B.7.4 DETERMINE VMAX AND THE VALUEOF THE MICHAELIS CONSTANT (KM ) BYGRAPHICAL MEANS AND EXPLAIN ITSSIGNIFICANCE.
The following can be deduced from the graph:•At low substrate concentration, the rate of the reaction is proportional tothe substrate concentration; enzyme is available to bind to the substrate.•As the substrate concentration is increased, the rate decreases and is nolonger proportional to the substrate concentration; some of the enzymeactive sites are occupied by substrate and is not available•At high substrate concentration, the rate is constant and independent ofsubstrate concentration; at this point the enzyme is saturated withsubstrate.
Michaelis- Menten equationTwo features of Michaelis-Menten kinetics to note:
1. The maximum velocity (Vmax)
• Definition :Maximum velocity of enzyme under thecondition of the experiment•Has the unit of rate.•Varies from one enzyme to another and with PH,temperature•The rate of reaction is sometimes expressed as theturnover number: the number of molecules of substratethat can be processed into products per enzyme moleculeper unit time•E.g : the enzyme catalase is a very fast enzyme withturnover rate of upto 100000 molecules of its substrate H2O2 per second
2. The Michaelis constant (Km)
•Definition: the substrate concentration at which thereaction rate is equal to one half its maximum value.•Has the unit of concentration•Varies with PH and temperature•The value of Km gives info about affinity of enzyme for itssubstrate.•A low value of Km that the reaction is going quickly even atlow substrate concentrations.•A higher value means that the enzyme has a lower affinityfor its substrate. graph
B.7.5 DESCRIBE THE MECHANISM OFENZYME ACTION, INCLUDING ENZYMESUBSTRATE COMPLEX, ACTIVE SITEAND INDUCED FIT MODEL.
enzymes are protein containing several hundred amino acids its conformation(3 dimensional shape) is determined by the interactions between all the R groups and is essential for its function Some enzymes require the binding of non- protein molecules for activity (co-factors) Organic co-factors are called coenzyme-metal ions Inorganic co-factors acts as precursors for coenzyme- vitamin
HOW DOES ENZYME FUNCTION? Act as catalyst that will increase the rate of chemical reaction without undergoing chemical change. Lowers the activation energy of the reaction route between substrate and product, so reaction can happen more quickly at the same temperature. This is because of the ability of enzyme to form a temporary binding to the substrate where it is held by relatively weak forces of attraction, forming an enzyme- substrate complex. Binding occurs at a small region of enzyme also known as active site.
Formation of complex depends on compatibility between the substrate and the R groups of the amino acids at the active site of enzyme. It involves hydrophobic attractions, dipole-dipole attractions, hydrogen bonds and ionic attractions. The binding in the complex puts a strain on the substrate molecule, and so facilitates the breaking and forming bond. Once the substrate has reacted, the product formed no longer fits in the active site, so it detaches. Enzyme remains unchanged and is able to catalyse further reaction.
ENZYMES ARE HIGHLY SPECIFIC FOR THEREACTION THEY CATALYSE. German chemist Emil Fischer proposed a model known as the lock-and-key mechanism. But recent work on proteomics has recognized that enzymes are less rigid structures. Danial Koshland suggested the theory ‘induced-fit mechanism’. In the presence of substrate, the active site undergoes some conformational changes, shaping itself to allow better fit.