Cofactors are metal ions or organic compounds that associate with enzymes and are necessary for their function. Coenzymes are organic cofactors like coenzyme Q10 and NAD+, which help carry electrons or groups between reactions. A key coenzyme is ATP, which contains high-energy phosphate bonds and transfers energy and phosphates during phosphorylation reactions, coupling endergonic and exergonic processes to drive cellular metabolism.

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5.5 How Do Cofactors Work?
• Cofactor: metal ion or organic compound that associates
with an enzyme and is necessary for that enzyme’s
function
– Examples: vitamins, minerals, metal ions
• Coenzyme: an organic cofactor
– Example: coenzyme Q10, NAD+

2. © Cengage Learning 2015
How Do Cofactors Work? (cont’d.)
Figure 5.18 Example of a coenzyme. Coenzyme Q10 (above) is
an essential part of the ATP-making machinery in your
mitochondria. It carries electrons between enzymes of electron
transfer chains during aerobic respiration. Your body makes it,
but some foods—particularly red meats, soy oil, and peanuts—
are rich dietary sources.

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How Do Cofactors Work? (cont’d.)

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How Do Cofactors Work? (cont’d.)
• The enzyme catalase has four tightly bound cofactors
called hemes
– Catalase’s substrate is hydrogen peroxide, a highly reactive
molecule that can be dangerous
– The heme in catalase breaks hydrogen peroxide into water
– Catalase is an antioxidant: prevents oxidation of other
molecules

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How Do Cofactors Work? (cont’d.)
iron atom
Figure 5.19 Heme. This organic molecule is part of the active
site in many enzymes (such as catalase). In other contexts, it
carries oxygen (e.g., in hemoglobin), or electrons (e.g., in
molecules of electron transfer chains).

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ATP — A Special Coenzyme
• ATP (adenosine triphosphate) functions as a cofactor in
many reactions
– Bonds between phosphate groups hold a lot of energy
– When a phosphate group is transferred via the process of
phosphorylation, energy is transferred along with it

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ATP — A Special Coenzyme (cont’d.)
ribose
adenine
three phosphate
groups
Figure 5.20 ATP, an important energy currency in metabolism.
A ATP. Bonds between its phosphate groups hold a lot of
energy.

8. © Cengage Learning 2015
ATP — A Special Coenzyme (cont’d.)
• ATP/ADP cycle:
– Process by which cells regenerate ATP
– ADP (adenosine diphosphate) forms when a phosphate group
is removed from ATP, then ATP forms again as ADP gains a
phosphate group
• The ATP/ADP cycle couples endergonic reactions with
exergonic ones

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ATP — A Special Coenzyme (cont’d.)
ribose
adenine
AMP ADP ATP
P P P
ADP + Pi
energy out energy in
Figure 5.20 ATP, an important energy currency in metabolism.

10. © Cengage Learning 2015
ATP — A Special Coenzyme (cont’d.)
organic compounds
(e.g., carbohydrates, fats, proteins)
ADP + Pi
oxidized
coenzymes
reduced
coenzymes
small molecules
(e.g., carbon dioxide, water)
Figure 5 .21 How ATP and
coenzymes couple
endergonic reactions with
exergonic reactions. Yellow
arrows indicate energy flow.
Compare Figures 5.8 and
5.20 C .