Enzymes

Unit 3: BioenergeticsHonors BiologyMonkemeier

Lowering Activation Energy! Catalysts in living systems, lower the

activation energy to speed up chemical reactions.

The agents that carry out most of the catalysis in living organisms are called enzymes.

The chemical reactions within living systems are regulated by controlling the points at which catalysis takes place.

RNA? Most enzymes are proteins, although

increasing evidence indicates that some enzymes are actually RNA molecules!

In 1981, Thomas Cech at the University of Colorado reported that reactions involving RNA molecules appear to be catalyzed by RNA itself.

RNA catalysts are called RIBOZYMES!

Ribozymes!

Research has revealed at least two kinds of Ribozymes.

Intramolecular catalysis: Ribozymes that have folded structures that catalyze reactions on themselves.

Intermolecular catalysis: Ribozymes that act on other molecules without being changesd themselves.

CONNECTING CONCEPTS!

The ability of RNA to act as a catalyst appears to provide a potential answer to the question – Which came first, the protein or the nucleic acid?

It now seems at least possible that RNA may have evolved first and may have catalyzed the formation of the first proteins!

Enzymes (as Proteins) The unique three-dimensional shape of an

enzyme enables it to stabilize a temporary association between substrates.

The area on the enzyme that matches the shape of its substrate(s) is known as the ACTIVE SITE.

By bringing two molecules together in the correct orientation, or by stressing particular chemical bonds of a substrate, the enzyme lowers the activation energy.

Enzyme – Substrate Complex

Induced Fit- enzyme changes shape slightly to accommodate or match shape of substrate.

Multi-enzyme Complexes Often several enzymes catalyzing different

steps of a sequence of reactions are associated with one another in non-covalently bonded assemblies called multienzyme-complexes.

An example of a multi-enzyme complex is the bacterial pyruvate dehydrogenase multienzyme complex.

This assembly of enzymes in bacteria is involved with cellular respiration.

Benefits of Multienzyme Complexes

They increase catalytic efficiency by In a series of sequential reactions, it makes

it easier for the product of one reaction to become the reactant of the next reaction.

Since the reactant never leaves the complex, it eliminates unwanted side reactions and keeps the needed chain of reactions moving in one direction.

All reactions that take place within the multienzyme complex can be controlled as a unit.

Environmental Factors that Affect Enzyme Function

Every enzyme has its own SPECIFIC range of temperature and pH at which it will perform at its maximum activity level.

Temperatures and pH outside the optimum range for an enzyme can denature or change the shape of the enzyme thereby altering its activity level.

Optimum Temperature and pH Ranges

Inhibitors A substance that binds to an enzyme and

decreases its activity is called an inhibitor. There are two kinds of inhibitors:

Competitive Inhibitors bind to the active site and do not allow the substrate to bind.

Noncompetitive Inhibitors bind to a site on the enzyme known as the allosteric site. The binding of the inhibitor to the allosteric site causes the enzyme to change shape and renders it inactive

Inhibitors: Competitive and Noncompetitive

Allosteric Site

A substance that binds to the allosteric site and reduces enzyme activity is known as an allosteric inhibitor.

A substance that binds to the allosteric site and increases enzyme activity is known as an allosteric activator.

Cofactors Enzyme function is often assisted by

additional chemical components known as cofactors.

These can be metal ions that are often found in the active site participating in catalysis.

Cofactors are different from coenzymes. Coenzymes are typically vitamins that act with the enzyme to perfom the catalysis.

Cofactors, Coenzymes, etc

Enzymes and Metabolism The sum of all chemical reactions in a

cell constitutes its metabolism. This is usually arranged in pathways

where sequential reactions either build up increasing complex molecules or break down complex molecules in steps.

These pathways are regulated and controlled by enzymes.