Introduction

Bacteria show an incredible diversity with regards to their use of different energy sources.

An overview of a hypothetical bacterial cell:

Substrates

Energy conservation (ATP and transmembrane potential)

Biosynthesis

Transport

Movement

Cell division

Energy and Carbon Metabolism: An overview

Chemotrophs Phototrophs

Chemo-lithotrophsChemo-organotrophs

Energy

Carbon metabolism

Heterotroph

Autotroph (CO2)

Basic principles

Mechanisms for the generation of ATP

Substrate level phosphorylation

Respiration

Photophosphorylation

The common denominators in energy metabolism are ATP and transmembrane potentials

Adenosine-5'-triphosphate (ATP) Figure 8.3

Oxidation / Reduction (Redox) reactions

Redox reactions:

Oxidation is the loss of electrons and reduction is the gain of electrons. Electrons cannot exist in solution and the loss of electrons must be coupled to the gain of electrons.

Reduction potential:

This is a quantitqtive measure of the tendency for a substance to give up electrons in biological systems. It is measured in volts and generally at pH = 7.0.

Half reactions

Half reaction are a convenient way of showing the reduced and oxidized form of a compound. Two half reactions are coupled to give a redox reaction.

Half reactions Table 8.1

The number of electrons transferred (n), and the electrode potential under standard conditions (E0') compared to the hydrogen half cell.

Relationship between free energy and reduction potential

Go = -nF Eh

Go = Change in Free energy

n = number of electrons in reaction

F = Faradays constant

Eh = E’ (oxidized) - E’ (reduced)

ATP hydrolysis releases –31.8 kJ / mole so we need at least this amount of energy to make a phosphodiesterase bond in ATP.

Nicotinamide adenine dinucleotide (NAD)

Respiration and fermentation

Fermentation: Energy generation by anaerobic energy-yielding reactions characterized by substate level phosphorylation and the absence of cytochrome-mediated electron transfer.

Respiration: Energy generation in which molecular oxygen or some other oxidant is the terminal electron acceptor. Among the latter are nitrate, sulfate, carbon dioxide and fumarate.

Energy and Carbon Metabolism: Fermentative organisms

Chemotrophs Phototrophs

Chemo-lithotrophsChemo-organotrophs

Energy

Carbon metabolism

Heterotroph

Autotroph (CO2)

Glycolysis

Substrate level phosphorylation

There are three basic reactions. 1 and 2 are found in most aerobic and anaerobic bacteria which grow on sugars. 3 is found mainly in anaerobic fermenting bacteria.

1. 1,3 bis-phosphoglycerate + ADP 3 phosphoglycerate + ATP

2. phosphoenol pyruvate + ADP pyruvate + ATP’

3. acetyl phosphate + AMP acetate + ATP

An example of substrate level phosphorylation

Acetyl-coenzyme A (acetyl-CoA)

The thioester bond between the β-mercaptoethylamine moiety of CoA and the acetyl groups (~) is an energy-rich bond.

Stickland reaction

Respiration and fermentation

Fermentation: Energy generation by anaerobic energy-yielding reactions characterized by substate level phosphorylation and the absence of cytochrome-mediated electron transfer.

Respiration: Energy generation in which molecular oxygen or some other oxidant is the terminal electron acceptor. Among the latter are nitrate, sulfate, carbon dioxide and fumarate.

Energy and Carbon Metabolism: respiration

Chemotrophs Phototrophs

Chemo-lithotrophsChemo-organotrophs

Energy

Carbon metabolism

Heterotroph

Autotroph (CO2)

Energy transducing membranes

Components of the electron transport chains

Flavo proteins: carry two electrons (e-) and two protons H+)

Iron sulphide proteins: carry one electron (e-)

Quinones: carry two electrons (e-) and two protons H+)

Cytochromes: carry one electron (e-)

Flavin nucleotides, components of flavoproteins

Iron-sulfur groups, components of nonheme iron proteins

Coenzyme Q (ubiquinone)

The heme portion of a cytochrome molecule

The electron transport chain operating during aerobic growth in Paracoccus denitrificans.

The electron transport chain of Thiobacillus ferrooxidans, which uses Fe2+ as an energy

source

The structure of ATP synthase, showing

the F0 and F1

subunits.

Diversity

In this lecture I have covered basic themes but at the begining of the lecture I said something about metabolic diversity.

This diversity is generated from variation in the electron donor (organic (thousands to choose from) or inorganic ( a few to chose from)) terminal electron acceptor organic (fermentation) or inorganic (respiration).

Use of light energy in the phototrophs.