Transcript energetics and metabolism biology 1

energetics
and metabolism
biology 1
• The chemistry of life is organized into
metabolic pathways
• Organisms transform energy
• The energy of transformation are subject to
the Laws of Thermodynamics
• Organisms live at the expense of free energy
• Cellular work is driven by ATP
• Enzymes act as catalysts to reactions
• How Enzymes work, and how they are
controlled
The chemistry of life = metabolism
• Metabolism is the sum total of an organism’s
chemical processes, requiring management
of
– Materials
– Energy
• Metabolic pathways
– Catabolic pathways break down complex
macromolecules to simpler products, releasing
energy
– Anabolic pathways utilize energy to construct
complex macromolecules from simpler ones
– Anabolism feeds catabolism, and visa-versa
Energy + H20 + CO2
anabolic
catabolic
CH2O + O2
Organisms transform energy...
• Energy = capacity to do work
• Potential energy; energy inherent to
matter due to location or arrangement
• Kinetic energy; energy in the process of
doing work through motion
• For example:
Sun’s Energy
(kinetic energy)
Transformation
via photosynthesis,
using CO2, H20
(plants)
C6H12O6
(chemical energy)
Energy transformation conforms
to the Laws of Thermodynamics
• 1st Law: Energy can neither be destroyed or
created
• 2nd Law: Energy transformations occur so as to
maximize entropy
• Organisms can be considered open systems.
Although metabolic processes may be
considered to decrease entropy within the
system, the net change to the system and its
surroundings should reflect an increase in
entropy
Organisms live at the expense of free energy
• Not all of a systems energy is available to do work.
The energy that is available is termed free energy
(G), where
G = H - TS
H = total energy
T = absolute temperature
S = Entropy
\TS = energy not available to do work
• Amount of energy harvested from a reaction is equal
to change in free energy from start to end of reaction
DG = DH - TDS
• Certain reactions occur spontaneously (don’t need
energy). Thus in these cases, DG < 0
Free energy and metabolism
Exergonic reaction
Endergonic reaction
Chemical products have less free energy
than the reactant molecule
Products store more free energy than
reactants
Reaction is energetically downhill
Reaction is energetically uphill
Spontaneous reaction
Non-spontaneous reaction (requires
energy input)
DG = negative
DG = positive
DG is the maximum am ount of work the
DG is the minimum amoun t of work
reaction can perform
required to drive the reaction
e.g., 6CO2 + 6H2O
DG = +2870 kJ/mol
DG = -2870 kJ/mol
C6H12O6 + 6O2
How does the cell harness energy
• ATP (adenosine triphosphate) is the
immediate source of energy that drives
cellular work
– Mechanical work (cilia, flagella, cytoplasmic flow,
etc.)
– Transport work (pumps)
– Chemical work (anabolism)
• ATP is a nucleotide with unstable phosphate
bonds. Hydrolysis of a phosphate group is an
exergonic reaction that releases energy
ATP + H2O DG = -55 kJ/mol ADP + Pi
ATP
• Provides energy through phosphorylation
(addition of temporary phosphate group to
reactant forming transitional compound)
• Is continually being regenerated (107/sec).
Regeneration is endergonic, needing
energy (from cellular respiration)
The role of enzymes in metabolism
• Enzymes act as catalysts
• Catalysts act to lower the activation energy of
a reaction
• Activation energy of a reaction represents the
energy needed to initially break chemical
bonds in reactants
• Addition of activation energy (EA) raises free
energy of reaction to a transitional state
• As new bonds form, free energy is released.
DG for the reaction is the difference in free
energy between the reactants and products
Enzymes as catalysts
• Very selective to a specific substrate, as dictated
by three-dimensional structure of protein
• Substrate binds to enzyme’s active site with weak
hydrogen or ionic bonds (lock-and-key
hypothesis), inducing a change shape of enzyme
– Active site may hold two or more reactants in place so
that they may react
– Induced fit of enzymes active site may distort reactant’s
chemical bonds, weakening them
– Active site may provide localized micro-environment
conducive to reaction
Enzyme activity
• Found freefloating in cytoplasm or
bound within a membrane or organelle
• Activity is maximal under certain
conditions of temperature (35°-40°C),
pH (6-8), and substrate concentration
(up to saturation)
• Some enzymes require a co-factor
(inorganic [Zn, Fe, Cu] or organic [coenzymes, vitamins]) that binds to
enzyme to induce correct shape
• Enzymes can be inhibited either
reversibly or irreversibly
– Competitive inhibitors block the active site
form the substrate by binding with it
themselves
– Non-competitive inhibitors bind to sites
other than the active site on the enzyme,
changing its shape so that is no longer
specific to original reaction (DDT,
antibiotics)
Regulation of enzyme activity
• Allosteric enzymes use non-competitive
inhibitors that bind to sites that activate or
inhibit the enzyme
– Binding of an activator to an allosteric site
activates the enzyme
– Binding of an inhibitor to an allosteric site inhibits
the enzyme
• Another common method of control is through
feedback inhibition
Enzyme 1
Threonine
Initial
substrate
Enzyme 2
A
B
Enzyme 3
Enzyme 4
C
Feedback inhibition
Enzyme 5
D
isoleucine
Endproduct and
allosteric inhibitor
of enzyme 1