An Introduction to Metabolism
An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics
A metabolic pathway begins with a specific molecule, which is then altered in a series of defined steps to form a specific product
Catabolic pathways release energy by breaking down complex molecules to simpler compounds
Anabolic pathways, also called biosynthetic pathways, consume energy to build complicated molecules from simpler compounds
Bioenergetics is the study of how energy flows through living organisms
Kinetic energy is the energy associated with the relative motion of objects. Potential energy is the energy that matter possesses because of its location or structure.
Heat or thermal energy is kinetic energy associated with the random movement of atoms or molecules
Chemical energy is a term used by biologists to refer to the potential energy available for release in a chemical reaction
Thermodynamics is the study of energy transformations that occur in a collection of matter
The first law of thermodynamics states that the energy of the universe is constant: Energy can be transferred and transformed, but it cannot be created or destroyed
The second law of thermodynamics states: Every energy transfer or transformation increases the entropy of the universe
The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously
Free energy is the portion of a system’s energy that can perform work when temperature and pressure are uniform throughout the system, as in a living cell
An exergonic reaction proceeds with a net release of free energy; G is negative
An endergonic reaction is one that absorbs free energy from its surroundings.
ATP powers cellular work by coupling exergonic reactions to endergonic reactions
Cells manage their energy resources to do this work by energy coupling, using an exergonic process to drive an endergonic one
ATP (adenosine triphosphate) is a nucleotide triphosphate consisting of the sugar ribose, the nitrogenous base adenine, and a chain of three phosphate groups
This recipient molecule with a phosphate group covalently bonded to it is called a phosphorylated intermediate. It is more reactive (less stable) than the original unphosphorylated molecule.
Enzymes speed up metabolic reactions by lowering energy barriers
An enzyme is a macromolecule that acts as a catalyst, a chemical agent that speeds up the rate of a reaction without being consumed by the reaction.
The initial investment of energy for starting a reaction is the free energy of activation, or activation energy (EA).
The reactant that an enzyme acts on is the substrate
The enzyme binds to a substrate, or substrates, forming an enzyme-substrate complex
The active site of an enzyme is typically a pocket or groove on the surface of the protein where catalysis occurs
This change leads to an induced fit that brings the chemical groups of the active site into position to catalyze the reaction.
Many enzymes require nonprotein helpers, called cofactors, for catalytic activity
Some reversible inhibitors resemble the substrate and compete for binding to the active site. These molecules are called competitive inhibitors
Noncompetitive inhibitors impede enzymatic reactions by binding to another part of the molecule
Regulation of enzyme activity helps control metabolism
In allosteric regulation, a protein’s function at one site is affected by the binding of a regulatory molecule to a separate site
This mechanism, called cooperativity, amplifies the response of enzymes to substrates, priming the enzyme to accept additional substrates.
A common method of metabolic control is feedback inhibition