Enzymes

Mrs: Jackie

Enzymes

• A biological catalyst that speeds up chemical reactions in the body by lowering the energy needed in the reaction

• Globular proteins are folded , enabling the enzyme to have an active site

The enzyme active site• Chemical reactions

occurs in the active site• Specific to its substrate

forming an enzyme-substrate complex

• Lock and key model- each enzyme can catalyzed a reaction of one specific substrate.

Enzyme active site• Lock and Key– Substrate and

enzyme lock precesily into each other

– A reaction takes place and the products formed

– The enzyme comes out unaffected and can be reused.

Induce fit model of enzyme action

• Hypothesizes that:– The active site of the enzyme is not a rigid pocket

for the substrate to fit in, but as the substrate enters the active site, it induces the enzyme to change its shape in a way that fits the substrate more tightly.

– This explain why some enzymes can catalyse the reaction of more than one substrate

Induce fit model : Hexocinase and glucose

Enzyme action and the activation energy

• For a reaction to start it needs activation energy

• Exergonic reaction- products contain lower amount of energy than the reactants-energy is released

Enzyme action and the activation energy

• Endergonic reaction- products have more energy than the reactants and thus energy is absorbed by the reactants to formed the products

Examples of enzymes catalyzed reactions

• Reactions in the small intestine– Starch + AMYLASE maltose units– Maltose + MALTASE glucose + glucose– Lactose + LACTASE glucose + galactose– Sucrose + SUCRASE glucose + fructose

– All these reactions required water these are _____ reactions.

Examples of enzymes catalyzed reactions

• In red blood cells– Carbon dioxide + water + carbonic anhydrase

carbonic acidThe enzymes facilitates gaseous exchange

• In the cells of organisms breaking of hydrogen peroxide into oxygen and water– Hydrogen peroxide + catalase oxygen +water

Factors affecting enzyme activity

• Temperature– The faster the temperature the faster is the

movement of molecules of enzyme and substrate and the higher the chances of collision between them.

– Enzyme activity increases as temperature increases. It doubles every 10 degrees increased in temp.

– At 40-45 degrees is the optimum temperature. Above this temperature the enzyme will denature

Factors affecting enzyme activity

• Temperature continues– At a temperature of 0 enzymes are inactive but

not denature. They can become active by increasing the temperature.

– Inactivation by freezing is reversible but not by high temperatures.

– Some enzymes resist extreme temperatures such as bacteria living in icy habitats in the Antarctics

Factors affecting enzyme activity

• pH– Most enzymes work best at neutral pH– Digestive enzymes like pepsin work best on an

acidic pH. – Extreme pH changes may denature the enzyme by

changing the shape of the active site due the breaking of hydrogen bonds resposible of the tertiary structure of enzymes

Factors affecting enzyme activity

• Substrate concentration– As the substrate concentration increases, the rate

of enzyme catalyzed reactions increases due to the formation of more substrate-enzyme complex.

– This is because the enzyme concentration is limited and it becomes a limiting factor

– All active sites of the enzyme become occupied by the substrate.

– If more enzymes are added the rate of the reaction will increase.

Factors affecting enzyme activity

Enzyme inhibitors

• Competitive inhibitors– Substances very similar in their structure to the

substrate so they compete with it on the active site– No reaction takes place but it slower the reaction– Normal process needed for controling enzymatic

reactions– Example

• MALONATE similar to the substrate succinate used in krebs cycle compete in the active site of dehydrogenase

• The rate of the reaction depends on the concentration of the substrate

Enzyme inhibitors

• Non competitive inhibitors– Carried out by substaces that are not similar to

the substrate, and so they do not compete with the substrate in the active site of the enzyme.

– They stick to another site of the enzyme.– Allosteric inhibitors- bind to the site of the

enzyme called the allosteric site. Alters the shape of the enzyme and a conformational change in its active site .• Examples Hg+2, Ag+,Cu2+, Sarin (nerve gas)

Enzyme inhibitors

• Feedback inhibition– The end product

inhibits an enzyme at the start of the metabolic pathway.

– Binds to the allosteric part of the enzyme

– Example • Inhibition of

phosphofructokinase by ATP

Enzyme inhibitors

Irreversible inhibitors

• Most are poisonous substances that enters the environment from the surrounding sites.

• Interfere with the binding site of the enzyme causinf the reaction to stop.

• Mercury, silver cyanide, nerve gas.– Cyanide inhibits cytochrome oxidase involve in aerobic

respiration in the mitochondria causing suffocation– Lead- binds to the sulphur part of the enzyme breaking

S-S linkages . Can be reverse by treatment

Irreversible inhibitors

– DDT- pesticide used in the past. Caused inhibition of certain enzymes from the nervous system.

Irreversible inhibitors such as antibiotics are helpful.Penicillin inhibits some bacterial enzymes needed

for building cell walls stopping bacterial growth and reproduction.

Homework

• Page 41, 43,44, 47