Enzymes

Just because a reaction is spontaneous, thatdoes not mean it will happen rapidly

Enzymes catalyze reactions by loweringactivation energy of the reaction

Enzymes lower activation energy of reactions

Where does activation energy come from?

This energy makes chemical bonds unstableenough to break

Can come from heat; but not in a physiologicalenvironment

Enzymes lower activation energy so that a reaction can proceed

They cannot change the direction of a reaction(cannot make an exergonic reactionendergonic- how DO those occur?)

Essential features of enzymes

Specificity for substrateDefined by active site

actually a small part of the enzyme“induced fit”

Is not itself consumed by the reaction

Is a protein, with protein properties

Modeling enzyme activity

What happens in “induced fit”?

Substrate activationconformational changeenzyme may donate or accept protons

affecting reactivity of substrate;may be why pH is so important

enzyme may donate or accept electronsforming (temporary bonds betweenenzyme and substrate)

“Catalytic event” results in formation of product

Product is released; enzyme may be used again.

Active site and catalysis

Environmental effects on enzyme function

• Heat tends to increase enzyme activity- up to a point

• Enzymes in our bodies are optimized for physiological conditions

• Optimal range may be narrow or broad

• Cofactors or coenzymes may also be required

Enzyme activity can be saturated

Regulation of enzyme activity

Temperature and pHSubstrate, enzyme concentration

Availability of cofactors, coenzymes

Inhibitorscompetitiveallosteric

Cofactors and coenzymes• Cofactors are

metallic ions– Might bind to active

site• Coenzymes are often

derived from vitamins– Usually act as carriers

of electrons, protons, or other molecules

Inhibition of enzyme activity

• Competitive inhibitors bind to the active site

• Allosteric- bind to another site on the enzyme and change its conformation

Competitive inhibitioncan be reversible or irreversible

can be overcome by adding more substrate

irreversible inhibitors bind covalentlyexample: neurotoxins bind toacetylcholinesterase and inhibittransmission of nervous impulses(many insecticides work this way)

reversible inhibitors bind noncovalently

Noncompetitive inhibitors do NOT bind at theactive sight (but do bind the enzyme)

Allosteric regulation

Covalent modification

Note: allosteric and active sites are different

Many biological reactions occur in pathways

A B C D E

E is the end product of the reaction

Feedback inhibition prevents excess accumulationof E, or depletion of A

Pathways and their regulation (usually allosteric)

Enzymes can be regulated by covalentmodification

Chemical groups are added or removed fromenzymes which affects their activity

Phosphorylation and dephosphorylation areamong the most ubiquitous

Kinases and phosphorylases do just that

Proteolytic cleavage can convert inactive“proenzymes” to active forms

Enzymes with related functions may becollected into specific parts of the cell

Mitochondrion- respiration

Endoplasmic reticulum- protein modification

Lysosome- degradation of waste products

Ribozymes- not all enzymes are proteins!

Cech et al., 1980’s (Nobel Prize, 1989)

RNA is processed by removal of segmentscalled introns

RNA was shown to excise the introns andbehave like enzymes, i.e., Michaelis-Menton kinetics

RNA may have been the first catalyst,proteins later

Summary• Enzymes are proteins• Enzymes interact with specific

substrates• Enzyme activity can be affected by

environmental factors• Enzyme activity can be inhibited• Enzymatic reactions are organized into

pathways in the cell