Enzymes

Objectives:

(a)

explain that enzymes are globular proteins that catalyse metabolic reactions;

(b)

explain the mode of action of enzymes in terms of an active site, enzyme/substrate complex, lowering of

activation energy and enzyme specificity;

Enzymes

… are biological catalysts in the form of globular proteins, often conjugated and often showing

quaternary structure.

Like all catalysts …

… enzymes speed up reactions by providing a pathway with a lower activation energy:

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

The reaction occurs …

… while the substrate is temporarily bound to the enzyme’s ACTIVE SITE.

The enzyme LACTASE The substrate ((in this case lactose) binds to the active site forming an ENZYME-SUBSTRATE COMPLEX

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_enzymes_work.html

Models of enzyme action

The LOCK AND KEY model

postulates an exact steric fit between

active site and substrate:

The INDUCED FIT model suggests only a rough initial fit

between active site and substrate, but that substrate

binding alters the active site to its catalytic form:

Action of the enzyme HEXOKINASE (glucose-6-phosphatase)

Hexokinase (glucose-6-phosphatase) catalyses the first reaction in the oxidation of glucose during respiration.

A molecule called adenosine triphosphate (ATP) donates a phosphate group to glucose, making the more reactive product glucose-6-phosphate.

http://www.learnerstv.com/animation/animation.php?ani=161&cat=Biology

Factors affecting the rate of enzyme-catalysed reactions: 1

What happens to rate between 0 and 40 oC, and why?

The rate increases proportionally with temperature because molecules have more energy and enzyme-substrate collisions are more frequent.

What happens above 45 oC?

Above this point the enzyme becomes DENATURED: the 3D structure of the active site is altered, and the substrate can no longer bind.

Effect of temperature:

Factors affecting the rate of enzyme-catalysed reactions: 2

Effect of pH:Any given enzyme has an optimum pH, and works efficiently in only a narrow range either side. Why is this?

Changes in pH affect the ionisation of acidic and basic amino acid side chains, and thus alter the enzyme’s tertiary structure. The enzyme is denatured outide its optimum pH range.

Factors affecting the rate of enzyme-catalysed reactions: 3

Effect of enzyme concentration:

Because enzyme molecules are almost always greatly outnumbered by substrate molecules, the rate of an enzyme-catalysed reaction is directly proportional to enzyme concentration.

Factors affecting the rate of enzyme-catalysed reactions: 4a

Effect of substrate concentration:

The more efficient an enzyme, the lower the substrate concentration necessary to reach substrate-saturation and maximum reaction rate (Vmax).

ENZYME INHIBITION

Factors affecting the rate of enzyme-catalysed reactions: 4b

More on effect of substrate concentration:

The substrate concentration giving 1/2Vmax is called the Michaelis constant (Km), and is a measure of the enzyme’s affinity for its substrate. The lower the value of Km, the more efficient the enzyme.

Factors affecting the rate of enzyme-catalysed reactions: 5

Effect of competitive inhibitors:

Here an enzyme (purple) is catalyzing the combination of two substrates (blue and green) into a product (red).

A COMPETITIVE INHIBITOR (yellow) has a similar shape to one of the substrates, and binds to the active site without reacting. While it is bound, the substrate cannot enter, and the reaction rate is slowed down.

Factors affecting the rate of enzyme-catalysed reactions: 6

Effect of non-competitive inhibitors:

A non-competitive inhibitor binds to a region of the enzyme molecule other than the active site.

When it is bound the tertiary structure of the enzyme is altered, and the substrate will no longer bind to the distorted active site.

Non-competitive inhibitors, like competitive inhibitors, usually bid reversibly.

Enzyme inhibitors in pharmacology

The image shows the ‘designer drug’ methotrexate in the active site of the enzyme DHFR (dihydrofolate reductase).

DHFR is especially active in rapidly dividing cells, making methotrexate an effective anti-cancer agent.

Effects of competitive and non-competitive inhibition

Competitive inhibition Non-competitive inhibition

Irreversible inhibition

Irreversible inhibitors, such as heavy metal ions, bind irreversibly to side chains on the enzyme molecule and distort its tertiary structure:

Allosteric control of enzymesThe susceptibility of enzymes to inhibition by molecules which do not resemble the substrate makes it possible to control their functioning by the presence or absence of non-active-site-directed inhibitors:

This makes possible FEEDBACK INHIBITION, in which the end-product of a metabolic pathway ‘switches off’ the first enzyme in the pathway:

Allosteric control of enzymes (2)

Enzymes may also be subject to ALLOSTERIC ACTIVATION:

Here a molecule other than the substrate has to bind to an allosteric site before the active site takes on its functional shape. For example, the chloride ion acts as an allosteric activator of salivary amylase.

Rate

0

20

40

60

80

100

120

0 20 40 60 80

Rate