Enzymes

Enzymes

• Enzymes are biological catalysts that speed up metabolic reactions by lowering activation energy, complex protein.

• More reaction happening requries more energy and thus more ATP if catalysed

• Cellular level:

  • Control metabolic pathways (e.g. respiration, protein synthesis, DNA replication)

  • Regulate rate of ATP production and other essential molecules

  • Help maintain normal cell function and homeostasis

• Whole-organism level:

  • Enable digestion of food into usable molecules

  • Support growth, movement, and repair

  • Affect overall energy availability and organism function

• Structure and function:

  • Enzyme-controlled reactions determine which molecules are made or broken down

  • This influences cell and tissue structure (e.g. proteins, membranes)

  • Also controls function by regulating metabolic activity and physiological processes

Intracellular enzymes (inside cells)

Catalyse reactions within cells

Example: catalase breaks down hydrogen peroxide into water and oxygen inside cells (protects cells from toxic buildup)

Extracellular enzymes

Catalyse reactions outside cells, often after being secreted

Example: amylase is released into the digestive system (and trypsin) and breaks down starch into sugars outside cells (e.g. in the mouth and small intestine)

Enzyme specifity

Enzymes are proteins with a specific tertiary structure, folded into a unique 3D shape.

This creates an active site, whose shape is specific and complementary to a particular substrate. This makes enzymes specific.

Enzyme hypothesis

Lock and key hypothesis:
The substrate fits exactly into the active site like a key in a lock
Explains enzyme specificity

Induced-fit hypothesis:
The active site is flexible and changes shape slightly when the substrate binds
This improves the fit and helps the reaction occur

Enzyme action

Substrate binds to the active site which is complementary to specific tertairy structure → forms an enzyme-substrate complex → enzyme-product complex is formed → product released

Enzyme catalyses the reaction, lowering activation energy (brings two substrates together, supply reactants and strains bonds)

Substrate is converted into product(s) → forms an enzyme-product complex

Products are released, leaving the enzyme unchanged and ready to reuse

Enzyme activity, temperature

Increasing temperature increases kinetic energy → more frequent succsessful collisions → higher rate of reaction

Above the optimum temperature, enzymes denature (active site changes shape/ tertiary structure threfore not specific) → activity drops rapidly

Temperature coefficient (Q10) shows how rate changes with a 10°C increase:

• Q10=R2/R1

• Typically Q10 ≈ 2 for many biological reactions (rate doubles for every 10°C rise within limits)

Enzyme activity, pH

Each enzyme has an optimum pH

Changes in pH alter bonding in the enzyme outside of the optimum → active site changes shape/ denatured which is change in tertiary shape

Extreme pH causes denaturation by → loss of activity

OH- and H+ ions react with enzymes/ interact with ionic and hydrogen bonds and proteins changes tertiary structure and denatures it by changing active site and changes function

Enzyme activity, enzyme concentration

Increasing enzyme concentration increases reaction rate (more active sites available)

Only increases until substrate becomes limiting

Enzyme activity, substrate concentration

Increasing substrate increases rate due to more frequent collisions

Eventually plateaus when all active sites are occupied (enzyme saturation)

Cofactors

Non-protein components needed for enzyme activity (make up part of the active site)

Often inorganic ions

If permenantly bound it is prosthetic group

Example: chloride ions (Cl⁻) act as a cofactor for amylase, helping it catalyse the breakdown of starch

Coenzymes (organic type of cofactor)

• Organic molecules that assist enzymes during reactions

• Often temporarily bind to the enzyme and help transfer chemical groups

• Many coenzymes are derived from vitamins in the diet (e.g. vitamin-derived coenzymes are essential for metabolism)

Competitive inhibitors

• Similar shape to the substrate

• Compete for the active site

• Block substrate binding

• Effect can be reduced by increasing substrate concentration

• Temporarily bind → reversible

Non-competitive inhibitors (reversible)

Bind to a different site (allosteric site), not the active site

Change the enzyme’s tertiary structure, altering the active site shape

Substrate may still bind, but reaction is slowed or prevented

Not affected by increasing substrate concentration

Usually reversible

Non-reversible (irreversible) inhibitors

Bind permanently (often covalently) to the enzyme

Destroy or block the active site/ tertiary structure permanently

Enzyme is permanently inactivated

Effect cannot be reversed by changing conditions

End product inhibition

A form of negative feedback control

The final product of a metabolic pathway acts as an inhibitor of an earlier enzyme in the pathway

Prevents overproduction of the product

Helps maintain balance (homeostasis) in cells