Co-factors and Enzyme Inhibition

Cofactor - a Substance that is Required by Enzymes for them to work.
Enzyme Inhibitor - a Substance that Stops Enzymes doing their job. (some are harmful, others can be used for medicines).

Cofactors and Coenzymes
Some Enzymes will Only Work if there is Another Substance Attached to it. These are Non-Proteins and are called Cofactors.

Some of these are Inorganic Molecules (or Ions). They work by Helping the Enzyme and Substrate to Bind Together. They Don't directly Participate in the Reaction, so are Not Used up or Change in any way. An example would be Chloride Ions (Cl-) which are Cofactors for Amylase (in Saliva)

Other Cofactors are Organic, these ones are called Coenzymes. Unlike the Inorganic Cofactors, these Do Participate in Reactions and Are Changed by it (They're like a second substrate). They often act as Carriers, moving Chemical Groups between Different Enzymes. They're Continually Recycled during this process. Vitamins are often Sources of Coenzymes.

If a Cofactor is Tightly Bound to the Enzyme, it's known as a Prosthetic Group. For Example, Zinc Ions: (Zn2+) are a Prosthetic Group for Carbonic Anhydrase (An Enzyme in Red Blood Cells) which Catalyses the Production of Carbonic Acid from Water and Carbon Dioxide.The Zinc Ions are a Permanent part of the Enzyme's Active Site.

Inhibitors
Enzyme Activity can be Prevented by Enzyme Inhibitors, which are Molecules that Bind to the Enzyme they Inhibit. There are two types of Inhibitors, Competitive and Non-Competitive:

COMPETITIVE INHIBITION

• These molecules have a Similar Shape to the Substrate molecules.
• They Compete with the Substrate molecules to Bind with the Active Site
• Instead, they Block the Active Site, so No Substrate molecules can Fit in it.
• The Level of Inhibition depends on the Relative Concentrations of  the Inhibitors and the Substrates. e.g. if the Concentration of the Inhibitor is High, it will take up nearly all of the Active Sites, meaning that Fewer Substrates will get to the Active Sites of the Enzymes. Vice Versa, High Conc. of Substrate, Higher Rate of Reaction.

NON-COMPETITIVE INHIBITORS

• These Bind to the Enzymes Away from the Active Site, the point where they attach is called the Allosteric Site.
• This causes the Active Site to Change Shape so Substrates can No Longer Bind to it.
• Increasing the Concentration of Substrate WILL NOT make any Difference to the Reaction Rate.

REVERSIBLE AND IRREVERSIBLE INHIBITORS

The Strength of the Bonds between the Enzyme and the Inhibitor decides which one they are:

• If they're Strong Covalent Bonds, the Inhibitor Cannot be Removed Easily and the Inhibition is Irreversible.
• If they're Weaker Hydrogen Bonds or Weak Ionic Bonds, the Inhibitor Can be Removed Easily and the Inhibition is Reversible.

DRUGS AND POISONS

Some Medicines are Enzyme Inhibitors, e.g.

• Some Antiviral Drugs, Inhibit the Enzyme that Catalyses the Replication of Viral DNA.
• Some Antibiotics (e.g. Penicillin) Inhibits the Enzyme that Catalyses the Formation of Proteins in Bacterial Cell Walls. This Weakens the Cell Wall and Prevents the Bacterium from Regulating its Osmotic Pressure. As a Result, The Cell Bursts and the Bacterium are Killed.

Metabolic Poisons Interfere with Metabolic Reactions (Reactions in Cells) which can cause Illness or even Death.

• Cyanide - an Irreversible Inhibitor of an Enzyme that Catalyses Respiration Reactions. Cells that Can't Respire will Die.
• Malonate - Inhibits another Enzyme that Catalyses Respiration Reactions.
• Arsenic - Also inhibits an enzyme with a silly long name we don't need to know, but we do need to know that it Catalyses Respiration Reactions.

METABOLIC PATHWAYS

A Metabolic Pathway is a Series of Connected Metabolic Reactions - The Product of the First Reaction Takes Part in the Second Reaction and so on. Each Reaction is Catalysed by a Different Enzyme.

Many Enzymes are Inhibited by the Product of the Reaction they Catalyse. This is called Product Inhibition.

End Product Inhibition is when the Final Product of a Metabolic Pathway Inhibits an Enzyme that has Catalysed a Previous Reaction.

End Product Inhibition can be used to Regulate the Pathway and Control the amount of End Product that gets made.

For Example, 

• Phosphofructokinase (That's a great scrabble word) Is an Enzyme that is Involved in the Metabolic Pathway that Produces ATP by Breaking Down Glucose.
• ATP Inhibits the Action of Phosphofructokinase - so a High level of ATP Inhibits the Production of ATP.

Both Product and End-Product Inhibition are Reversible, So when the Level of Product starts to Drop, the Level of Inhibition will start to Drop too, so the Enzyme can start to Function again so More Product can be made.