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Enzymes - PowerPoint PPT Presentation

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Enzymes. Enzymes, substrates and active sites. Enzymes are globular proteins that work as catalysts Substrates are the substances that enzymes convert into products during metabolic reactions.

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enzymes substrates and active sites
Enzymes, substrates and active sites

Enzymes are globular proteins that work as catalysts

Substrates are the substances that enzymes convert into products during metabolic reactions.

Active site is the region on the surface of an enzyme to which substrates bind and which catalyzes the reaction.


enzyme substrate complex
Enzyme – substrate complex

Once a substrate has been locked into the active site, the reaction is catalyzed

The products are released and the enzyme is used again.

enzymes are specific to their substrates
Enzymes are specific to their substrates
  • Enzymes are highly specific and work only on specific substrates (enzyme substrate specificity)
  • Two theories explain the substrate specificity of enzymes:
    • Lock and key model
    • Induced fit model
making and testing theoretical models
Making and testing theoretical models

The German scientist Emil Fisher introduced the lock and key model in 1890. Daniel Koshland suggested the induced-fit model in 1959 in the US. The conformational changes predicted by Koshland’s model were subsequently observed using high-resolution X-rays analysis of enzymes and other newly developed techniques. Although much experimental evidence has accumulated confirming predictions based on the induced-fit model, it is still just viewed as a model of enzyme activity.

Why has the lock and key model been superseded by the induced-fit model?

Could there ever be a situation in which two models are both retained?

Why is the induced-fit model not now called the Law of Induced Fit?

enzymes lower the activation energy of a reaction hl
Enzymes lower the activation energy of a reaction (HL)

Activation energy is the amount of energy that must be put into a reaction to make it occur.

An enzyme stresses the bonds in the substrate(s), reducing the activation energy required for the reaction to occur.


Enzymes are globular proteins with a tridimentional structure

Their structure can be altered by several factors

When the shape of their active site is changed considerabily they will not function.

Denaturation is changing the structure of a protein (enzyme) so that it can not carry out its function.



factors affecting enzyme activity
Factors Affecting Enzyme Activity

Effect of pH on enzyme activity

  • Enzymes only operate in a narrow range of pH values. The optimum pH
  • If there is a deviation from the optimum pH, hydrogen bonds in amino acids are broken an the enzyme is denatured.
  • Different enzymes have different optimum pH and work in different pH environments.


factors affecting enzyme activity1
Factors affecting enzyme activity

Effect of temperature on enzyme activity

  • As temperature increases the rate of reaction increases. The particles have more kinetic energy, therefore collide and react more often.
  • Above the optimal temperature denaturation of the enzymes occur, the active site changes and looses its function.
  • A thermophile, such as bacteria in deep sea vents, is an organism that is able to withstand higher temperatures before enzymes denature.


factors affecting enzyme activity2
Factors Affecting Enzyme Activity
  • Substrate concentrations
    • Increasing the substrate concentration increases the rate of reaction.
    • At the optimum concentration of substrate molecules, all the active sites are full and working at a maximum efficiency
    • Any increase in concentration beyond the optimum will have no added effect as there is no extra active sites to be used.


lactose intolerance
Lactose Intolerance

Lactose, the sugar found in milk causes allergies in some people because they are unable to produce enough of the enzyme lactase.

lactose - lactase-- glucose + galactose

Global estimates of lactose intolerance

how can we cope with lactose intolerance
How can we cope with lactose intolerance
  • Take a lactase supplement
    • Produced industrially using the Aspergillusnigerfungus
  • Drink lactose free milk
    • Milk is treated with lactase (produced by A. niger) and pre-digested before being packaged.
    • Lactase can be either added directly to milk or be inmmobilized on a surface or beads of a porous material.



metabolic pathways hl
Metabolic Pathways (HL)

Chains or cycles of enzyme catalyzed reactions

The Krebs cycle (cell respiration) and the Calvin cycle (photosynthesis) are examples of metabolic pathways that form cycles



metabolic pathways hl1
Metabolic Pathways (HL)

Protein synthesis is a metabolic pathway that occurs in a chain.


enzyme inhibition hl
Enzyme Inhibition(HL)
  • Enzymes can be inhibited by other molecules.
  • The process can be:
    • Competitive
    • Non – competitive
      • Allosteric interactions
      • Allosteric site



end product inhibition of metabolic pathways hl
End – product inhibition of metabolic pathways (HL)

Metabolic pathways can be switched off completely in cells where there is an excess of product.

The end product inhibition prevents a build up of intermediate products.