Energetics & Enzymes
- All eukaryotic organisms use the same basic metabolic pathways for converting energy. Functions carried out by organisms, procaryotic and eucaryotic, are forms of WORK. In order to complete the work energy is required and must be converted from CHEMICAL POTENTIAL ENERGY into KINETIC ENERGY and various forms of potential energy. Biological systems are also chemical systems, thus energy conversions involve chemical reactions. ENERGY CHANGES IN BIOLOGICAL SYSTEMS FOLLOW THE SAME PHYSICAL LAWS AS DO ALL CHEMICAL REACTIONS.
- Metabolic pathways or sequences take place in small increments. This prevents potential damage to cells if too much heat energy was liberated all at once. These small steps increase overall efficiency and make it easier to control the processes. It also creates many more sites where interruptions can have an impact on a reaction.
- Metabolic pathways generally release energy, often in the form of heat.
- The Laws of Thermodynamics describe energy transfers and conversions. It might be more appropriate to call the Laws of Energetics since they deal with all forms of energy.
- The energy within chemical bonds is potential energy and when a bond is broken the potential energy is converted to kinetic energy. Heat is the most common form of energy released.
- The FIRST LAW OF THERMODYNAMICS provides a look at the quantitative aspects of energy relationships in a system. The first law tells us that energy is conserved but does not tell us about how much of that energy is in a form usable for work of a system.
- The SECOND LAW OF THERMODYNAMICS tells us that systems left to themselves tend to become disorganized or chaotic. Since no conversion of energy is ever 100% efficient there is an inefficient transfer of energy through ecosystems which requires a continuous fuel supply for organisms.
- Heat is defined as the random motion of atoms and molecules.
- ENTROPY is the term used to describe the disorder,chaos, disorganization or randomness of a system. As entropy increases, the organization of a system decreases. Death might be viewed as irreversible positive entropy.
- Chemical energy is used to maintain and sustain the activities of cells.
- REACTANTS are the starting materials of a reaction. They combine or are rearranged to form PRODUCTS.
- EXERGONICreactions release energy. The reactants have more energy than the products. Hydrolytic reactions would be an example.
- ENDERGONIC reactions require the input of energy to proceed. They are synthesis reactions where energy is stored in the newly formed bonds.
- Exergonic and Endergonic reactions are coupled reactions. Energy released from exergonic reactions are used for endergonic reactions.
- Metabolic pathways or sequences are coupled reactions using ATP and other energy carrier molecules as intermediates in these energy conversions.
- A certain amount of energy is necessary to start a reaction. this is known as Activation or Starting energy. Once started the reaction can proceed on its own and release energy.
- ENZYMES are ORGANIC CATALYSTS. They affect the rate of a reaction,usually speeding it up, without being used up or consumed in the reaction itself. thus they are recyclable molecules.
- Activation energy, also known as starting energy, is the energy required to begin a reaction. The match to start the fire. Enzymes reduce the activation energy necessary to start a biochemical reaction within a living system.
- Enzymes have a specific structural arrangement known as the ACTIVE SITE which will fit specific SUBSTRATES. A substrate is a molecule or part of a molecule that is acted upon by an enzyme. The shape of the active site and the substrate are more or less complementary.
- When an enzyme binds with a substrate a temporary enzyme-substrate complex is formed. This joining induces the molecules involved to change their shapes and bond together or be broken apart.
- Metabolism is the sum total of the biochemical reactions and processes that acquire and convert energy and matter so that new materials can be processed and energy utilized. The process of building new molecular parts such as the building blocks of carbohydrates, lipids, proteins and nucleic acids is sometimes called BIOSYNTHESIS.
- ATP and GTP are energy carrier molecules that provide energy for cellular reactions.
- Metabolism consists of anabolic pathways which are involved in synthesis and catabolic pathways that are involved in breakdown. Catabolic pathways release energy from energy rich compounds.
- Energy released from catabolic reactions is used to convert ADP to ATP or GDP into GTP.
Activity or work within the cell is powered by the energy released when ATP or GTP is converted back into ADP or GDP.
- The process of attaching a phosphate group to a molecule is known as PHOSPHORYLATION. It is analogous to the setting of a switch to the "ready" position. When the bond holding the phosphate group in place is broken energy is released, thus moving the switch from the "ready" position to the "go" position.
- OXIDATION-REDUCTION reactions involve the transfer of electrons from one molecule to another. These electrons are often energy-rich.
- Oxidation involves the loss or removal of electrons.
- Reduction is the addition or gain of electrons. This reduces the net charge or makes the molecule more negative or less positive.
- Within living systems such as cells, special energy carrier molecules move hydrogen atoms in the form of electrons and hydrogen ions. The energy from these electrons often are used to phosphorylate ADP or GDP.
- Homeostatic control of metabolism is maintained by feedback systems, usually negative feedback.
- Some enzymes have more than one binding site for attachment of molecules such as cofactors or coenzymes.
- If an active or binding site is blocked, the enzyme will not be able to catalyze its normal reaction. Sometimes the site is blocked by an intermediate or end product of the metabolic pathway the enzyme helps to regulate.
- Enzyme activity may be controlled by the presence of a product in a feedback system. This is sometimes known as endproduct inhibition.
Most of the energy of a biological system is found in the chemical bond. An understanding of the energy relationships in biological form and function, then, depends on understanding of the features of chemical energy.
The energy within chemical bonds is potential chemical energy. When a bond is broken, its potential energy is expressed as kinetic energy, usually in the form of heat, sometimes light. The amount of energy within a given bond is related to the attraction of the atoms for one other.
A chemical reaction changes atomic combinations. A reaction always involves one or more chemical parts that undergo change:(the REACTANTS) and one or more chemical entities that result from the change (the PRODUCTS). All chemical reactions proceed until a balance is established between reactants and products. The balance represents the EQUILIBRIUM of the reaction. It is determined by the attraction of the chemical entities for one another and the TEMPERATURE and PRESSURE of the system. The equilibrium is an DYNAMIC (HOMEOSTATIC) one. The reactants are constantly forming products, and the products are constantly forming reactants.
A chemical reaction may be viewed as a system that is undergoing an irreversible change in state towards a state of equilibrium. The change that is occurring during such a reaction involves an alteration of the energy relationships of the system as well as the alteration of the particle relationships.
The "Laws" of Thermodynamics or Energetics
THE FIRST LAW OF THERMODYNAMICS is sometimes called the LAW OF CONSERVATION OF ENERGY. It may be stated in a variety of ways.
- The total energy of the universe remains constant - there is neither a gain nor a loss in the total amount of energy transferred or transformed. This implies that:
- Energy is neither created nor destroyed in any physical or chemical process but may be transformed (e.g. heat, light, electrical, mechanical, chemical). This implies that:
- Matter and energy are interconvertible (photosynthesis, respiration); E = mc2. Therefore:
- The term ENERGY CRISIS more correctly involves forms and transformations or conversions, NOT amounts.
- The energy in a system can be accounted for in the kinetic energy of that system, plus the bond energy of the product(s).
While the first law states that energy is conserved, it says nothing about the state of the energy resulting from an energy conversion in regards to its ability to do work. The SECOND LAW OF THERMODYNAMICS permits one to determine something about the work capacity of the energy resulting from a chemical reaction. In a sense, the second law deals with USABLE and UNUSABLE ENERGY relationships. It is sometimes stated that:
- The ENTROPY of the universe increases - energy transformations or conversions involve a degradation of energy from a concentrated form to a dispersed form (usually heat). Because energy dispersed as heat is unavailable for most work, this implies that:
- No conversion or transformation of energy from one form to another is ever 100% efficient. This explains why:
- Organisms have a need for a continuous fuel supply in the body (food). This need is the result of the inefficient transfer of energy through organisms in the food chain or web. We might summarize the second law as stating:
|Change in the amount of ||Change in the|| Change in the|
|USABLE ENERGY (FREE ENERGY)||=||TOTAL ENERGY || - || UNUSABLE ENERGY|
It would seem clear that as the entropy of a system increases, the amount of energy available for work decreases. Put another way, as entropy increases, the degree of order of a system decreases. Scientists' wives have often suggested that young children are entropy's little helpers.
Enzymes and Energy
Chemical reactions within biological systems are governed by the same laws of thermodynamics. It must be realized that biological systems are semi-isolated, in the sense that they do not have an endless source of energy and are, therefore, dependent on conserving the energy within the system for efficient function. One of the primary means developed to deal with this problem within the scope of physical laws is a reduction of the ENERGY OF ACTIVATION of a reaction by means of enzymes. this reduction then permits the reaction to occur with less energy input on the part of the biological system. THE ENZYME IMPARTS NO NET ENERGY TO THE SYSTEM.
ENZYMES serve as ORGANIC CATALYSTS affecting the rate of biochemical reactions without being used up in the process; they normally speed up reactions which are already thermodynamically possible and allow them to proceed at a rate which makes life, as we understand it, possible.
It has been calculated that enzyme catalysis speeds up reactions by a factor or 1 x 109. This means that reactions in cells occur a billion times more rapidly than they would if enzymes were not present.
ENZYMES normally operate by reducing the ACTIVATION ENERGY required to start reactions thus reducing the thermal energy (high temperature) which would otherwise be needed but detrimental to living systems. In general enzymes function by providing a convenient surface for bringing together the reactants and then becoming separated from them.
Enzymes influence various types of reactions:
- larger molecules may be synthesized from smaller ones
- larger molecules may be hydrolyzed into smaller ones
- atoms may be exchanged between molecules
- atoms may be rearranged within molecules
Some enzymes operate in two parts: an APOENZYME which is the protein portion and a COENZYME which is an organic molecule often constituted of a vitamin and a phosphate combination. Inorganic molecules which aid enzymes are often called COFACTORS. Some enzymes require the presence of specific ions (salivary amylase requires Cl-) to carry out their job.
Enzymes influence reactions by:
- reducing the amount of activation energy required
- providing a surface upon which the possibility of
contacts between reactants is increased
When enzymes fail to act properly many different types of problems may arise.
The substance or molecule which is acted upon by an enzyme is known as the SUBSTRATE.
The speed at which chemical reactions occur is known as the RATE OF REACTION. Reaction rate is more precisely defined as:
the amount of substrate acted upon/unit of time (usually min.)
Thus, when a great deal of substrate is altered by an enzyme every minute, the reaction is said to be proceeding at a rapid rate.
In enzyme reaction rates, the rate depends on the CONCENTRATION of the enzyme and the CONCENTRATION of the substrate (CONCENTRATION rather than AMOUNT). Concentration refers to amount in a given volume of solution. As previously mentioned, it has been calculated that enzyme mediated reactions occur 1 x 109 times faster than the same reactions without enzymes.
In most enzyme reactions, enzyme concentration is small compared to the substrate concentration. Therefore, the rate of the reaction becomes proportional to the concentration of the enzyme. If the enzyme concentration is doubled, the reaction rate is doubled.
At low substrate concentrations, the rate of the reaction is proportional to the substrate concentration, but at higher substrate concentrations the reaction rate is independent of substrate concentration. That is, further increase in the amount of substrate present per unit volume does not cause the reaction to proceed at a faster rate because all available enzymes are already saturated or tied up in reactions. This was first described mathematically in 1913 by Michaelis and Menten as:
rate = V(S)/K+S where V and K are constants
V = maximum velocity of saturated enzyme-substrate complex
K = Michaelis constant
S = substrate concentration
when S is very small the denominator is not affected very much and rate = V/K(S) when
S is very large the denominator becomes effectively equal to S and the rate becomes
effectively equal to V or independent of substrate concentration
this assumes no product inhibition
S + E <=======> ES <=======> ES* <=======> EP <=======> P + E
ES = enzyme-substrate complex; an intermediate compound
ES* = activated complex
Most of these reactions are essentially reversible, and the direction of the reaction depends upon the concentration of the reactants in relation to the concentration of the products
The enzyme imparts no net energy to the system. The portion of the enzyme at which the substrate combines is known as the ACTIVE SITE of the enzyme and represents a spatial arrangement of atoms complementary or nearly complementary to a specific portion of the substrate.
The rate at which the reactants are converted to products, in enzyme catalyzed systems, is controlled by pH, enzyme concentration, temperature, substrate concentration, and product concentration. The interaction of these factors imposes a delicately balanced system of controls on the rate of metabolic activity within the living system.
There is a committee which oversees the proper naming of enzymes. Enzymes are commonly named by attaching the suffix -ASE to a stem word which designates either (1) the substrate they affect; (2)
reaction type which they catalyze; or (3) the type of bond holding the molecule together. For example: proteinase, lipase, maltase, sucrase, amylase, OR oxidase, hydrolase, transferase, mutase, OR peptidase, esterase.
General Characteristics of Enzymes
- Chemically, all known enzymes are proteins
- Usually they are soluble in water, or dilute saline
- They are usually most active within the small range
of temperature tolerated by living cells
- Their influence is very specific with respect to:
- type of reaction
- type of substrate
- In general their influence is reversible, with the
speed and direction of the reaction depending upon:
- concentration of enzyme
- concentration of substrate
- concentration of products
- pH - each enzyme functions best within a specific
range of pH
- temperature - each functions within its own range
- inhibiting substances (enzyme poisons such as
Pb++ [lead], Hg++ [mercury]; inhibitors such as
References to Consult
On-line Study Guide for the Audesirk text. You do not have to register to use it. Simply go to the chapter of interest.
For Starr & Taggart users - go to Student Resources and then find the picture of your textbook. Click on that picture.
Another excellent online text