An Introduction to Metabolism

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Slide 60

Cofactors

Cofactors are nonprotein enzyme helpers

Cofactors may be inorganic (such as a metal in ionic form) or organic

An organic cofactor is called a coenzyme

Coenzymes include vitamins

Slide 61

Enzyme Inhibitors

Competitive inhibitors bind to the active site of an enzyme, competing with the substrate

Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective

Examples of inhibitors include toxins, poisons, pesticides, and antibiotics

Slide 62

Fig. 8-19

(a) Normal binding

(c) Noncompetitive inhibition

(b) Competitive inhibition

Noncompetitive inhibitor

Active site

Competitive

inhibitor

Substrate

Enzyme

Slide 63

Concept 8.5: Regulation of enzyme activity helps control metabolism

Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated

A cell does this by switching on or off the genes that encode specific enzymes or by regulating the activity of enzymes

Slide 64

Allosteric Regulation of Enzymes

Allosteric regulation may either inhibit or stimulate an enzyme’s activity

Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site

Slide 65

Allosteric Activation and Inhibition

Most allosterically regulated enzymes are made from polypeptide subunits

Each enzyme has active and inactive forms

The binding of an activator stabilizes the active form of the enzyme

The binding of an inhibitor stabilizes the inactive form of the enzyme

Slide 66

Fig. 8-20

Allosteric enyzme

with four subunits

Active site

(one of four)

Regulatory

site (one

of four)

Active form

Activator

Stabilized active form

Oscillation

Non-

functional

active

site

Inhibitor

Inactive form

Stabilized inactive

form

(a) Allosteric activators and inhibitors

Substrate

Inactive form

Stabilized active

form

(b) Cooperativity: another type of allosteric activation

Slide 67

Fig. 8-20a

(a) Allosteric activators and inhibitors