Chapter 6. Introduction to Metabolism.
Energy and Life.
Chemistry of life is
organized into metabolic pathways.
Energy transformation is
governed by two laws of thermodynamics
Organisms live at the
expense of free energy.
ATP powers chemical
reactions by coupling exergonic reactions to endergonic reactions.
Enzymes speed up
Enzymes are substrate
Active site is catalytic
physical and chemical environment affects enzyme activity.
Control of Metabolism.
Metabolic control often
depends on allosteric regulation.
helps order metabolism.
Energy and Life.
Metabolism is the total chemical activity of a living organism.
interactions between molecules within the orderly cell environment.
reactions occur every instant.
of Life is organized into metabolic pathways.
and complex road map of the thousands of chemical reactions that take place
inside the cell (Fig 6.1).
is concerned with managing the material and energy recourses of the cell.
things down to release subunits and energy or building complex molecules
(macromolecules) and using energy.
types of reactions:
Catabolic reactions-breakdown complex molecules into simpler ones releasing energy.
Anabolic reactions link together simple molecules to form complex molecules storing
energy in chemical bonds.
Protein synthesis from
types of energy are kinetic and potential.
is the energy of action.
Energy that does work,
(alters the state or motion of matter) can exist as heat, light, electric and
Potential energy is the energy of state or position.
Stored energy: chemical
bonds, concentration gradients and electrical potentials are all important in
transformations of life are subject to two laws of thermodynamics.
of Thermodynamics: Energy is neither created or destroyed it just changes
Law of Thermodynamics: Not all energy is used and disorder tends to
increase (entropy increases).
An organism can increase its order at the expense of
the order around it (Fig 6.4).
When energy is created some is unavailable to do work.
No reaction is 100%
live at the expense of free energy.
that proceed without energy input are considered spontaneous while those
that do not proceed without an energy input are considered nonspontaneous.
energy (G) is the portion of a systems energy that can perform work when temperature
is uniform throughout.
Free because it is available to do work (Fig. 6.5).
Total energy is called enthalpy (H), unusable
energy is entropy (S).
This is affected by absolute temperature
(T), in Kelvin (K) (K = oC + 273).
We are interested in the change in free energy
therefore the equation is DG=DH-TDS, from a starting state to ending state.
This equation indicates whether free energy is
released or consumed by a chemical reaction (Fig 6.6).
-DG = energy released (exergonic), considered
+DG = energy consumed (endergonic), considered
We will be able to
follow the release or consumption of energy during chemical reactions by
following the change (D) in free energy (G).
Ex: Exergonic reaction (cellular respiration):
+ 6O2 ΰ 6CO2 + 6H2O; DG = -686 kcal/mol
For every mole of
glucose (186 g) broken down by respiration 686 kcal of energy are made
available (released) for work.
Reverse of this would be
photosynthesis and would cost 686 kcal (+686 kcal/mole glucose made, powered
by light energy).
If a reaction runs spontaneously from reactant A ΰ product B then the reverse (B ΰ A) requires energy.
At some concentration of A and B; the forward and
reverse reactions take place at the same rate this is a chemical equilibrium
(DG = 0).
No observable change in
the system, even though individual reactions are still occurring.
of free energy change and
thermodynamics are important for understanding how cells function, specifically
how biochemical reactions occur.
Exergonic reactions drive endergonic reactions (energy
coupling), a small molecule (ATP) is responsible for mediating most
energy coupling in cells.
This will be important
when we discuss how energy is made in cells.
powers cellular work by coupling exergonic reactions to endergonic reactions.
triphosphate or ATP is the energy form in the cell used for action.
of the nitrogenous base adenine fused to ribose which has three phosphates (not
one) attached (Fig 6.8).
Hydrolysis of ATP yields adenosine diphosphate
(ADP) and an inorganic phosphate ion (Pi = HPO2-4)
as well as free energy (DG = -13 kcal/mol or 50 kj/mol ( -7.3/-30 under lab
The equilibrium is far to
the right (products) at equilibrium there is ten million times as much ADP as
ATP, ATP is continuously regenerated (Fig 6.10).
Why is there more energy
in the P-P bonds?
exergonic and endergonic reactions.
Many enzyme-catalyzed reactions (exergonic) can
provide the energy to convert ADP to ATP (cellular respiration).
breakdown of this ATP then yields energy for other reactions (Fig 6.9).
Can be through a phosphorylated intermediate.
less than one minute in the cell and on average 40 kg of ATP per day is
produced by a person at rest.
DG may indicate how far the reaction proceeds
to completion but does not indicate the speed of the reaction.
reactions may be fast or slow.
Ex: Sucrose hydrolysis
is spontaneous but will happen imperceptibly until the enzyme sucrase is
Enzymes (biological catalysts) speed up metabolic
reactions by lowering energy barriers.
catalyst is any substance that speeds up a reaction.
biological catalysts are proteins called enzymes.
There are also RNA enzymes called ribozymes (Ch.
barriers must be overcome for a reaction to proceed.
Includes breaking and reforming of chemical bonds.
Ex: Sucrose hydrolysis requires breaking the chemical bond
between glucose and fructose then forming water from H+ and OH-.
barrier represents the amount of energy needed to start the reaction, the activation
energy (EA) (Fig 6.12).
The activation energy moves the reactants from stable
to unstable, a state called the transition state.
Exergonic reactions need very little activation energy
while endergonic reactions need more.
The activation energy is often recovered so it does
not affect the DG.
Enzymes act to lower the activation energy, however, they
have no effect on equilibrium or DG.
are substrate specific.
catalysts do not show specificity like protein enzymes do.
are substrates and they bind to the active site of an enzyme.
from the 3-dimensional structure of the enzyme and substrate.
E + S ΰ ES ΰ E + P.
This active site is not rigid, the substrate can cause
the enzyme to change the shape of its active site, induced fit.
Ex: Like a handshake.
Ex: Hexokinase uses this
model for fitting around its substrate glucose.
Some active sites are more rigid and they fit like a
lock and key (lock and key model).
Ex: The active site of
lysozyme neatly fits its substrate.
the following mechanisms in order to change a reactant to a product: Orientation
of substrates, add charges to substrates or induce strain.
cells physical and chemical environment affects enzyme activity.
activity can be altered by environment (Ex: temp. and pH), chemicals and
concentration of substrate.
concentration will affect reaction rate.
The more substrate the more possible collisions which
leads to more reactions per unit time (reaction rate).
Eventually in an enzyme catalyzed reaction, the enzyme
becomes saturated at a certain substrate concentration and rate levels
require added molecules for function.
are inorganic ions (copper, zinc and iron) that bind temporarily to enzymes and
are essential for function.
of enzyme function can act reversibly or irreversibly (Fig 6.17).
Irreversible inhibition occurs when an inhibitor binds via a covalent bond
(often at the active site).
Reversible inhibition can occur at the active site (competitive
inhibition) or at a second site on the enzyme (noncompetitive inhibition).
inhibition occurs when the inhibitor
and substrate compete for the same active site.
inhibition occurs when the inhibitor
binds outside the active site but in doing so renders the active site ineffective.
Enzymes are affected by
environmental conditions (Fig 6.16).
and salinity can affect the function of an enzyme.
The Control of
must be regulated in order to maintain homeostasis.
is therefore organized into pathways: A
+ an enzyme yields B; B + an enzyme yields C and so on.
be regulated by their own production (gene level) or by enzyme activity.
control often depends on allosteric regulation (Fig 6.18).
enzymes (made of subunits) have a different response to substrate concentration
then other enzymes, they respond to fluctuating levels of regulators.
enzymes have a catalytic subunit and a regulatory subunit giving
extreme levels of control.
inhibition (common) is the switching off of a metabolic pathway due to the
production of its end product (Fig 6.19).
(Fig 6.20)is similar to allosteric activation in that binding of one
substrate stimulates catalytic activity.
may change the conformation of the other subunits and may amplify the response.
are not just bags of enzymes there are specific compartments for specific
enzymes as we saw in Chapter 7.