Enzymes and Enzyme Systems Quiz

Study Notes

Enzymes are biological catalysts, speeding up biochemical reactions within cells
Active sites are located in pockets within the enzyme's three-dimensional structure:
- Substrate binding site: where the substrate molecule binds to the enzyme
- Catalytic site: where the catalytic reaction takes place
- Allosteric site: a site where other molecules can bind and influence enzyme activity
Lock and key model: The substrate fits perfectly into the active site like a key in a lock.
Induced fit model: The substrate induces a conformational change in the enzyme to fit the substrate.

Absolute specificity: The enzyme acts on only one substrate (e.g., uricase acting on uric acid).
Dual specificity: Two types:
- The enzyme acts on two different substrates but catalyzes the same type of reaction (e.g., xanthine oxidase acting on hypoxanthine and xanthine).
- The enzyme acts on one substrate, but catalyzes two different reactions (e.g., isocitrate dehydrogenase acting on isocitrate).
Stereo-specificity: The enzyme is specific to a specific isomer of a substrate (e.g., L-amino acid oxidase acting on L-amino acids only).
Relative specificity (group specificity): The enzyme acts on a group of compounds that share the same type of bond (e.g., lipase hydrolyzing ester linkages).
Structural specificity: The enzyme is specific to a particular bond and the chemical groups around it (e.g., trypsin breaking peptide bonds near arginine or lysine).

Enzyme kinetics: The study of enzyme reaction rates.
[S]: Substrate concentration
Vo: Initial velocity of a reaction
Vmax: Maximal velocity of a reaction
Km: Michaelis constant, representing the substrate concentration at which the reaction rate is half-maximal.
Significance of Km: Reflects the enzyme's affinity for its substrate.

Temperature: Enzyme activity increases with temperature until an optimal point (typically around 37°C - 40°C).
pH: Enzymes have an optimal pH range for maximal activity (e.g., 7.4 for most human enzymes).
Enzyme concentration: Increased enzyme concentration leads to a faster reaction rate.
Substrate concentration: Increased substrate concentration leads to faster reaction rate up to a point.
Cofactors and coenzymes: These molecules are often required for enzyme activity and can increase the reaction rate.
Product concentration: Can inhibit enzyme activity through feedback regulation.

Competitive inhibitors: Structurally similar to the substrate and bind to the active site, blocking substrate access. Examples: Allopurinol (inhibits xanthine oxidase) and methotrexate (inhibits dihydrofolate reductase).
Non-competitive inhibitors: Bind to a site other than the active site, altering the enzyme's shape and activity.

Zymogens: Inactive precursor forms of enzymes. They are activated by:
- Release of inhibitory polypeptide by cleavage or conformational change.
- Association with a cofactor (e.g., Mg2+).
- Association with another protein.
Significance of zymogens: Protects cells and transport systems, allows for storage, and provides regulatory mechanisms.

Isoenzymes: Multiple forms of the same enzyme with different physical properties.
Same catalytic activity, but different electrophoretic mobility, inhibitor sensitivity, and tissue distribution.
Examples: Lactate dehydrogenase (LDH), Creatine kinase (CK), Acid phosphatase (ACP), and Alkaline phosphatase (ALP).

Catalyzes the conversion of lactate to pyruvate in the presence of NAD+.
Tetrameric protein, comprised of two subunits (H and M), with five isoenzymes: LDH-1 (H4), LDH-2 (H3M1), LDH-3 (H2M2), LDH-4 (H1M3), and LDH-5 (M4).
Elevated levels in blood indicate damage to specific organs:
- Myocardial infarction: LDH-1 and LDH-2 predominate.
- Viral hepatitis: LDH-5 and LDH-4 predominate.
- Other diseases: LDH levels can be elevated in a variety of other conditions, including blood diseases and malignancies.