Energy, Enzymes, and Biological Reactions: The Fundamentals of Life
Energy, Enzymes, and Biological Reactions
Life on Earth could not have evolved without catalysts – most of which are proteins called enzymes.
Enzymes speed up the rates of reactions by many millions of times without the need for an increase in temperature.
Enzymes are essential in metabolism, the biochemical modification and use of organic molecules and energy to support the activities of life.
Energy
Energy is defined as the capacity to do work.
Energy exists in many different forms, including heat, chemical, electrical, mechanical, and radiant energy (including light, gamma rays, and X-rays).
Energy can be converted from one form to another – in plants, the radiant energy of sunlight is transformed into chemical energy in the form of organic molecules (potential energy).
All forms of energy can exist in one of two states (kinetic or potential), which are interconvertible.
Kinetic Energy
Kinetic energy is the energy of an object in motion.
Examples: a falling rock, electricity, and light.
Potential Energy
Potential energy is stored energy.
Examples: a rock at the top of a hill, chemical energy, gravitational energy, and stored mechanical energy.
Thermodynamics
Thermodynamics is the study of energy and its transformations.
When discussing thermodynamics, scientists refer to a system, which is the object under study – everything outside a system is its surroundings.
There are three types of systems:
- Isolated
- Closed
- Open systems
All living organisms are open systems.
Laws of Thermodynamics
First Law of Thermodynamics
Energy can be transformed from one form to another, or transferred from one place to another, but it cannot be created or destroyed. Also called the principle of conservation of energy. In any process that involves energy change, the total amount of energy in a system and its surroundings remains constant.
For most organisms, the ultimate source of energy is the sun – plants capture kinetic energy of light and convert it to the chemical potential energy of complex organic molecules.
Chemical potential energy stored in sugars and other organic molecules is used for growth, reproduction, and other work of living organisms.
Eventually, most of the solar energy absorbed by green plants is converted into heat.
Second Law of Thermodynamics
The total disorder (entropy) of a system and its surroundings always increases (although the total energy in the universe does not change).
Living organisms seem to decrease in entropy as they grow – but when nutrients and waste products are considered, total energy remains constant and entropy increases.
Energy (which is largely unusable by living organisms) and radiated into space.
Reactions tend to be spontaneous if the products have less potential energy than the reactants.
For a reaction to be spontaneous, ΔG must be negative.
As a system moves toward equilibrium, its free energy becomes progressively lower and reaches its lowest point when the system achieves equilibrium (ΔG = 0).
Many reactions have a ΔG that is near zero and are readily reversible by adjusting the concentration of products and reactants slightly.
Reversible reactions are written with a double arrow:
A + B ↔ C + D
reactants products
Many reactions in living organisms never reach equilibrium because living systems are open – the supply of reactants is constant and products do not accumulate.
The ΔG of life is always negative – organisms constantly take in energy-rich molecules and use them to do work.
Organisms reach equilibrium, ΔG = 0, only when they die.
Exergonic Reaction
Reaction that releases free energy.
ΔG is negative because the products contain less free energy than the reactants.
Endergonic Reaction
Reactants must gain free energy from the surroundings to form the products.
ΔG is positive because the products contain more free energy than the reactants.
Metabolic Pathways
A metabolic pathway is a series of reactions in which the products of one reaction are used immediately as the reactants for the next reaction in the series.
In a catabolic pathway (or a single catabolic reaction) energy is released by the breakdown of complex molecules to simpler compounds; overall ΔG is negative.
In an anabolic pathway (or an anabolic reaction or biosynthetic reaction), energy is used to build complicated molecules from simpler ones; overall ΔG is positive.
Adenosine Triphosphate (ATP)
The nucleotide adenosine triphosphate (ATP) consists of the five-carbon sugar ribose linked to the nitrogenous base adenine and a chain of three phosphate groups.
The negative charges of the phosphate groups repel each other strongly, making the bonding arrangement unstable.
Removal of one or two phosphate groups is a spontaneous reaction that releases large amounts of free energy.
The breakdown of ATP is a hydrolysis reaction which results in the formation of adenosine diphosphate (ADP) and a molecule of inorganic phosphate (Pi).
ATP + H2O → ADP + Pi
ADP can be hydrolyzed to adenosine monophosphate (AMP).
ATP synthesis from ADP and Pi is an endergonic reaction that uses energy from the exergonic breakdown of carbohydrates, proteins, and fats (food).
The continual hydrolysis and resynthesis of ATP is called the ATP/ADP cycle.
Approximately 10 million ATP molecules are hydrolyzed and resynthesized each second in a typical cell.
Activation Energy
Even when a reaction is spontaneous (negative ΔG), the reaction will not start unless a small amount of activation energy (Ea) is added.
Enzymes
The most common biological catalysts are proteins called enzymes, which increase the rate of reaction by lowering the activation energy of the reaction.
Enzymes do not alter the ΔG of the reaction.
The free energy stays the same; the difference is in the path the reaction takes.
Cells have thousands of different enzymes, found in different areas inside and outside of the cell.
The name of an enzyme typically refers to its substrate or type of reaction, and ends in –ase (e.g., proteinases).
In enzymatic reactions, an enzyme combines briefly with reacting molecules and is released unchanged when the reaction is complete.
The reactant that an enzyme acts on is called the substrate.
Each type of enzyme catalyzes the reaction of a single type of substrate molecule or group of closely related molecules (enzyme specificity).
The substrate interacts with a small pocket or groove in the enzyme molecule, called the active site.
When the substrate binds at the active site, both enzyme and substrate molecules are distorted – this makes the chemical bonds in the substrate ready for reaction (induced fit).
Once an enzyme-substrate complex is formed, catalysis occurs – the substrate is converted into one or more products.
Many enzymes require a cofactor, a nonprotein group that binds to the enzyme, for catalytic activity.
- Some are metallic ions, including iron, copper, magnesium, zinc, and manganese.
- Other cofactors are small organic molecules (coenzymes) which are often derived from vitamins.
- Some coenzymes bind loosely to enzymes.
- Others (prosthetic groups) bind tightly.
Enzymes stabilize the transition state through three major mechanisms:
- Bringing the reacting molecules together.
- Exposing the reactant molecules to altered charge environments that promote catalysis.
- Changing the shape of the substrate molecules.
Enzymatic Activity
Changes in concentration of substrate and other molecules that bind to enzymes can alter enzyme activity.
Changes in temperature and pH can have a significant effect on enzyme activity.
When enzymes are saturated with substrate, further increases in substrate concentration have no effect on reaction rate.
Enzyme Inhibitors
Enzyme inhibitors are nonsubstrate molecules that bind to an enzyme and decrease its activity.
Competitive Inhibition
Inhibitors bind to the active site, blocking access for the normal substrate – slowing or stopping the reaction.
Noncompetitive Inhibition
Inhibitors bind at a location other than the active site – reducing the ability of the active site to bind substrate.