Enzyme Action

What you should learn

How biochemical reactions are catalysed by enzymes.

The precise role of active sites. Types of enzyme inhibition. The factors affecting the rate and

direction of biochemical reactions and metabolic pathways.

Activation energy

The energy needed to either make or break chemical bonds in the reactant chemicals (enzyme substrates) is called the activation energy.

When they have absorbed sufficient energy (e.g. heat), chemical bonds break and the reactants reach an unstable transition state.

A reaction can now occur.

Enzymes and activation energy

Enzymes (biological catalysts):-

1. Lower the activation energy required for a chemical reaction.

2. Speed up the rates of reactions.3. Remain unchanged after the

reactions are complete.

Activation energy

Activation energy

Enzymes therefore allow reactions to occur very quickly and at temperatures low enough to sustain life (5–40oC)

Induced fit and Orientation

Active sites are not rigid but are flexible and dynamic.

The uptake of a substrate ‘induces’ a slight change in the shape of the enzyme and it’s active site, making for a more precise fit.

This induced fit increases the chance of a reaction taking place by . . . .

. . . . . orientating the substrates into the best position to react.

Induced fit

Now . . . . . calm down!! and

Watch the animation:-

http://web.chem.ucsb.edu/~molvisual/ABLE/induced_fit/index.html

The effect of increasing substrate concentration.

Lets start with a fixed concentration of enzyme which is higher than the substrate conc. (i.e. more enzyme molecules than substrate).

As we increase the substrate concentration, more enzyme/substrate complexes can form.

This speeds up the rate of reaction

The effect of increasing substrate concentration.

The effect of increasing substrate concentration.

But when the enzyme is no longer in excess, the number of complexes is at a maximum.

The rate of reaction levels off and remains constant no matter how much more substrate is added.

The effect of increasing substrate concentration.

Enzyme Inhibition

Inhibitors are chemicals which slow down or stop an enzyme controlled reaction.

They can be competitive inhibitors.

Or they can be non competitive inhibitors.

Competitive inhibitors

Can bind to the active site of the enzyme, thereby competing with the substrate.

This prevents the substrate from binding.

Competitive inhibitors

Competitive inhibitors

Watch the link:- http://bcs.whfreeman.com/thelifewire/content/chp06/0602001.html

And http://www.youtube.com/watch?v=PILzvT3spCQ

Competitive inhibitors

The effect of a competitive inhibitor is reduced if the substrate concentration is increased.

Non - competitive inhibitors

Bind to the enzyme but not at the active site.

Alter the shape of the active site. Substrate molecules cannot bind. Non competitive inhibitors are not

influenced by change in substrate concentration.

Examples include cyanide and heavy metals such as lead, copper and mercury.

Non - competitive inhibitors

Now watch the link:- http://www.youtube.com/watch?v=PILzvT3spCQ

Competitive and non-competitive inhibitors summary

Feedback (end product) inhibition.

A third type of inhibition occurs when an end product binds to an enzyme that catalyses a reaction early in the pathway.

Feedback (end product) inhibition.

‘Feedback’ or ‘end point’ inhibition is another way in which metabolic pathways can be regulated.

It is an example of ‘negative feedback’ control and helps avoid wasteful overproduction of products.

Now watch this clip:- http://highered.mcgraw-hill.com/olcweb/cgi/

pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120070/bio10.swf::Feedback%20Inhibition%20of%20Biochemical%20Pathways