Wednesday, 16 May 2012

NOTES: Enzyme


The role of enzymes in organisms

Metabolisms are chemical reactions that occur within a living organism.
Enzymes regulate almost all the cellular reactions.
Enzymes are biological catalysts that speed up biochemical reactions in the cell.

The general characteristics of enzymes

1.    Enzymes are proteins which are synthesised by living organisms
2.    Enzymes alter or speed up the rates or chemical reactions but remain unchanged at the end of reactions.
3.    Enzymes have specific sites called active sites to bind to specific substrates.
4.    enzymes are highly specific; each enzyme can only catalyse one kind of substrate.
5.    Enzymes are needed in small quantities; because they are not used up but released at the end of  a reaction.
6.    Enzyme can catalyse specific reactions both in the forward and in the reverse directions. Most enzyme-catalysed reactions are reversible.
7.    Many enzymes require helper molecules, called cofactors, to function
(inorganic cofactor-ferum and copper; organic cofactor-vitamin B)
8.    Enzyme activities can be slowed down or completely stopped by inhibitors (heavy metals such as lead and mercury)


Naming of enzymes

1.                The name of enzyme is derived from the name of the substrate it catalyses.
2.                The name of most enzymes are derived by adding suffix –ase to the name of the substrates they hydrolyse.  For example:

Substrate
Enzyme
Lactose
Lactase
Sucrose
Sucrase
Lipid
lipase

3.                However, there are some enzymes that were named before a systematic way of naming enzymes was formulated.  For example, pepsin, trypsin and rennin.

Synthesis of enzymes

1.                Ribosomes are the sites of protein synthesis.  Since enzymes are proteins, ribosomes are also the sites of enzymes synthesis.
2.                The information for the synthesis of enzymes is carried by the DNA.  The different sequences of bases in the DNA are codes to make different proteins.  During the process, messenger RNA is formed to translate the codes into a sequence of amino acids.  These amino acids are bonded together to form specific enzymes according to DNA’s codes.

Intracellular enzymes and extracellular enzymes

1.    Enzymes are synthesised by specific cells.
2.    Enzymes which are produced and retained in the cell for the use of the cell itself are called intracellular enzymes.  These  enzymes are found in the cytoplasm, nucleus, mitochondria and chloroplasts.  For example, the enzymes oxireductase catalyses biological oxidation and reduction in mitochondria.
3.    Enzymes which are produced in the cell but secreted from the cell to function externally are called extracellular enzymes.  For example, digestive enzymes produced by the pancreas are not used by the cell in the pancreas but are transported to the duodenum, which is the actual site of the enzymatic reaction.

Production of extracellular enzymes

1.    Many enzymes produced by specialised cells are secreted outside the cell.  For example, pancreatic cells secrete pancreatic amylase outside the cells to be transported to the target organ  (duodenum).

The production of extracellular enzymes
a.     The nucleus contains DNA which carries the information for the synthesis of enzymes.
b.    Protein that are synthesised at the ribosomes are transported through the space within the rough endoplasmic reticulum (rough ER).
c.     Proteins depart from the rough ER wrapped in vesiscles that bud off from the membranes of the rough ER.
d.    These transport  vesicles then fuse with the membrane of the Golgi apparatus and empty their  contents into the membranous space.
e.     The proteins are further modified during their transport in the Golgi apparatus, for example, carbohydrates are added to protein to make glycoproteins.
f.      Secretory vesicles containing these modified proteins bud off from the Golgi apparatus and travel to the plasma membrane.
g.     These vesicles will then fuse with the plasma membrane before releasing the protein outside the cell as enzymes.


Mechanism of enzyme action

1.      Each enzyme molecule has a region with a very precise shape called the active site.
2.      The substrate molecule fits into the active site of the enzyme like a key into a lock.
3.      Various types of bonds including hydrogen bonds and ionic bonds hold the substrate(s) in the active site to form an enzyme-substrate complex.
4.      The enzyme then changes the substrate(s) either by splitting it apart (as in hydrolysis) or linking them together (as in condensation).
5.      Once formed, the products no longer fit into the active site and escape into the surrounding medium, leaving the active site free to receive other substrate molecule.

Enzyme + substrateàenzyme-substrate complexàenzymes + products

6.      The explanation of enzyme action is known as the ‘lock and key’ hypothesis, where the substrate is like a key whose shape is complementary to the enzyme or lock.
7.      The ‘lock and key’ hypothesis is able to explain:
a.      why enzymes are specific, and
b.     why any change in the shape of enzyme alters its effectiveness.

Factors affecting enzyme activity

(i) Temperature
1.      At low temperature, the enzyme-catalysed reaction progresses slowly.
2.      An increase in temperature leads to an increase in the rate of reaction because the kinetic energy of the enzyme and substrate molecules produces more collisions, and therefore more enzyme-substrate complexes are formed.
3.      For every increase in temperature of 10° C, the rate of reaction is doubled.
4.      The rate of reaction increase up to a maximum at the optimum temperature.
5.      The optimum temperature of human enzymes is about 37° C.
6.      Above the optimum temperature, the rate of reaction falls quickly because the bonds maintaining the structure of the enzymes start to break and the active site loses its shape.
7.      The enzyme-substrate complexes can no longer be formed and the enzyme is denatured.
8.      Most enzyme-catalysed reactions stop at about 60° C.

(ii) pH

1.      Most enzymes are effective in a narrow pH range only.
2.      The optimum pH is the particular pH at which the rate of reaction is fastest(maximum).
3.      A change in pH can alter the charges on the active sites of the enzymes and the substrate surfaces.  This can reduce the ability of both molecules to bind each other.
4.      Unlike the effects of temperature on enzymes, the effects of pH on enzymes are normally reversible.  When the pH in the environment reverts to the optimum level for the enzymes, the ionic charges on the active sites are restored.  Thus, the enzyme resume their normal function.
5.      Pepsin function most effectively in an acidic medium at pH of about 2.  Pepsin is found in the stomach where the conditions are acidic.
6.      Trypsin functions most effectively in an alkaline medium at about pH 8.5.  It is found in the duodenum where conditions are alkaline.

 (iii) Substrate concentration


1.      Initially, an increase in substrate concentration increases the chance of enzyme-substrate collisions and, therefore,the rate of reaction increases.
2.      Eventually, all the active sites are filled at any one time and the rate remains constant.  The enzyme molecules are said to be saturated and the reaction has reached its maximum rate.
3.      The concentration of enzyme become a limiting factor.  At this stage,the only way to increase the rate of reaction by increasing the concentration of enzyme.

(iv) Enzyme concentration
1.      As the concentration of enzyme increases, there is more chance of enzyme-substrate collisions.  The rate of reaction increases linearly as long as no other factors  are limiting( there is abundant supply of substrate; pH, temperature and pressure are constant).  Thus, the rate of reaction is directly proportional to the concentration of the enzyme present until a maximum rate is achieved.  After the maximum rate, the concentration of substrate becomes a limiting factor.
2.      Figure 4.15 (pg 75 textbook) shows a linear graph is obtained when we plot a graph of the rate of reaction against enzyme concentration.  When the enzyme concentration is doubled, the rate of reaction will also doubled, provided that substrate concentrations are in excess .(As more active sites are available, more substrate can be converted to products-Figure 4.16pg 75 textbook). 


 

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