An Introduction to Metabolism

Page
5

Chemical

Transport

Mechanical

To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one

Most energy coupling in cells is mediated by ATP

Slide 37

The Structure and Hydrolysis of ATP

ATP (adenosine triphosphate) is the cell’s energy shuttle

ATP is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups

Slide 38

Fig. 8-8

Phosphate groups

Ribose

Adenine

Slide 39

The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis

Energy is released from ATP when the terminal phosphate bond is broken

This release of energy comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves

Slide 40

Fig. 8-9

Inorganic phosphate

Energy

Adenosine triphosphate (ATP)

Adenosine diphosphate (ADP)

P

P

P

P

P

P

+

+

H2O

i

Slide 41

How ATP Performs Work

The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP

In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction

Overall, the coupled reactions are exergonic

Slide 42

Fig. 8-10

(b) Coupled with ATP hydrolysis, an exergonic reaction

Ammonia displaces

the phosphate group,

forming glutamine.

(a) Endergonic reaction

(c) Overall free-energy change

P

P

Glu

NH3

NH2

Glu

i

Glu

ADP

+

P

ATP

+

+

Glu

ATP phosphorylates

glutamic acid,

making the amino

acid less stable.

Glu

NH3

NH2

Glu

+

Glutamic

acid

Glutamine

Ammonia

∆G = +3.4 kcal/mol

+

2

1

Slide 43

ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant

The recipient molecule is now phosphorylated

Slide 44

Fig. 8-11

(b) Mechanical work: ATP binds noncovalently

to motor proteins, then is hydrolyzed

Membrane protein

P

i

ADP

+

P

Solute

Solute transported

P

i

Vesicle