CHAPTER 6: Enzymes summary

Enzymes are protein catalysts that increase the rate of a chemical reaction by lowering the activation energy

They are defined as biocatalysts synthesized by living cells that speed chemical reactions

Classes of enzymes

  • Oxidoreductases – Oxidation to reduction
  • Transferases – Group transfer
  • Hydrolases – Hydrolysis
  • Lyases – Addition and elimination
  • Isomerases – Interconversion of isomers
  • Ligases – Condensation

Factors affecting enzymatic reactions

  • The concentration of enzyme – Increase in enzyme concentration increases reaction speed
  • Substrate concentration – More substrate implies a higher rate of product formation
  • Temperature – An increase in temperature increases enzymatic reaction. An increase above the optimum temperature will, however, denature the enzyme.
  • pH – Enzymatic reaction is higher around the optimum pH and decreases below or above optimum pH
  • Product concentration – Accumulation of products decreases enzymatic reaction
  • Activators – Presence of cofactors increases enzymatic reaction
  • Time
  • Light and radiation

Enzyme Active Site

The active site is a small region on the enzyme at which the substrate binds and participates in a chemical reaction

Features of an active site
  • The active site exists as a result of the tertiary protein structure
  • The active site is made up of enzymes far apart from each other in the linear sequence
  • They are regarded as clefts on the enzyme
  • The active site has a catalytic site and substrate binding site
  • Active sites are specific in their function
  • Substrates bind enzymes with weak noncovalent bonds
  • Binding of the substrate on the active site forms an enzyme-substrate complex

Enzyme Inhibition

There are three categories of enzyme inhibition
  • Reversible inhibition
  • Irreversible inhibition
  • Allosteric inhibition

Reversible inhibition – Competitive inhibition where a substance that resembles the substrate binds on the active site. Non-competitive inhibition whereby the inhibitor binds on a site other than the active site.

Irreversible Inhibition – Inhibitor binds covalently on the enzyme, inactivating them in an irreversible manner

Allosteric inhibition – inhibitor binds the active site changing its conformation

Enzyme specificity

  • Stereospecific – Acts on one isomer only
  • Reaction specificity
  • Substrate specificity – Either on one substrate, structurally related substrates or a broad category of substrates


Non-protein substance that is associated with enzyme function. The functional enzyme is called holoenzyme, which is made up of protein part called apoenzyme and the non-protein part called coenzyme

Thermodynamics of enzymatic reactions

  • Isothermic reaction – negligible change in energy
  • Exothermic reaction – Releases energy to the surroundings
  • Endothermic reaction – Consumes energy

Enzymatic regulation by negative feedback inhibition – This is a mechanism in which the end product of a reaction regulates the initial stages of the reaction, mostly by deactivating the enzyme.


Are catalysts that speed up chemical reactions by Lowe activation energy

Why are enzymes important?
They allow reactions to occurring that would not occur under normal given environmental conditions

The largest component of an enzyme is the?
Core or apoenzme

Is the largest part and is made of proteins and contains an active site

Induced fit model
Once substrate binds to active site, enzyme slightly changes shape to bind substrate more firmly. This places strain on bonds in substrate which lowers activation energy

What it the function of coenzymes?
A small non-protein organic molecules that are synthesized from vitamins. Not specific to an individual reaction but transfer hydrogens, electrons or small function groups

What are cofactors?
Inorganic ions such as metallic ions that adjust the shape of the active site upon association with the apoenzyme

Dehydration synthesis
Large molecules are made by removing water. Example: proteins

Molecules are split/digested by adding water. Example: glucose into fructose and sucrose

When a molecule gains electrons (or hydrogens. Example:

When a molecule loses electrons or hydrogens. Example:

When a phosphate is added to a molecule. Example: ADP into ATP

List the 5 classification of enzymes

  1. Endoenzyme
  2. Exoenzyme
  3. Inductive/Inducible
  4. Constitutive
  5. Repressive

Are enzymes that do their job inside the cell

Are enzymes that are secreted and their job is to breakdown large molecules outside

Constitutive enzymes
Are enzymes that always produced. An example is esculinase.

Inductive enzymes
Are enzymes that are produced only when needed

Repressive enzymes
Are enzymes that are not switched on or produce when the product of the enzyme pathway is present

List some examples of Exenzymes
Amylase, caseinase, lipase, DNAse, Tryptophanses, Urease,

List things that damage enzymes
Hydrolysis, acids and strong alkalis, oxidaiton, hydrogen peroxcide, attaching atoms of chemical groups and extreme temperatures

Feedback Inhibtion
Is when an enzyme binds toa substrate and a product is created. If concentrations of this product rise a feedback loop occurs. In high concentrations the final product binds to the allosteric site on the enzyme changing its shape so it can no longer bind to its substrate

Allosteric site
The place on an enzyme where a molecule that is not a substrate may bind, thus changing the shape of the enzyme and influencing its ability to be active.

Competitive Inhibition
Is when two molecule compete for the same active site.

Two examples of competitive inhibition

  1. Sulfa drugs
  2. Nucleosid emimics (AZT and Acyclovir)

What conditions affect enzymes activitiy

  1. Anything that denatures a protein
  2. Feedback inhibition
  3. Competitive inhibition
    Competitive inhibitions
  4. Concentration of the enzyme’s substrate

List some examples of exoenzymes
Amylase, Lipase, DNse, Tryptophanse, Uurease

List an example of constitutive enzyyme

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