Enzymes

Enzymes

• biological catalysts• speed up chemical

reactions without being consumed

• usually proteins with tertiary or quaternary structure

Enzyme Action

• enzymes bind to specific reactants called substrates

• substrates bind to active sites on enzymes, forming the enzyme-substrate complex

Enzyme-Substrate Complex

Induced-Fit Model

• in the “induced-fit” model, the enzyme alters its shape to better “fit” substrate

• contrast with the “lock and key” hypothesis

Example: Sucrase

What reactions do these enzymes catalyze?

• lactase

• breaks down lactose (sugar found in milk)

• ATPase

• breaks off a phosphate group from ATP, resulting in ADP + an inorganic phosphate + energy

• amylase

• breaks down starch (in saliva & intestine)

What about this one?

• ATP synthase

• synthesizes ATP from ADP and inorganic phosphate

Cofactors and Coenzymes

• some enzymes require cofactors to function as catalysts

• cofactors are non-protein molecules or inorganic ions

• coenzymes are organic cofactors

Cofactors

Factors AffectingRate of Enzyme Activity

• substrate concentration

• enzyme concentration

• temperature

• pH

Enzyme Regulation

Competitive Inhibition

Competitive Inhibition

competitive inhibitors• have a similar structure to the

substrate• bind to the enzyme’s active site and

block the substrate from binding

Irreversible Inhibitors

Many drugs & pesticides act as irreversible inhibitors of enzymes

• cyanide (inhibits key enzyme in cellular respiration)

• antibiotics (inhibit bacterial enzymes)

Allosteric Regulation

• some enzymes have allosteric sites

• allosteric regulators either inhibit or stimulate an enzyme’s activity

Allosteric Regulation

Allosteric Regulation

Allosteric Regulation

• allosteric activators promote the action of the enzyme

• allosteric inhibitors prevent the action of the enzyme (i.e. noncompetitive inhibition)

Feedback Inhibition

• often used to regulate multi-step metabolic pathways

• the product of a pathway usually acts as an allosteric inhibitor of the first enzyme of the pathway

• http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter8/animations.html#

Commercial Uses of Enzymes

• cheese• lactose-free dairy

products• cheese• corn & wheat syrup

sweeteners• etc.