Transcript (energy releasing) reactions

Chapter 8 Metabolism
Essential Concepts
--- chemical energy is necessary to life in that it allows
living organisms to drive endergonic (energy requiring) reactions
using energy captured from exergonic (energy releasing) reactions
--- electrons can’t flow in a vacuum, oxidation reactions
must always be coupled to reduction reactions
--- phosphate bonds are efficient ways to transport energy,
they carry a relatively large amount of energy and are stable
enough 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 phosphate
containing 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 reduced
compounds (organic or inorganic) pump protons and
create a charge separation.
Charge separation across a ion impermeable membrane
A Respiratory Chain
Other NADH producing
reactions
Glycolysis
Glucose  Pyruvate + 2 NADH
Electron transport
Other oxidation reactions
(produce more NADH)
OR
Biosynthetic reactions
Produce NAD+
Charge separation (pmf)
ATP generation or other
processes (flagellar rotation)
transport
Chemiosmotic Theory
Peter Mitchell’s idea that energy could be stored in a
transmembrane ion gradient
went very much against accepted theory of the time
two components:
DpH – pH differential across the membrane
-- usually about 1.0 pH unit
DY -- charge potential across the membrane (-160 mV)
pmf - proton motive force (-240 mV for E. coli)
DmH - 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 must
replace 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 (DEh)= Eo (oxidized) – Eo (reduced)
(DEh)= 0.82 V – ( - 0.42 V) = 1.24 V
Net energy = 1.24 V
Using
DG = (-nF)( DEh)
DG = (-2)(-96.48 kJ/V)(1.24 V)
DG = +239 kJ
This is essentially aerobic respiration, how so?
Big Gulps:
In the previous reactions O2 is the terminal electron acceptor
and 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, and
oxygen 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
DG = 142.8 kJ
Fe3+ + e-  Fe2+
DG = 105.1 kJ
0.77 V