Enzymes

Biological CatalystsBiological Catalysts

Proteins that change the rate of Proteins that change the rate of cellular reactions without being cellular reactions without being

consumed in the reactionconsumed in the reaction

What happens in a chemical reaction? Bonds are brokenBonds are broken New bonds are formedNew bonds are formed

Energy Investment in a Reaction Reactants must absorb energy from their Reactants must absorb energy from their

surroundings for their bonds to breaksurroundings for their bonds to break Products release energy when their new Products release energy when their new

bonds are formedbonds are formed The initial investment of energy for The initial investment of energy for

starting a reaction is starting a reaction is ACTIVATION ENERGY, EACTIVATION ENERGY, EAA

The reaction is

Exergonic if the new bonds release more Exergonic if the new bonds release more energy than was invested in the breaking energy than was invested in the breaking of bondsof bonds

Endergonic if the new bonds release less Endergonic if the new bonds release less energy than was invested in the breaking energy than was invested in the breaking of bondsof bonds

Why do molecules such as proteins, DNA, and other molecules that are in the cell which are free in energy and have the potential to decompose simultaneously not breakdown continually?

ANSWER: The activation energy for these reactions is too high at cellular temperatures.

BIG IDEA!!!!!!

Enzymes speed chemical reEnzymes speed chemical reactions by lowering the actiactions by lowering the activation energy barriervation energy barrier

Enzymes cannot

Change Change ΔGΔG Make an endergonic reaction Make an endergonic reaction

exergonicexergonic They can only HASTEN They can only HASTEN

REACTIONS THAT WOULD REACTIONS THAT WOULD OCCUR ANYWAY!OCCUR ANYWAY!

Enzymes are substrate specific

Substrate—the reactant an enzyme acts Substrate—the reactant an enzyme acts on.on.

The substrate “fits” the reactant(s) and The substrate “fits” the reactant(s) and hastens the move to producthastens the move to product

Enzymes distinguish related compounds, Enzymes distinguish related compounds, even isomerseven isomers

An enzyme’s specificity is a function of An enzyme’s specificity is a function of its shapeits shape

Only a region of enzyme binds to the substrate This region is called the This region is called the ACTIVE SITEACTIVE SITE It is a groove on the surface of the proteinIt is a groove on the surface of the protein The shape of the enzyme at the active site The shape of the enzyme at the active site

matches the shape of the substratematches the shape of the substrate This area is not rigid, the enzyme changes This area is not rigid, the enzyme changes

its shape slightly to increase the fit. This its shape slightly to increase the fit. This is called is called INDUCED FITINDUCED FIT

Active Site

How do enzymes lower activation energy? Providing a template for the substrates to Providing a template for the substrates to

come together in the proper orientationcome together in the proper orientation Stressing the substrate molecules, Stressing the substrate molecules,

stretching & bending critical chemical stretching & bending critical chemical bondsbonds

Providing an environment that is Providing an environment that is conducive to a particular type of reactionconducive to a particular type of reaction

What factors affect enzyme activity? The concentration of the The concentration of the

substratesubstrate The concentration of the enzymeThe concentration of the enzyme TemperatureTemperature pHpH

What factors affect enzyme activity? Cofactors—nonprotein helpers usually bound Cofactors—nonprotein helpers usually bound

to the active site. Examples include metal ions to the active site. Examples include metal ions and coenzymes (vitamins)and coenzymes (vitamins)

Competitive inhibitors—Chemicals that Competitive inhibitors—Chemicals that resemble the normal substrate molecule & resemble the normal substrate molecule & compete for admission to the active site.compete for admission to the active site.

Noncompetitive inhibitors—Bind to enzyme Noncompetitive inhibitors—Bind to enzyme away from active site & change protein away from active site & change protein configurationconfiguration

The Control of Metabolism

Cellular metabolism is regulated by Cellular metabolism is regulated by controlling when and where the cell’s controlling when and where the cell’s various enzymes are activevarious enzymes are active

Regulatory molecules bind to Regulatory molecules bind to ALLOSTERIC SITESALLOSTERIC SITES—a specific —a specific receptor site on the enzyme away from receptor site on the enzyme away from the active sitethe active site

Allosteric Regulation

Allosterically regulated enzymes are Allosterically regulated enzymes are constructed from one more subunitsconstructed from one more subunits

Each subunit has its own active site with Each subunit has its own active site with allosteric sitesallosteric sites

Feedback Inhibition

The switching off of a metabolic pathway The switching off of a metabolic pathway by its end product—which acts as an by its end product—which acts as an inhibitorinhibitor