1.4 - Enzymes Flashcards
(a)
Metabolism
A series of enzyme controlled reactions of the body.
(a)
Metabolic pathway
A sequence of enzyme-controlled reactions in which a product of one reaction is a reactant in the next.
(a)
Anabolic reactions
Build up molecules e.g. protein synthesis
(a)
Catabolic reactions
Break molecules down e.g. digestion
(b)
Enzyme
A biological catalyst; a protein made by cells that alters the rate of a chemical reaction without being used up by the reaction.
(b)
Protein nature of enzymes
Enzymes are tertiary structure proteins and so they are a very specific 3D shape. This includes an active site which is held together by peptide, hydrogen, ionic and disulphide bonds.
(c)
Intracellular enzymes
These work inside cells
(c)
Extracellular enzymes
Secreted from cells for use outside of the cell.
(d)
Active sites
An enzyme acts on its substrate, with which it makes temporary bonds at the active site, forming an enzyme-substrate complex. When the reaction is complete, products are released, leaving the enzyme unchanged and the active site ready to recieve another substrate molecule.
(d)
Lock and key theory
The unique shape of the active site means that enzyme can only catalyse one type of reaction. Other moelcules, with different shapes, will not fit. ‘Enzyme specificity’ means that an enzyme is specific for its substrate. The substrate is imagined fitting into the active site as a key fits into a lock.
(e)
The theory of induced fit as illustrated by lysozyme
The induced fit theory is an alternative
theory of enzyme action – lysozyme is
proposed to function in this way. In this theory, the active site and substrate are notf ully complementary in shape. Reactive groups in these areas align and the substrate forces its way into the active site. Both areas
change structure slightly, the bonds in the substrate weakens and the reaction occurs at a lower activation energy.
(f)
Activation energy
Enzymes are catalysts, which means they lower the activation energy of reactions but remain unchanged in the reaction.
(f)
Activation energy defintion
The minimum energy required that must be put into a chemical system for a reaction to occur.
(g)
Effect of temperature on the rate of reaction
At low temperatures, enzymes have low kinetic energy, so there are fewer successful collisions between the enzyme and substrate, meaning fewer products are made.
As the temperature rises, the kinetic energy increases, causing more frequent collisions and more enzyme-substrate complexes, which speeds up the reaction. This continues until the enzyme reaches an optimum temperature where it works best.
However, if the temperature gets too high, the enzyme’s structure starts to break down, causing its active site to lose shape. This change means the substrate can no longer fit, no more reactions happen, and the enzyme is denatured.
(g)
Effect of pH on the rate of enzyme action
Most enzymes have an optimum pH. Small changes from the optimum, either above or below optimum pH, make small reversible changes in the enzyme molecule reducing its efficiency. Large changes in pH can
disrupt ionic and hydrogen bonds in the enzyme causing permanent changes to the shape of the active site. This prevents the formation of enzyme-substrate complexes, denaturing the enzyme.
(g)
Substrate concentration
As more substrate is added, the reaction rate increases because there are more chances for the enzyme and substrate to collide and react. At low levels of substrate, the reaction speed depends on how much substrate is available. However, once there is enough substrate that all enzyme active sites are busy, adding more substrate won’t increase the reaction rate. At this point, the reaction rate levels off (plateaus), and now the amount of enzyme limits how fast the reaction can go.
(g)
Enzyme concentration
Assuming an excess of substrate, any
increase in enzyme concentration increases the rate of reaction as more active sites are available for reactions.
(g)
The importance of
buffers for maintaining a constant pH
Buffers are essential for maintaining a constant pH in biological systems because they resist changes in pH when small amounts of acid or base are added. They work by neutralizing excess H⁺ or OH⁻ ions, keeping the pH within a narrow range.
(h)
Competitive inhibitors
Competitive inhibitors are complementary in shape to the active site of the enzyme. They therefore prevent the formation of enzymesubstrate complexes by blocking the active site. They do not bind permanently.
(h)
Non-competitive inhibitors
Non-competitive inhibitors bind to the
enzyme away from the active site at an ‘allosteric’ site. This alters the shape of the active site so no enzyme-substrate complexes can be formed. Some inhibitors bind reversibly, while others bind irreversibly.
(i)
Immobilised enzymes
Enzymes can be attached to an inert matrix such as cellulose microfibrils or
sodium alginate beads. Industrial processes use immobilised enzymes, allowing enzyme reuse and improving stability Immobilised enzymes are used in biosensors and to create lactose-free milk.
(i)
The importance of immobilised enzymes
- Increased stability so will denature at higher temperature and can be used efficiently over a wider range of pH.
- Products uncontaminated with enzyme.
- Enzymes easily added and removed, therefore giving control over reactions. Alternatively, they are recovered for re-use.
