Enzymes

Enzymes

All enzymes are globular proteins and round in shape

They have the suffix "-ase"

Intracellular enzymes are found inside the cell

Extracellular enzymes act outside the cell (e.g. digestive enzymes)

Enzymes are catalysts → speed up chemical reactions

Reduce activation energy required to start a reaction between molecules

Substrates (reactants) are converted into products

Reaction may not take place in absence of enzymes (each enzyme has a specific catalytic action)

Enzymes catalyse a reaction at max. rate at an optimum state

Induced fit theory

Enzyme's shape changes when substrate binds to active site

Amino acids are moulded into a precise form to perform catalytic reaction effectively

Enzyme wraps around substrate to distort it

Forms an enzyme-substrate complex → fast reaction

E + S → ES → P + E

Enzyme is not used up in the reaction (unlike substrates)

Changes in pH

Affect attraction between substrate and enzyme and therefore efficiency of conversion process

Ionic bonds can break and change shape / enzyme is denatured

Charges on amino acids can change, ES complex cannot form

Optimum pH

pH 7 for intracellular enzymes

Acidic range (pH 1-6) in the stomach for digestive enzymes (pepsin)

Alkaline range (pH 8-14) in oral cavities (amylase)

pH measures the conc. of H+ ions - higher conc. will give a lower pH

Enzyme Conc. is proportional to rate of reaction, provided other conditions are constant. Straight line

Substrate Conc. is proportional to rate of reaction until there are more substrates than enzymes present. Curve becomes constant.

Increased Temperature

Increases speed of molecular movement → chances of molecular collisions → more ES complexes

At 0-42 °C rate of reaction is proportional to temp

Enzymes have optimum temp. for their action (varies between different enzymes)

Above ≈42°C, enzyme is denatured due to heavy vibration that break -H bonds

Shape is changed / active site can't be used anymore

Decreased Temperature

Enzymes become less and less active, due to reductions in speed of molecular movement

Below freezing point

Inactivated, not denatured

Regain their function when returning to normal temperature

Thermophilic: heat-loving

Hyperthermophilic: organisms are not able to grow below +70°C

Psychrophiles: cold-loving

Inhibitors

Slow down rate of reaction of enzyme when necessary (e.g. when temp is too high)

Molecule present in highest conc. is most likely to form an ES-complex

Competitive Inhibitors

Compete with substrate for active site

Shape similar to substrates / prevents access when bonded

Can slow down a metabolic pathway

[EXAMPLE] Methanol Poisoning

Methanol CH3OH is a competitive inhibitor

CH3OH can bind to dehydrogenase whose true substrate is C2H5OH

A person who has accidentally swallowed methanol is treated by being given large doses of C2H5OH

C2H5OH competes with CH3OH for the active site

Non-competitive Inhibitors

Chemical does not have to resemble the substrate

Binds to enzyme other than at active site

This changes the enzyme's active site and prevents access to it

Irreversible Inhibition

Chemical permanently binds to the enzyme or massively denatures the enzyme

Nerve gas permanently blocks pathways involved in nerve message transmission, resulting in death

Penicillin, the first of "wonder drug" antibiotics, permanently blocks pathways certain bacteria use to assemble their cell wall component (peptidoglycan)

End-product inhibition

Metabolic reactions are multi-stepped, each controlled by a single enzyme

End-products accumulate within the cell and stop the reaction when sufficient product is made

This is achieved by non-competitive inhibition by the end-product

The enzyme early in the reaction pathway is inhibited by the end-product

The metabolic pathway contains a series of individual chemical reactions that combine to perform one or more important functions. The product of one reaction in a pathway serves as the substrate for the following reaction.

Cells & Molecules

Cell Division

Cell Types

Cell Ultrastructure

Enzymes

Gene Technology

Genes, DNA, RNA

Large Molecules

Plasma Membrane

Respiration

Content

Enzymes

Changes in pH

Increased Temperature

Decreased Temperature

Inhibitors

End-product inhibition