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ENYMES
IB Biology
Lecture 2
What is an Enzyme?
An enzyme is a catalytic PROTEIN– It is effective in small amounts– It is unchanged by the reaction– It speeds up the rate of reaching
equilibrium in a reversible reaction
Lock and Key Hypothesis
Due to its precise shape and distinctive chemical properties, each enzyme is specific for a certain substrate or a VERY small group of substrate molecules.
Induced Fit Hypothesis
Some enzymes change shape when combined with a substrate
The active site is then molded into a precise conformation
The bonds of the substrate are stretched to make the reaction easier (less energy needed)
Enzyme changes shape after it connects with the substrate.
Three factors that affect the rate of enzyme-catalyzed reactions
Temperature—As temperature increases, molecules are moving faster and are more likely to collide and react.
Different enzymes have different optimum temperatures (ex: bacteria in hot springs, plants of the tundra, enzymes in our bodies)
Substrate Concentration
Increasing the amount of substrate will speed up the rate of reaction.
However, there comes a point when there is more substrate than enzyme so adding more substrate molecules will no longer increase the rate of reaction
pH—each enzyme has a range of PH in which it functions efficiently
Denaturation
Denaturation is a structural change in a protein that alters its 3-D shape and causes the loss of its biological properties
Denaturation may be cause by changes in temperature and pH
Heat—exposure to heat causes atoms to vibrate violently and this disrupts bonds within globular proteins, and causes changes in the chemical characteristics—usually the change is irreversible (ex: raw vs. cooked egg white)
pH—small changes in pH also alter the shape of proteins. However the structure MAY re-form when optimum pH is restored
Industrial uses of enzymes
Lactose-Free Milk
The enzyme lactase helps digest lactose in milk Many adults do not produce lactase, so drinking milk
causes diarrhea and/or gas Lactose-free milk can be produced using lactase
– Whole-cell preparations may not be appropriate for food– Adding and removing enzymes to each product is
expensive
So immobilized enzymes are used to make the milk
Methods of immobilizing enzymes
Metabolism consists of chains (linear sequences) and cycles of enzyme-catalzyed reactions
Metabolism=anabolic reactions + catabolic reactions
Anabolic reactions—larger molecules built from smaller molecules (ex: protein synthesis)—ENERGY REQUIRING/ ENDOTHERMIC
Catabolic reactions—larger molecules are broken down (ex: digestion)—ENERGY RELEASING/ EXOTHERMIC
Enzymes lower the activation energy of exothermic reactions
Competitive Inhibition
Example: Carbon dioxide should combine with Rubisco during photosynthesis but can be competitively inhibited by oxygen
Non-competitive Inhibition
Example: Nerve gas, Sarin blocks acetyl cholinesterase in synapse transmission
Allosteric Enzymes
Allosteric enzymes have 2 sites. – Active site of the enzyme– Additional site where another substance can lock in– When the other substance is locked in, the active site is
non-functional
End Product Inhibition—a specific type of Allosteric Inhibition
As the end product accumulate, the steps in the product are stopped