Module 1 - Metabolism Flashcards
Metabolism
refers to the enzyme-catalyzed chemical reactions that occur in a cell to either breakdown biomolecules to obtain energy in a form that can be used by the body (ATP), or reactions that synthesize biomolecules that our bodies need.
the sum total of Catabolism and Anabolism
Catabolism
refers to those chemical reactions that breakdown biomolecules to produce energy
Anabolism
refers to chemical reactions that lead to the synthesis of biomolecules that require the input of energy
The primary functions of metabolism are:
To obtain chemical energy in the form of ATP from the degradation of energy-rich nutrients from the environment or from captured solar energy. For example, some of the food you eat is used to move your muscles or use your brain, both of which require energy.
To convert molecules that we ingest as food into the building blocks of larger molecules needed in the body
To assemble these small building block molecules into larger molecules such as proteins, nucleic acids, lipids, polysaccharides
To synthesize and degrade biomolecules that have specialized functions in cells
A metabolic pathway
a term used to describe a series of linked reactions that begins with a particular biomolecule and converts it into another biomolecule in a carefully defined fashion and one that is regulated.
can be linear, branched, or circular
metabolic pathways can be interconnected; that is, an intermediate of one pathway can also be an intermediate in one or more other pathways
amphibolic pathways
pathways that can function anabolically and catabolically, depending on the energy conditions in the cell
Rate-limiting
An important concept is that the first committed step, which is often the first reaction in the pathway, is usually a point of regulation for that pathway.
It is also important to note that reactions that are regulated in a pathway are often rate-limiting steps i.e. slow reactions that can be increased by a variety of mechanisms.
Living organisms require a continual input of free energy for three major purposes:
(1) the performance of mechanical work, such as muscle contraction and cellular movement;
(2) the active transport of biomolecules and ions; and
(3) the synthesis of macromolecules from simple precursors.
Enzymes
enzymes only speed up the rate of a chemical reaction, but do not change the equilibrium constant for reactions and thus cannot make a thermodynamically unfavorable reaction proceed.
There are only two thermodynamic properties of a reaction that one needs to consider in order to understand how enzymes work
the free energy difference (ΔG) between the products and the reactants,
the free energy required to initiate the conversion of reactants to products.
free energy difference (ΔG)
determines whether a reaction will take place spontaneously i.e. whether it is possible
free energy
determines the rate at which the reaction will proceed
if ΔG is negative
Reactions can take place spontaneously
Exergonic
reactions that release energy
positive ΔG
Reactions that cannot proceed unless there is an input of energy, often involving ATP cleavage
Endergonic
reactions that require energy
What is so special about ATP that it is used as the carrier of energy in the cell?
It has high phosphoryl-transfer potential, meaning that it readily transfers its phosphate group to water.
the standard free energy of hydrolysis of ATP, where the terminal phosphate group is cleaved off to yield ADP and inorganic phosphate (Pi), is very negative at -30.5 kJ/mol.
ATP + H2O ←—→ ADP + Pi
ΔG°’ = - 30.5 kJ/mol
there are four reasons for the large standard free energy change
Electrostatic repulsion - at pH 7, ATP carries four negative charges which repel one another when they are in close proximity
Resonance stabilization - Pi, one of the products of ATP hydrolysis, has a greater resonance stabilization than any of the phosphates in ATP
Increase in Entropy - hydrolysis of ATP results in two molecules instead of a single ATP molecule
Stabilization by hydration -water binds to ADP and Pi which stabilizes these molecules and makes the reverse reaction less favorable
where does the energy that ATP possesses come from?
The carbon in the fuels we eat (primarily glucose and fats) is oxidized to CO2 and the energy released is used to regenerate ATP from ADP and Pi.
substrate-level phosphorylation
ATP synthesis that results from the transfer of a phosphate group from one high phosphoryl-transfer potential compound to ADP
Activated carriers
Many activated carriers are small organic molecules that function as coenzymes.
Activated carriers of electrons for fuel oxidation
electrons released from oxidation of fuels are transferred to special carriers, and the reduced forms of these carriers then transfer the electrons to oxygen.
One of these carriers is NAD+.
The second activated carrier for fuel oxidation is FAD.
Activated carriers of electrons for biosynthesis
In many biosynthetic reactions, the precursor molecules are more oxidized than the final product, so there is a need for electrons as well as for ATP.
In biological systems, the primary electron donor used in biosynthetic processes is NADPH
NADH is used primarily for the generation of ATP, while NADPH is used for reductive biosynthesis
Activated carrier of two-carbon fragments
Another central molecule in metabolism is Coenzyme A (or simply CoA), which is a carrier of 2-carbon units termed acetyl groups as well as longer carbon units referred to as acyl groups.
Acetyl groups are attached to CoA through the reactive sulfhydryl group, to form the molecule acetyl CoA
most activated carriers that act as coenzymes are derived from vitamins, particularly the B vitamins
