The Chemistry of Life: Metabolism
Metabolism refers to the sum of all chemical reactions that occur within a living organism to maintain life. These reactions are organized into metabolic pathways.
Anabolic Pathways: Synthesize complex molecules from simpler ones, requiring an input of energy (endergonic). Example: photosynthesis, protein synthesis.
Catabolic Pathways: Break down complex molecules into simpler ones, releasing energy (exergonic). Example: cellular respiration.
Enzymes: The Catalysts of Life
Nearly every metabolic reaction is catalyzed by an enzyme.
Function: An enzyme is a biological catalyst, typically a protein, that speeds up the rate of a specific chemical reaction without being consumed in the process.
Mechanism: Enzymes work by lowering the activation energy (Ea) of a reaction—the energy barrier that must be overcome for the reaction to proceed.
Enzyme Structure and Function
Active Site: A specific three-dimensional region on the enzyme where the reactant molecule, called the substrate, binds.
Substrate: The molecule upon which an enzyme acts.
Specificity: The shape and chemical properties of the active site give an enzyme its high specificity, meaning each enzyme typically catalyzes only one type of reaction.
Induced-Fit Model:
This model describes how enzymes and substrates interact. It proposes that the active site is not a rigid lock, but is flexible. When the substrate binds, the active site changes shape slightly to create a tighter, more optimal fit. This 'induced fit' strains the bonds of the substrate, helping to lower the activation energy and catalyze the reaction.
Factors Affecting Enzyme Activity
The rate of an enzyme-catalyzed reaction is influenced by several factors:
Temperature: Each enzyme has an optimal temperature at which it functions most efficiently. At very high temperatures, the enzyme can denature—lose its three-dimensional shape, including the active site—and become inactive.
pH: Each enzyme also has an optimal pH. Extreme changes in pH can alter the charge of amino acids in the active site, disrupting substrate binding and causing denaturation.
Substrate Concentration: At low concentrations, the rate increases as substrate concentration increases. However, the rate eventually plateaus when the enzyme's active sites become saturated with substrate.
Inhibitors:
Competitive Inhibitors: Molecules that resemble the substrate and bind to the active site, blocking the actual substrate from binding.
Noncompetitive Inhibitors: Molecules that bind to a different site on the enzyme (an allosteric site), causing the active site to change shape and become less effective.