Chapter 3: Enzymes

A catalyst: substance which speeds up the rate of a reaction without itself undergoing permanent change. 

Enzymes 

• Globular proteins
• Increase the rate of reaction 

Enzyme-substrate complex 

• Lock and key theory 
• The substrate molecule is held within the active site by bonds that temporarily form between the R groups of the amino acids of active site and substrate molecules 
• Induced fit mechanism 
1. Substrate shape is not exactly complementary to the shape of the active site 
2. Active site changes its shape slightly when substrate binds 
3. This allows them to fit better
4.  Forms enzyme-substrate complex 
5. Products leave 

• Lysozyme is a natural defence against bacteria (peptidoglycan/ murein cell wall) that is found in tears, saliva 

Activation energy 

• An enzyme requires lower activation energy compared to without enzyme 
• When a chemical reaction takes palce, bonds within the reactants are broken and new bonds are made to form products 
• Breaking bonds require energy 

The course of a reaction 

1. Substrate but no product 
2. Substrates fit into empty active sites on the enzyme molecules 
3. Substrate is rapidly broken down to its products 
4. Substrate ↓ Product ↑

Initial rate of reaction is usually the first 30 seconds. 

Factors affecting the enzyme action 

The effect of enzyme concentration 

• Enzyme > Substrate, increasing the enzyme concentration does not affect 
• Substrate concentration is the limiting factor
• When Substrate > Enzyme, enzyme concentration ↑, ROR ↑ 
• Vmax = maximum velocity (level off)

The effect of substrate concentration 

• Substrate concentration ↑ each enzyme molecule works as fast as it could 
• Repeating with more substrate does not result in higher ROR, as enzymes molecules cannot bind with substrate 
• Substrate ↓ ROR ↑ 
• Addition of more substrate will have no effect on the ROR because more queuing up 

Temperature and enzyme activity 

•  Temperature ↑ collision ↑ substrate molecules enter the active site more often 
• At the enzyme's optimum temperature, the ROR is at its maximum 
• As temperature ↑ above the optimum temperature, enzyme is increasingly denatured 
• Denature - Hydrogen bonds holding the shape breaks, substrate no longer fits 

pH and enzyme activity 

• At extreme pH, the enzyme is denatured 
• Bonds holding the shape are disrupted 

Enzyme inhibitors 

Substrate binds into the active site

Inhibitors slow down ROR 

Competitive inhibitor (reversible) 

• Similar shape, binds with the active site 
• Conc. substrate ↑ inhibitor ↓ ROR ↑ 

Non-competitive inhibitor (reversible) 

1. Attach to allosteric site (inhibitor) and active site (substrate) 
2. When inhibitors bind with allosteric site, substrates are not allowed to enter the active site, as the correct shape to catalyse the reaction does not exist. 
3. Disrupts the normal arrangement of hydrogen bonds and hydrophobic interactions holding the molecule 

Competitive inhibition 
Higher Km with inihibitor 
Low affinity for the substrate, slows down the affinity to bind with substrate (Vmax not affected) 

Non-competitive inhibition 
Lower Vmax, Km not affected 
End-product inhibition 

• Controls metabolic reactions 
• Reactions start up again when concentration of the end product falls to a sufficiently low level 

Comparing enzyme affinities - the ability for enzymes to convert substrates to products: 

• Higher the affinity of the enzyme for the subtrate, the lower the substrate concentration needed for this to happen 
• Higher the affinity, the more likely the product will be formed when a substrate molecule enters the active site, rather than the substrate simply leaving the active site again before a reaction takes place. 
• The value of Km depends on:
• Temperature 
• pH 
• Type of substrate 
• Overall presence of inhibitors, poison, pollutants 

Vmax - maximum rate only when the enzyme is saturated with substrate 

 

Significance of Vmax and Km values 

• Enables computerised models of biochemical pathways 
• Can be compared quantitatively 
• Can design better catalsts (future) 
• Performance of the same enzyme from different organisms can be compared 

Immobilised enzymes - Attached with an insoluble support. Enzyme is then held in place during the reaction. Yeat with specific enzymes can also be immobilised. 

• Enzyme lactase can be immobilised using alginate beads 
• Lactase hydrolyses the lactose in the milk to glucose and galactose --> lactose free 

Advantages: 

• Can be reused again and again 
• More tolerant to pH and temperature changes 
• Continuous production 

Examples: 

• Trapping within a gel (alginate beads) 
• Covalent binding to a solid support (nylon)