Enzymes

Topic 2.1

What are Enzymes?The chemical reactions that take place inside an organism, i.e. its’

metabolism, must be controlled. The rate of these metabolic reactions is controlled by enzymes.

Enzymes are globular proteins (3-D spherical shape). They are usually named by adding the suffix –ase to the name of the substrate it helps metabolise.

E.g. Maltase – helps break down the disaccharide maltose Lactase – helps break down the disaccharide lactose Amylase – helps break down the polysaccharide starch (amylose)

• Enzymes regulate the metabolic processes that occur in cells• They are catalysts, they speed up reactions and are not used

up themselves in the reaction • The energy needed to start a reaction is the ACTIVATION

ENERGY• Enzymes lower the activation energy, so reactions can take

place at acceptable temperatures in living organisms

WITH ENZYME – when a substrate binds to an enzyme, the Activation Energy is lowered because they split the reaction into smaller stages

Activation EnergyActivation energy is the amount of energy required to

start off a chemical reaction, where the reactant(s) are turned into product(s).

The reaction will not occur unless ‘extra energy’ is given temporarily to the molecules:

Substrate(s)Substrate

Product(s)

Activation Energy

Pote

ntia

l Ene

rgy

(of

mol

ecul

es)

Time

Substrate(s)Substrate

Product(s)

Activation Energy

Time

Pote

ntia

l Ene

rgy

(of

mol

ecul

es)

WITHOUT ENZYME – to change into a product the energy of the substrate must be briefly raised by an amount known as the Activation Energy

http://www.sumanasinc.com/webcontent/animations/content/enzymes/enzymes.html

Why do enzymes increase the rate?

How Enzymes WorkEnzymes are globular proteins, coiled into a precise 3-D shape,

with hydrophilic side chains on the outer parts of the molecule (making them soluble).

Enzymes have a special feature called an active site, where a substrate molecule(s) bind to, forming an ‘enzyme-substrate complex’. The substrate is then converted to product(s).

Enzyme + Substrate Enzyme + Product(s)Enzyme-substrate

Complex

Enzyme

Substrate (has complimentary shape)

Active Site

Here side-chains of amino acids in the active site temporarily bind with

the substrate

Product

HypothesesThis leaves the active site unchanged and ready to bind

with another substrate molecule.

This was traditionally called the ‘Lock and Key’ hypothesis because the substrate fits into a rigid active site like a key into a lock.

This explains why enzymes are very specific (the substrate must fit the active site exactly to be catalysed).

However this theory has since been developed into the ‘Induced Fit’ hypothesis. Here the active site is believed to change shape slightly so that the enzyme moulds itself around the substrate. (See fig 3, p.43)

Now answer Qu.2 from p.43 in your book)http://www.sumanasinc.com/webcontent/animations/content/enzymes/enzymes.html

Models of enzyme action: lock-and-key

Models of enzyme action: induced fit

Enzymes: true or false?

What factors affect enzymes?The rate of an enzyme-controlled reaction is affected by several factors:

Each enzyme works best within a range of conditions, and this range is different for each enzyme.

temperature

pH

enzyme concentration (you must know in detail!)

substrate concentration.

Enzymes are also affected by the presence of inhibitors.

Effect of enzyme concentration on rate

Rate of reaction experiment

Effect of temperature on enzymes

Effect of pH on enzymes

Factors affecting rate of reaction