Chapter 8 Metabolism

Essential Concepts--- chemical energy is necessary to life in that it allows

living organisms to drive endergonic (energy requiring) reactionsusing energy captured from exergonic (energy releasing) reactions

--- electrons can’t flow in a vacuum, oxidation reactionsmust always be coupled to reduction reactions

--- phosphate bonds are efficient ways to transport energy,they carry a relatively large amount of energy and are stableenough to move around the cell but not too stable to be easily broken

--- ion impermeable membranes can be used to establish charge separation (electrochemical potential) as a way to store

energy and to convert electrochemical energy into chemical energy

Different Ways to the Same End:

Strategies for producing ATP:

Substrate-level Phosphorylation: Use a high energy phosphatecontaining molecule to transfer phosphate to ADP--- usually involves addition of free (inorganic) phosphate to a molecule and then rearrangement to increase the energy of the phosphate group

Photophosphorylation: Use energy captured from light to pump

protons and create a charge separation

Respiration: Us energy captured from the oxidation of reducedcompounds (organic or inorganic) pump protons and create a charge separation.

Charge separation across a ion impermeable membrane

A Respiratory Chain

Glucose Pyruvate + 2 NADH

Electron transport

Charge separation (pmf)

ATP generation or other processes (flagellar rotation)

transport

Other oxidation reactions (produce more NADH)

ORBiosynthetic reactions

Produce NAD+

Glycolysis

Other NADH producingreactions

Chemiosmotic Theory

Peter Mitchell’s idea that energy could be stored in atransmembrane ion gradient went very much against accepted theory of the time

two components:

pH – pH differential across the membrane -- usually about 1.0 pH unit

-- charge potential across the membrane (-160 mV)

pmf - proton motive force (-240 mV for E. coli)

- proton activity (-32 kJ/mol)

Uncouplers -

An aerobic electron transport chain

ATP Synthase

Recycling is Good!

--- At the heart of most respiratory chains is the concept that you mustreplace the oxidizing/ reducing equivalents that you use in the pathway.

--- So electron transport actually has two functions:

1.) reduce NADH to NAD+ to replenish NAD+ pool

2.) produce ATP via proton pumping and charge separation

NAD+ + 2H+ + 2e- NADH -0.32 V

Redox Reactions:

Occur in half reactions (either an oxidation or a reduction)

H2 2H+ + 2e- -0.42 V (requires energy)(reduction)

Which is great, but . . . electrons can’t be in solution alone

So we combine the oxidation with an oxidation reaction

½ O2 + 2H+ + 2e- H2O +0.82 V (produces energy) (oxidation)

Total energy (Eh)= Eo (oxidized) – Eo (reduced)

(Eh)= 0.82 V – ( - 0.42 V) = 1.24 V

Net energy = 1.24 V

Using G = (-nF)( Eh)

G = (-2)(-96.48 kJ/V)(1.24 V)

G = +239 kJ

This is essentially aerobic respiration, how so?

Big Gulps:

In the previous reactions O2 is the terminal electron acceptorand 239 kJ is the maximum energy that can be extracted from this system.

However, living systems cannot take H2 and ½ O2 directly to H2O in one step, too much energy is released. Living systems have a solution to this problem:

1.) Break the redox system down into multiple smaller steps, each of which release a manageable amount of energy

2.) Use mobile electron carriers to link these smaller reactions

These unified systems likely evolved from simpler, less contiguous sets of reactions.

Alternate Electron Acceptors

--- oxygen generates one of the largest gaps between electron donor and acceptor, and so is the most favorable terminal

electron acceptor for respiratory chains.

--- however, many bacteria can grow in the absence of oxygen, andoxygen was not originally present on Earth

Some other electron acceptors and their energy yields:

N03- + 2e- + 2H+ N02

-+ H20 0.42 V

total voltage = 0.42 V – (- 0.32 V) = 0.74 V

G = 142.8 kJ

Fe3+ + e- Fe2+ 0.77 V

G = 105.1 kJ