ENZYMES AS BIOLOGICAL CATALYSTS
• A catalyst is a substance that increases the rate, or velocity, of a chemical reaction without itself being
changed in the overall process.
• Most biological catalysts are proteins.
• Enzymes are biological catalysts.
→ Enzymes accelerate and control biochemical reaction rates.
→ Exert kinetic control over thermodynamic potentiality
→ Enzymes are globular proteins that increase the rate of specific biochemical reactions.
→ Each enzyme operates on a limited number of substrates of similar structure to generate products under
well-defined conditions of concentration, pH, temperature, etc.
→ In metabolism, groups of enzymes work together in sequential pathways to carry out complex molecular
transformations, e.g. the multi-reaction conversion of glucose to lactate (glycolysis).
• As catalysts, each enzyme acts on a specific substance, called the enzyme’s substrate (or substrates, when
there are two or more reactants), and each catalyzes only one reaction.
⤷ Trypsin catalyzes hydrolysis of peptide bonds in proteins and polypeptides.
⤷ Sucrose (table sugar) is the substrate of the enzyme sucrase, which catalyzes the hydrolysis of
sucrose to glucose and fructose.
⤷ Catalase, an enzyme present in many cells, increases the rate of H2 O2 decomposition about 1 billion-
fold over the uncatalyzed rate.
– Hydrogen peroxide is produced in some cellular reactions and is a dangerous oxidant thus, selective
pressures have resulted in the evolution of catalase to defend cells against the damaging effects of
H2 O2 .
– This example shows that the rate of a favorable reaction depends greatly on whether a catalyst is
present and on the nature of the catalyst.
• Enzymes are among the most efficient and specific catalysts known.
• Catalysts accelerate the approach to equilibrium for a given reaction.
• Catalysts do not change the thermodynamic favorability of a reaction.
• As catalysts, enzymes typically accelerate chemical reactions by lowering their activation energy.
• Enzymes have a high specificity and produce more than a 90% yield as they do not require numerous
steps compared to normal catalysts that require numerous steps which consecutively decreases the yield.
∴ with each step you introduce to the chemical reaction, the decrease in the yield obtained.
DISTINCTIVE FEATURES OF ENZYMES
1. Catalytic power
2. Specificity
3. Regulation
4. Mild conditions
Normal chemical reaction conditions without
an enzyme
TJW NOTES
,ENZYME NOMENCLATURE
Enzymes are divided into six major classes, with subgroups and sub-subgroups to define their functions
more precisely.
The major classes are as follows:
1. Oxidoreductases ~ catalyze oxidation–reduction reactions.
2. Transferases ~ catalyze transfer of functional groups
from one molecule to another.
3. Hydrolases ~ catalyze hydrolytic cleavage.
4. Lyases ~ catalyze removal of a group from, or addition of
a group to, a double bond, or other cleavages involving
electron rearrangement.
5. Isomerases ~ catalyze intramolecular rearrangement.
6. Ligases~ catalyze reactions in which two molecules are joined.
TJW NOTES
, To illustrate, consider the enzyme that catalyzes this reaction:
ATP + D-glucose⎯⎯→ADP + D-glucose-6-phosphate
• A phosphate group is transferred from ATP to the C-6-OH group of glucose, so the enzyme is a transferase
(class 2, Table 13.1).
• Subclass 7 of transferases is enzymes transferring phosphorus-containing groups, and sub-subclass 1 covers
those phosphotransferases with an alcohol group as an acceptor.
• Entry 2 in this sub-subclass is ATPD-glucose-6-phosphotransferase, and its classification number is
E.C.2.7.1.2.
• In use, this number is written preceded by the letters E.C., denoting the Enzyme Commission.
CHEMICAL REACTION RATES AND THE EFFECTS OF CATALYSTS
Reaction Rates, Rate Constants, and Reaction Order
FIRST-ORDER REACTIONS
let us first consider the simplest possible reaction, the Irreversible conversion of substance A to substance B:
A→B
• The reaction rate, or velocity (𝑣), at any instant as the rate of formation of the product.
ⅆ𝐵
• In this case, the rate is the increase in concentration of B with time: 𝑣 = ⅆ𝑡
• The units of v are concentration per unit time (e.g., molar per second: M • s−1 , where [B] symbolizes
molar concentration of B).
• If we note that, for every B molecule formed, an A molecule must disappear, it is clear that 𝑣 can equally
ⅆ𝐴
well be written as 𝑣 = − ⅆ𝑡 where the negative sign indicates [A] is decreasing with time.
• Over time, as molecules of A are consumed, the number of molecules left to change is diminished, and the
rate decreases as the reaction proceeds.
TJW NOTES
