitation of the enzymes break the hydrogen bonds, ionic bonds and hydrophobic interactions that stabilize the specific 3D conformation of the enzyme.
• The enzyme becomes denatured as the shape of the enzyme's active site is altered and is no longer complementary to that of the substrate.

Image taken from: http://alevelnotes.com/content_images/i71_gcsechem_18part2.gif

pH

• Enzymes work best at their optimum pH level.
• A change in pH level will result in the alteration of the ionic charge of the R groups of the amino acid residues.
• This breaks the ionic bonds and hydrogen bonds that are responsible for maintaining the specific 3D conformation of the enzyme.
• The enzyme becomes denatured as the shape of the enzyme's active site is altered and is no longer complementary to that of the substrate.

Image taken from: http://alevelnotes.com/content_images/i72_enzyme_ph_graph.gif

Enzymes

What are enzymes?

• large biological molecules which catalyse chemical reactions in living organisms
• responsible for the metabolic processes which sustain life
• enzymes can be proteins or ribonucleic acid (RNA)

Properties of enzymes:

• lower the minimum amount of energy needed to start a reaction (activation energy)
• has active site of specific shape, which is complementary to its substrate 
• does not alter properties of end products of reaction
• highly efficient in small amounts
• denatured easily due to high heat

How do enzymes work?

1. After an effective collision between a substrate and an enzyme (where the substrate binds to the active site of the enzyme), an enzyme-substrate complex is formed. 
2. The substrate molecule is held in the active site by interactions such as hydrogen and ionic bonds between the R groups of the amino acids and the substrate molecule. 
3. Enzyme catalyses conversion of substrate to the end product. 
4. The alteration of chemical conformation results in the product being released from the active site since it is no longer complementary to the structure of the active site. 
5. The active site is then available for other substrates to bind to it.

"Lock and Key" Hypothesis

• there is an exact fit between the substrate and the active site of the enzyme
• the enzyme (lock) has a unique shape complementary to the substrate (key)
• enzymes are very specific; only substrates that are exactly complementary to its active site are able to bind with it

image taken from: http://katysstudynotes.files.wordpress.com/2010/11/enzymes.png

Induced Fit model

• active site of enzyme is flexible 
• active site is able to mould itself around the substrate to make the fit better
• active site returns to original shape after the products are released 

Illustration of the induced fit model

image taken from: http://zebrafish.umdnj.edu/Pre-Enrollment/Resources/Biochemistry/Enzymes%20and%20Metabolism/Images/enzyme_fit_diagram.png