Enzymes and Their Functions

Enzymes

• Enzymes hasten chemical reactions in organic matter
• Catalyze specific chemical reactions
• Found in large quantities within cells
• Increase membrane permeability, allowing them to enter the bloodstream
• Measured by activity, not absolute value
• Appear in serum after cell injury, degradation, or storage
• Specific to a substrate, converting it to a defined product
• Used clinically as a sign of organ damage

Enzyme Components

• Active site: Water-free cavity where substrate interacts
• Allosteric site: Cavity other than the active site, binding regulator molecules

ES Complex

• Physical binding of a substrate to the enzyme's active site

Factors Affecting Enzymatic Reactions

• Enzyme Concentration: Higher concentration results in faster reaction rate within the substrate range
• Substrate Concentration: Reaction rate increases with substrate up to saturation. Further increases have no effect
• Cofactors:
  • Coenzymes: Organic molecules aiding substrate fit. Increased coenzymes increase reaction velocity. Examples include NAD and NADP
  • Activators: Inorganic ions altering enzyme shape for proper binding. Examples include calcium, zinc, chloride, magnesium, and potassium.
  • Metalloenzymes: Inorganic ions attached to the enzyme. Examples include catalase and cytochrome oxidase.

Inhibitors

• Competitive Inhibitors: Bind to the active site, competing with the substrate. Reversible if substrate surpasses inhibitor concentration
• Noncompetitive Inhibitors: Do not compete with the substrate (bind to an allosteric site), causing irreversible inhibition.
• Uncompetitive Inhibitors: Bind to the enzyme-substrate complex (ES), increasing inhibition with increased substrate

Isoenzymes

• Enzymes with identical, catalytic reactions yet varying structures and amino acid sequences

Temperature

• Optimal temperature is 25-37 degrees Celsius
• Denaturation occurs above 40 degrees Celsius
• Inactivation above 60-65 degrees Celsius
• Temperature coefficient (Q10): a 10-degree increase in temperature leads to roughly a twofold increase in enzyme activity

pH

• Physiological range is 7-8
• Extreme pH denatures enzymes and alters charge in the amino acid residues within the active site

Storage

• Low temperatures (refrigeration/freezing) result in reversible inactivity.
• Repeated freezing/thawing denatures proteins
• -20 degrees Celsius is best for long-term storage
• 2-8 degrees Celsius is suitable for substrate and coenzyme storage
• Room temperature is good for LD4 and LD5 storage

Hemolysis

• Increased enzyme concentration due to erythrocyte rupture

Lactescence/Milky Specimen

• Increased enzyme concentration

Enzyme Nomenclature

• Standardized by the Enzyme Commission (EC) using a classification system
• Classified by biochemical function, substrate catalyzed, class of reaction.
• Individual identification numbers (AMS EC 3.2.1.1)

Other factors

• Hemolysis and lactescence can cause elevated enzyme concentration
• Enzyme theory includes lock-and-key and induced-fit models
• Enzyme kinetics deals with rate of reactions and affects of factors like substrate and enzyme concentrations, temperature, etc