Enzymes continued
• Some enzymes work together to complete a series of reactions to produce a given product:
• A + B ------- C ------- D ------------ E
• Collectively, these steps are known as a metabolic pathway
Enzyme 1 Enzyme 2 Enzyme 3
A metabolic pathway
The entire set of all chemical reactions being performed by an organism is
called its metabolism
• Anabolism = chemical reactions that expend energy to make or transform chemical bonds
• Catabolism = chemical reactions that harvest energy by breaking chemical bonds
Anabolism involves endergonic reactions
Catabolism involves exergonic reactions
Control of Metabolism
• Coenzymes
• Inhibitors
• Allosteric regulation
• Feedback Inhibition
• Cooperativity
Coenzymes
• Coenzymes are bound at the active site in order to interact with the substrate and play an essential role in the catalysed reaction.
• They act as carriers of a variety of chemical groups. .
Most water-soluble vitamins are components of coenzymes
Vitamin Coenzyme Deficiency
Thiamine (B1)Thiamine
pyrophosphateBeriberi (weight
loss,other problems
Riboflavin (B2) FAD+ Mouth lesions, dermatitis
Nicotinic acid (niacine) NAD+ Pellagra (dermatitis,
depression)
Pantohtinic acid Coenzyme A Hypertension
Biotin Biotin Rash, muscle pain
Inhibitors
• Specific for an enzyme
• Can be reversible or non-reversible
• Competitive inhibitors
• Non-competitive inhibitors
Competitive vs. Non-competitive inhibitors
Many inhibitors are used as pesticides or drugs
Regulating the rates of enzyme-driven reactions
• Cells use inhibitors and activators to turn off and on enzymes
• Many enzymes are controlled by an allosteric site remote from the active site
Allosteric regulation of enzyme activity
Many allosterically controlled enzymse show quaternary structure
Enzyme 1 Enzyme 2 Enzyme 3Inter-mediate
Inter-mediate
X ProductStart ofpathway
Presence of product inhibits enzyme 1
Feedback inhibition
Many enzymes are actually regulated by the end products of the reaction they catalyze
This prevents too much product from being made
An example of Feedback inhibition