Ch.12 Enzymes

Chemical "lubricants" that make chemical reactions go nice and easy

• Many chemical reactions can't occur at 37 C (too cold)
• Many chemical reactions require added heat energy

• Substances that lessen the amount of energy required for chemical reactions to occur.
• Not used up in reactions.
• Protein catalysts
• Required for numerous metabolic processes of all cells.
• Cells are damaged, leak out into the plasma

Decrease the amount of energy required to activate chemical reactions.

• 1: Amino acid sequence
• 2: Interaction between 2 locations on the protein chain
• 3: Folding of chains
• 4: 2 or more separate polypeptide chains

Physical location on the enzyme molecule which interacts with the substrate molecule.

Non-active site, but which may interact with other substances to change the overall enzyme 3D shape

Structurally different enzymes (proteins) but which catalyze the same chemical reactions

• Non-protein substance required for normal enzyme activity.
• Maintain enzyme 3D structure, critical for enzyme function.

• Activators: Inorganic- Mg, Ca
• Coenzyme: Organic- NADH

Enzyme + coenzyme (prosthetic group)= Active enzyme

Enzyme-coenzyme=Inactive enzyme

• Chemical reactions occur spontaneously if the energy for reactants is higher than products.
• The amount of energy required to stimulate molecules to break their chemical bonds and form new bonds is the activation energy.
• Increasing the temperature can generate the activation energy. Problem for living cells.

• Absolute: catalyzes only 1 specific substrate
• Group: Catalyzes reactions of a particular chemical group
• Bond: Catalyzes reactions of particular chemical bonds
• Steroisomerism: Catalyzes reactions of steroisomers

Enzyme activity proportional to substrate concentration.

• As substrate concentration increases, the reaction rate increases.
• As substrate concentration decreases, the reaction rate decreases.
• Reaction rate is affected by substrate concentration.

Not a good place to measure enzyme activity.

• Substrate concentration is in excess.
• Substrate "saturates" enzymes.

• All enzyme reacts with the excess substrate.
• Chemical reaction rate goes to max velocity.
• Reaction rate dependent on enzyme activity.

Enzymes are not measured in terms of concentration, but activity.

Suited for enzyme measurement because this is where the reaction rate is dependent on the enzyme's ability to catalyze a chemical reaction.

Activity is measured under Zero Order conditions because this is where the rxn rate is dependent on the work of the enzyme.

• Numberical constant for each enzyme and substrate under defined conditions.
• Expresses a relationship between the reaction rate and substrate concentration

• Enzymes are proteins and subject to changes in 3D structure from pH changes.
• pH must be carefully controlled in enzymatic reactions

• Reaction rates vary dramatically with temp changes.
• Reactions rates may double per 10 C increase in temp.
• Temp. must be defined and regulated for enzymatic reactions.
• 37 C, common standardized temperatures

• Activators (Ca, Fe, Mg, Mn, Zn)
• Coenzymes (Vitamins and nucleotide phosphates)

• Substances that bind at the enzyme's active site.
• Competes with substrate for the active site.
• Addition of additional substrate increases the reaction rate.

• Substances that bind at an enzyme's non-active site
• Enzymes 3D shape is altered, decreasing enzyme activity
• May bind substrate, but additional substrate has no effect on rxn rate

Substances that bind with enzyme-substrate complex (rare)

• Enzymes not directly measured, measured in terms of catalytic activity.
• Activity is proportional to concentration.

• Increase of product
• Decrease of substrate
• Decrease of co-enzyme
• Increase of altered co-enzyme

• Some enzyme testing utlizes other enzymatic reactions in a sequence of reactions.
• These other accesory enzymes are not being measured.
• Auxillary enzymes are used as reagents and added in excess so they do not become rate limiting factors in the overall process.

All enzymatic reactions in a coupled assay must be performed at zero order kinetic.

• Zero Order Kinetics
• Substrate concentration is measured at set timed intervals to determine enzyme activity.
• A slight delay from the beginning of the reaction to max velocity is the lag phase.

Extreme elevations in enzyme activity may deplete the substrate between measured intervals

Cause falsely decreased activity results

• Continuous measurements of substrate-product concentration are recorded by the spectrophotometer of an automated analyzer.
• Substrate depletion is detected because the analyzer is always "looking" and will see any sudden dramatic changes