Metabolism: Enzymes & Reactions (HL IB Biology)

Enzymes as catalysts

• Most chemical reactions either do not occur spontaneously or occur very slowly
• In laboratory or industrial settings, some chemical reactions require some sort of catalyst in order to form a sufficient concentration of product molecules
  
  • Other conditions that may speed up the reaction rate include:
    • High temperatures or pressures
    • Extremes of pH
    • High concentrations of the reactants
• Cells are very sensitive to extreme temperatures, pressures and pH-levels, so the chemical reactions occurring in them cannot be sped up by these means
• Enzymes are proteins that act as biological catalysts in cells and allow chemical reactions to occur at a suitable rate in the conditions found in living organisms
  • They are reusable, so only a small number is needed to catalyse reactions
  • They remain unchanged by the reactions that they catalyse
• Without the presence of enzymes, the rate of chemical reactions in organisms would be too low to support life
• To form product molecules, the reactants would need to collide at the correct angle and speed in order for a reaction to occur
  • The chances of this occurring under normal conditions would be so low, that this would be an insignificant event
• Enzymes ensure that molecules (called substrate molecules) are orientated correctly and close enough for a reaction to occur
• The cell has control over the enzymes being produced, which in turn gives the cell control over the chemical reactions occurring in the cytoplasm

What is metabolism?

• Metabolism is a catch-all term used to describe all the chemical reactions that take place within cells and organisms
• Metabolism can be thought of as the chemical reactions of life
  • The molecules involved are metabolites
• Many reactions of metabolism take place in multiple stages
  • Each stage is catalysed by a separate enzyme
• A series of interlinked metabolic reactions is called a metabolic pathway
• Metabolic reactions can be classified broadly as anabolic or catabolic

Role of enzymes in metabolism

• Enzymes are globular proteins
• Critical to the enzyme's function is the active site where the substrate binds
• Enzymes are specific to the substrate
  • The shapes of the enzyme and substrate and their chemical properties are complementary, to allow the substrate to fit into the active site, like two jigsaw pieces fitting together
  • This is called enzyme-substrate specificity
• Due to this specificity, thousands of enzymes are needed throughout an organism, to carry out individual chemical reactions
• This means that control over metabolism can be exerted through these enzymes

Anabolic reactions

• Anabolic reactions are involved with the building of large molecules from smaller ones
• Examples include;
  • Photosynthesis, where CO₂ and water are built up into complex sugars
  • Protein synthesis, where amino acids are joined together in sequence
  • The formation of glycogen by linking glucose molecules together
• Anabolic reactions often include condensation reactions
• Anabolic reactions are endergonic (they require an input of energy to take place)
  • Energy-storing products are the end result

Enzyme-catalysed anabolic reactions diagram

Anabolic reactions involve the linking of more than one substrate molecule to form a more complex product molecule

Catabolic reactions

• Catabolic reactions are involved with breaking down large molecules into smaller, simpler ones
• These reactions are often carried out to release energy for cellular processes and for the excretion of waste
• Examples include:
  • Respiration, where CO₂ and water are produced from the oxidation of sugars
  • Deamination of proteins to release urea
  • Breakdown of macromolecules into monomers during digestion
• Catabolic reactions often include hydrolysis reactions
• Catabolic reactions are exergonic (free energy is released for cellular processes or as excess heat)

Enzyme-catalysed catabolic reactions diagram

Catabolic reactions happens when a single substrate molecule is drawn into the active site and broken apart into two or more product molecules

Comparison of anabolism and catabolism table

Anabolism	Catabolism
Requires an input of energy (endergonic)	Releases energy (exergonic)
Builds large molecules from small ones	Breaks down large molecules into smaller ones
Used to store energy in chemical form	Used to release chemical energy as heat and for other activities such as movement and active transport
Involves condensation reactions	Involves hydrolysis reactions
Used for growth, repair and energy storage	Performs several activities such as digestion, excretion and energy supply
Both are made up of enzyme-catalysed reactions
Both are coupled to ATP, the principle energy carrier in cells