A catalyst is a substance that accelerates the rate of a chemical reaction without being consumed or permanently altered in the process. In living organisms, this principle is applied by molecules known as biological catalysts, which enable the complex chemistry of life to occur rapidly and precisely. Without them, the biochemical reactions required for survival would happen too slowly to sustain life. They are fundamental to every metabolic pathway and cellular function.
What Biological Catalysts Are
Biological catalysts are specialized molecules, most commonly proteins called enzymes. Enzymes are folded into intricate three-dimensional shapes that create a specific pocket on their surface known as the active site. The molecule an enzyme acts upon, the reactant, is called the substrate, which temporarily fits into this active site. This extreme selectivity means one enzyme typically only binds to and modifies one particular type of substrate, a property known as high specificity.
While most biological catalysts are protein-based enzymes, a small but important group consists of ribonucleic acid (RNA) molecules known as ribozymes. Ribozymes also possess a folded structure that allows them to accelerate reactions. Regardless of their chemical nature, all biological catalysts are regenerated unchanged after the reaction is complete, allowing a single molecule to repeat the process millions of times per minute.
How Enzymes Accelerate Reactions
Chemical reactions require an initial input of energy, called the activation energy, to get started and convert reactants into products. Enzymes accelerate reactions by providing an alternate pathway with a significantly lower activation energy barrier. They accomplish this by interacting with the substrate at the active site to form a temporary enzyme-substrate complex. This interaction strains the chemical bonds within the substrate, pushing it toward the unstable transition state.
The Lock-and-Key model suggested a perfect, rigid fit between the enzyme and substrate, but the more accurate description is the Induced Fit model. In this model, the enzyme’s active site changes its shape slightly upon binding to the substrate, tightening around it for a more effective fit. This conformational change optimizes the environment and orientation needed to stabilize the transition state intermediate. By stabilizing this state, the enzyme reduces the energy needed for the reaction to proceed rapidly at normal body temperature.
Environmental Conditions That Impact Function
The three-dimensional structure of an enzyme is responsible for its function and is highly sensitive to its immediate surroundings. Enzyme activity increases with temperature up to an optimal point, typically around 37°C in humans, due to increased molecular collisions. Temperatures rising much higher than this optimal range can cause the enzyme to denature, meaning its structure unfolds and it loses function permanently.
The acidity or alkalinity of the environment, measured by pH, also profoundly affects enzyme activity because it alters the electrical charges of amino acid residues in the active site. Each enzyme has a specific optimal pH at which it operates most efficiently, reflecting its location in the body. For example, the digestive enzyme pepsin works best in the highly acidic stomach (pH 1.5), while trypsin in the small intestine functions best at an alkaline pH of about 8.
Small non-protein molecules called cofactors can bind to an enzyme to assist its catalytic function. Conversely, inhibitors can bind to the enzyme to slow or stop its activity, providing a mechanism for cellular regulation.
Core Functions in Human Biology
Biological catalysts are the driving force behind virtually every process that maintains life, from the largest system functions to the smallest cellular events. In the digestive system, specific enzymes break down large, complex food molecules into smaller, absorbable units. Amylase starts the breakdown of starches in the mouth, while lipases and proteases break down fats and proteins further down the tract.
Metabolic functions, involving the conversion of energy, are entirely dependent on highly organized chains of enzyme-catalyzed reactions. Cellular respiration, the process that extracts usable energy (ATP) from glucose, involves dozens of separate enzymatic steps. Biological catalysts are also essential in genetic processes, where enzymes like DNA polymerase and helicase facilitate the rapid and accurate replication and repair of the genetic code.