Terminology:

Website on Energy and Enzymes

Class Info


¤ For starters, these are terms with which you should be quite familiar by the time our study of enzyme is complete: (click the Class Info link above for a full description of each term)
amino acid, protein, monomer, polymer, primary through quaternary structure, enzyme, biological catalyst, substrate, reactants, active site, energy of activation, product, "lock and key", induced fit, denaturation, competitive inhibition, allosteric inhibition

1. Definition

¤Enzymes have the unique ability in living systems to speed up reactions by lowering the activation energy normally required to carry out the reaction. Enzymes, therefore, are catalysts. The energy of activation is the energy needed to cause a chemical change in the bonds of the substrate. An enzyme itself is not changed in the reaction but can continue to carry out more reactions. A graph of this process is seen below:

external image lowerEa.jpg
Important parts of the graph:
- free energy must increase (energy in a system that can perform work when temp and pressure are the same throughout the cell)
- the top of the graph in which the molecules reach their "transition state", i.e. they have absorbed enough thermal energy from the surroundings to be reactive (NOTE: endergonic reactions often coupled with exergonic reactions)
- downhill portion of the graph represents free energy being lost by the molecules and their resulting stabilization; energy released to surroundings as bonds stabilize
-(NOTE: exergonic reactions occur spontaneously but may occur too slowly to be effective or relevant to a living system; enzymes speed these reactions)
Endergonic and Exergonic Reactions Animation

2. How Enzymes Lower the Energy of Activation:

- cannot increase temp in living systems to increase rate of reactions
- enzymes allow molecules to absorb enough energy to reach transition state
- substrate, active site, enzyme- substrate complex, induced fit
- one enzyme can catalyze 1000 substrate molecules per second
  • enzyme provides active site where substrate binds in ideal orientation
  • induced fit causes stress
  • ideal conditions "inside" enzyme to weaken substrate bonds or force bonds to form
inducedfit.gif
Image credited to: http://chsweb.lr.k12.nj.us/mstanley/outlines/enzymesap/Enzymesap.htm

3. What affects the rate of the reaction and enzymatic activity?

  • increasing substrate can increase the rate of the reaction, but only to a certain point. As the substrate is used up the collisions between enzyme and substrate decrease
  • more enzyme can be added if the present amount of enzymes are saturated
  • each enzyme has an optimal temp and pH under which it works (for animals, body temp and a pH between 6-8)
  • some enzymes require the presence of a cofactor (a metal ion or a vitamin molecule)
pHonEnzymes.gif
Image credited to: http://chsweb.lr.k12.nj.us/mstanley/outlines/enzymesap/Enzymesap.htm

4. Enzyme Inhibition:

Two main types:
  1. Competitive Inhibition- molecule or ion competes for active site with normal substrate molecule(s); must increase substrate to overcome this
external image 573compinhibit.gif

external image allostericnotes.gif2. Noncompetitive Inhibition- molecule or ion binds to a site on the enzyme which isn't the active site causing the enzyme to change its shape and distort its active site

5. Regulation of Enzyme Activity:

  1. Allosteric Regulation-
  • can be activating or inhibiting- molecule will bind to enzyme and cause temporary stabilization of active form or temporary conformational change which inhibits the enzyme; cooperativity occurs when one substrate molecule binds to just ONE active site of a multi- active site enzyme causing a conformational change, so other active sites can bind substrate as well
cooperativity.jpg
  • Regulation of ATP production in cell- when ATP gets low, more ADP can bind to enzymes that control ATP production and activate them; when ATP levels are high, ATP binds instead to these enzymes and inhibits them
2. Feedback Inhibition- product of metabolic pathway binds to enzyme at start of pathway which switches the pathway off
feedbackinhibition.jpg

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external image allostericnotes.gif