METABOLISM: OVERVIEW

METABOLISM-is the collection of enzyme catalyzed reactions that convert substrates that are external to the cell into various internal products.

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CHARACTERISTICS OF METABOLISM

1. Varies from organisms to organism

2. Many common characteristics

3. Affected by environmental conditions

» a) O2 availability: Saccharomyces cerevisiae

Aerobic growth on glucose → more yeast cells

Anaerobic growth on glucose → ethanol

» b) Control of metabolism is important in bioprocesses

TYPES OF METABOLISM

1. Catabolism

-metabolic reactions in the cell that degrade a substrate into smaller / simpler products

Glucose → CO2

-Reactions that release energy by breaking complex molecules into simpler ones

X—Y →X + Y + energy

Exergonic reaction

• Energy is captured / stored in high energy bonds of ATP & similar molecules

• Involves electron transfer (oxidation-reduction)

2. Anabolism

-metabolic reactions that require energy to synthesize larger or complex molecules from simpler ones

X + Y +energy →X—Y

endergonic reaction

• Needed for growth, reproduction, repair, movement, transport, etc.

• Where does the energy come from?

ENERGY & CARBON SOURCES

• All living things need energy

• All living things need Carbon

– Why? To synthesize all organic molecules

Microbes are extremely versatile in the ways in which they acquire energy & carbon.

OBTAINING CARBON

• Auto- (self)

– get carbon from CO2 to synthesize organic molecules

• Hetero- (other)

– get carbon from pre-made organic sources

OBTAINING ENERGY

•Photo-

-capture the energy of light

•Chemo-

-capture energy from chemicals

In metabolism, the energy carrier is often electrons, moving through redox reactions.

REDOX REACTIONS

• Oxidation & reduction reactions are always coupled so we call them RedOx

In redox reactions,

• Electrons are transferred from one atom/molecule to another

– Simultaneously 2 reactions: red/ox: electron gain/electron loss

• The electrons carry energy (so redox reactions are energy transfers)

• In a chain of reactions, the electrons must have a final resting place (a terminal electron acceptor).

- Often, this is oxygen.

Reduction:

• net charge is reduced (made more negative) because electrons are gained

• Energy is gained (reduced compound has more energy)

• Often, hydrogen is gained, oxygen is lost

For example, think: Hydrocarbons.

• Totally reduced

• Saturated with hydrogen

• No oxygen

• Lots of energy stored Propane

Oxidation:

• Electrons are lost

• Energy is lost

• Often, the electrons are transferred to oxygen

• Oxygen is NOT the only electron acceptor around

• There must always be an electron acceptor and an electron donor in redox reactions (coupled)

For example, think: Hydrocarbons burning

•The molecule gains oxygen/loses hydrogen (yielding CO2 & H2O)

•Energy is released (heat)

•Oxygen is the electron acceptor

If it has hydrogens, it likely canbe oxidized as an energy source(food) by some type of bacteria!

• Electron donor = reducing agent

-The atom or molecule that is oxidized

- It causes something else to be reduced (hence the name reducing agent)

• Electron acceptor = oxidizing agent

-The atom or molecule that is reduced

-It causes something else to be oxidized (oxidizing agent)

ELECTRON CARRIERS IN METABOLISM

• NAD (nicotinamide adenine dinucleotide)

- Derived from vitamin niacin

• FAD (flavin adenine dinucleotide)

-Derived from vitamin riboflavin

Both carriers cycle between oxidized (NAD+, FAD)and reduced states (NADH, FADH2)

ATP: Adenosine triphosphate

•is a nucleotide (yes, the same one that goes into DNA!)

•Has 3 phosphate groups attached (tri-)

•The distal, third phosphate group can be hydrolyzed to release a significant amount of energy:

ATP ↔ADP + Pi + energy

Where ADP = adenosine diphosphate

Pi = inorganic phosphate (PO43-)

•Release of the next phosphate group also releases a lot of energy (ADP AMP + Pi + energy)

•This process is not used as much by living things

One reason why so much energy is released by hydrolysis of the 2nd and 3rd phosphate groups: Electrostatic repulsion

-Each phosphate group is negatively charged; binding them together takes a lot of energy

{The “first” phosphate, attached to the adenosine, can also be cleaved off but the energy release is not impressive: no repulsion from other phosphates!}

ATP AS GLOBAL ENERGY CURRENCY

• The energy of ATP hydrolysis can be captured to perform the work of the cell

• ATP is like money that can “buy” almost any energy-requiring activity

• This reaction is reversible– i.e., an input of energy (such as the energy derived from glucose) can be used to make ATP from ADP & Pi

• Phosphorylation of ADP to make ATP is a key feature of metabolism