Biochemistry Wk 9 Flashcards
What are enzymes
• Enzymes are biologically active proteins that accelerate the breakdown of food that is eaten.
• Enzymes are biological catalysts. They accelerate reactions, but are not consumed or changed in reactions.
• They are involved in all processes essential for life such as DNA replication and transcription, protein synthesis, metabolism and signal transduction, etc.
The six major classes of enzymes
Oxidoreductases- catalyse oxidation-reduction reactions. (Dehydrogenase)
Transferases- catalyse transfer of C-,N-, or P-, containing groups
Hydrolases- catalyse cleavage of bonds by addition of water (urease)
Lyases- catalyse cleavage of C-C,C-S, and certain C-N bonds (decarboxylase)
Isomerases- catalyse rearrangement of optical or geometric isomers such as mutase
Ligases- catalyse formation of bonds between carbon and O,S,N coupled to hydrolysis of high energy phosphates such as carboxylase
Enzymes and Their Substrates,
Enzymes are 3D structures
Enzymes and Their Substrates, Continued
Consider hexokinase, an enzyme whose job is to transfer a phosphate group from the high energy molecule, adenosine triphosphate,
ATP, to D-glucose.
Enzymes and Their Substrates, Continued
The Active Site
• Active site is lined with amino acid side chains
• The active site is the functional part of an enzyme where catalysis occurs
Enzymes and Their Substrates, Continued
• Glucose, the reactant for hexokinase, fits snugly in the active site. In an enzyme reaction, the reactant is called the substrate.
• Enzymes have specific substrates, a property known as substrate specificity. For example, the active site of hexokinase reacts with
D-glucose, but will not react with L-glucose.
• Enzymes are specific for one enantiomer of the substrate.
Enzymes and Their Substrates, Continued
Co factors- are inorganic substances such as Mg2+
Coenzymes- are small organic molecules derived from vitamins. Riboflavin found in the coenzyme flavin adenine dinucleotide (FAD) is a coenzyme
Enzyme related definitions
Active site- the site in the 3D protein structure at which the substrate binds and is converted to a product
Prosthetic group- a tightly bound cofactor that remains stably bound to the enzyme during the reaction
Apoenzyme- an inactive enzyme, without its cofactor
Holoenzyme- an enzyme with a bound cofactor
Enzymes and Their Substrates, Continued
Enzyme–Substrate Models
• A substrate is drawn into the active site by intermolecular attractions like hydrogen bonding.
• Hydrogen bonding orients the substrate properly within the active site.
• The initial interaction of the enzyme with the substrate is called the enzyme–substrate complex (ES). This complex forms prior to catalysis.
Enzymes and Their Substrates, Continued
1.
There are two enzyme–substrate models:
In the Lock-and-key model, the active site is thought to be a rigid, inflexible shape that is an exact complement to the shape of the substrate. The substrate fits in the active site much like a key fits in a lock.
In the induced-fit model, the active site is flexible, has a shape roughly complementary to the shape of its substrate, and undergoes a conformational change, adjusting to the shape of the substrate when the substrate interacts with the enzyme.
Enzymes as catalysts
Most chemical reactions can be described as the conversion of a substrate to a product. This process often includes a transition state, an intermediate form between substrate and product. This intermediate usually has a higher free energy than the substrate. For this reaction to happen an input of energy required to overcome the barrier. This is the Ea. This accepts the ROR the, the greater the Ea, the slower the reaction
Gibbs Free Energy (G) is used to describe the useful energy in a reaction or the energy capable of doing work.
Catalysts
They increase the ROR by lowering the Ea for the reaction. This means that less energy is required to start the reaction. The rate of reaction is increased
Enzymes affect the activation energy. The activation energy is the difference in free energy between the substrate and the transition state. The transitions state is the intermediary state of the reaction, when the molecule is neither a substrate or product. The transition state has the highest free energy, making it a rare and un-stable intermediate.
An enzyme helps catalyze a reaction by decreasing the free energy of the transition state. As a result, more product will be made because more molecules will have the energy necessary for the reaction to occur and the reaction will occur at a faster rate
Factors That Affect Enzyme Activity
If allowed to sit untouched, the flesh of sliced apples will turn brown by a process known as oxidation, caused by an enzyme.
• If lemon juice is sprinkled on the sliced apple, the vitamin C in the lemon juice will inhibit the formation of this brown color by changing the pH of the environment of the enzyme.
• Enzyme reactions are affected by reaction conditions such as substrate concentration, pH, temperature, and the presence of inhibitors.
Factors That Affect Enzyme Activity, Continued
Substrate Concentration
Maximal velocity: The rate or velocity of a reaction (v) is the number of substrate molecules converted to product per unit time. The rate of an enzyme-catalyzed reaction increases with substrate concentration until a maximal velocity (Vmax) is reached . At a constant concentration of enzyme, an increase in substrate concentration will cause an increase in the enzyme activity up to the point where the enzyme becomes saturated with substrate.
A condition known as steady state is when an enzyme is operating under maximum activit
Factors That Affect Enzyme Activity, Continued
pH
Factors That Affect Enzyme Activity, Continued
• When the enzyme environment is changed by pH, its tertiary structure is disrupted, altering the active site and causing the enzyme’s activity to decrease.
• Enzymes are most active at a pH known as their optimum pH.
• At optimum pH, the enzyme maintains its tertiary structure and its active site.
