Cells and Energy - Part 3

Enzymes

 

Enzymes accelerate the rate of reactions.

Normally most of the reactions in metabolic pathways would occur very slowly if at all without the help of enzymes. Spontaneous reactions (such as explosions) are not useful to living things because

  1. they cannot be controlled
  2. they release too much energy too quickly or
  3. they are ineffecient wasting much of the potential energy as heat

 

How does life cope with energy barriers and run-away reactions.

  • exergonic reactions with high activation energy are tamed by enzymes which provide an alternative path with lower energy of activation.

  • Arrow A represents free energy
  • Arrow B is the energy of activation of the catalyzed reaction
  • Arrow C is the energy of activaton of the uncatalyzed reaction

  • endergonic reactions, which by themselves are impossible at biologically acceptable temperatures can be coupled to exergonic reactions. Enzymes can take the phosphate from ATP and use it to "charge" a molecule in an endergonic pathway. This phosphorylation step is used several times in glycolysis.

Characteristics of Enzymes

 

  1. Enzymes are biological catalysts and are usually globular proteins. Some RNA molecules made by the cell have catalytic properties and are appropriately called ribozymes.

 

  1. Enzymes catalyze reactions by lowering a thermodynamic barrier called the energy of activation. Activation energy is the energy that must be absorbed by reactants for their bonds to break. Enzymes provide a less energetic path to a transition state in which bonds can be made or broken more frequently and using less energy at a given temperature. Even an exergonic reaction, in which delta G is negative, energy must first be absorbed to reach the transition state. The activation energy is a barrier essential to life because it prevents the energy-rich molecules of the cell from decomposing spontaneously.

 

  1. Enzymes and their substrates are induced to fit together. The electric charge and shape of the enzyme's active site puts stress on one of the substrate's covalent bonds.

 

  1. Enzymes are not consumed in the reaction they catalyze. Because they remain unchanged they can be used over and over again. Eventually the enzyme will lose its function, either by natural decay or by other enzymes within the cell. This is an important regulatory function since few metabolic processes used continuously would simply waste energy.

 

  1. Each enzyme may catalyze hundreds of reactions every second. The speed of the enzyme can vary dramatically. Lysozyme, which breaks down bacterial cell walls, processes one substrate molecule every two seconds whereas another enzyme, carboanhydrase can change over a million substrate molecules during the same 2 seconds.
 
  1. Enzyme activity is usually controlled by feedback inhibition. This generally involves one of the products of the reaction reducing or stopping the activity of the enzyme as its concentration of that product increases.

product D combines with Enzyme1 preventing formation of B

For a general discussion of equilibrium and feedback systems you should visit the Fundamentals of Geology site.
  1. Enzymes catalyze both forward and reverse reactions equally well. Only by quickly removing products from the reaction site can a reaction go to completion. For instance if one of the products is a gas it can escape and by so doing prevent the reverse reaction.

 

HCl + Na2CO3 NaCl + H2O + CO2 (gas)

notice that the reverse reaction is impossible if carbon dioxide is not supplied.

NaCl + H2O + CO2 (gas) HCl + Na2CO3

 

  1. Enzymes are highly selective combining with a specific substrate. Substrates are 3-dimensional objects (molecule) which are complementary to the enzyme's active site. The substrate is temporarily bound to its enzyme. The bond between enzyme and substrate is weak, either hydrogen or ionic in nature. Enzymes can be poisoned by molecules which are similar to the normal substrate but which remain in the active site for unacceptably long periods of time.
Illustration of temporary Enzyme-Substrate bond verses inhibition with a poison.
  1. Enzyme activity is sensitive to environmental changes in pH, temperature, or salt concentrations. (See next section)

Modified July 10, 2005