Enzyme Kinetics and Km Value Quiz

Course Information

• Course Title: Foundation Course For Health Sciences II
• Course Code: MEDF 1012A
• Instructor: Dr. Yeung Hang Mee, Po
• Email: poyeung@cuhk.edu.hk
• Office: Choh-Ming Li Basic Medical Sciences Building 6/F Room 610P

Learning Objectives

• Describe the structures and components of enzymes
• Understand enzyme kinetics, including the Michaelis-Menten equation and Lineweaver-Burk plot
• Explain different types of enzyme inhibitions with examples
• Review multiple factors affecting enzyme activity

Biocatalyst

• A catalyst speeds up a chemical reaction, compared to the same reaction without a catalyst
• A catalyst is not consumed in the reaction and can be recycled
• A catalyst increases the rate of a reaction by lowering the activation energy

Structures and Components of Enzymes

• Most enzymes are proteins
• Enzymes require cofactors (inorganic) or coenzymes (organic)
• Some enzymes are ribonucleic acids (RNA) called ribozymes
• Ribozymes catalyze specific biochemical reactions
• Ribozymes are found in the ribosome where they join amino acids to form protein chains

General Properties of Enzymes

• Enzymes have highly specific reactants called substrates that bind to the active site
• Trypsin is formed in the small intestine when its proenzyme form, trypsinogen, produced by the pancreas, is activated
• Trypsin cleaves peptide chains mainly at the carboxyl side of the amino acids lysine or arginine
• Enzymes operate effectively at specific pH and temperature ranges
• Pepsin (stomach) works effectively at very acidic environments
• Trypsin (activated at duodenum) works effectively at alkaline environments
• Alkaline phosphatase (bones and liver) works effectively at alkaline environments
• Most enzymes (made of proteins) are denatured at high temperatures due to changes in their 3D structure

Cofactors

• Cofactors are inorganic components that are bound to enzymes for activity
• An inactive enzyme without a cofactor is called an apoenzyme
• Examples include inorganic metal ions such as zinc, copper, and ferric ions

Coenzymes

• Coenzymes are organic components bound to enzymes for activity
• An inactive enzyme without a coenzyme is also called an apoenzyme
• Examples include flavin adenine nucleotide (FAD) and nicotinamide adenine dinucleotide (NADH)

Classifications of Enzymes

• Each enzyme is assigned a four-digit classification number and a systematic name according to the reaction catalyzed
• Specific examples listed include ATP-glucose phosphotransferase (EC 2.7.1.1) (hexokinase)

Classifications of Enzymes (continued)

• Enzymes are classified based on the type of reaction they catalyze
• Listed classes include Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, and Ligases

Activation Energy in Catalyzed Reactions

• Enzymes can provide alternative pathways with lower activation energy for chemical reactions

Enzyme Kinetics

• The Michaelis-Menten equation reflects the change in velocity of enzymatic reactions under different substrate concentrations

Michaelis-Menten Equation

• Vmax is the maximum velocity an enzyme can reach
• Km is a specific value of substrate concentration where the velocity of enzymatic reaction reaches half Vmax
• Km value indicates substrate affinity for the enzyme

Transformation of Michaelis-Menten Equation into Lineweaver-Burk Plot

• The Lineweaver-Burk plot is a linear transformation of the Michaelis-Menten equation
• The plot allows for easy determination of Km and Vmax values

Enzyme Inhibitions

• A reversible inhibition occurs when the inhibitor rapidly dissociates from the enzyme or enzyme-substrate complex
• Types include competitive, non-competitive, and un-competitive inhibition
• An irreversible inhibition occurs when the inhibitor very slowly dissociates from the enzyme or enzyme-substrate complex

Reversibly Competitive Inhibition

• A competitive inhibitor binds to the active site of an enzyme, competing with the substrate
• This results in an apparent increase in Km, but Vmax remains unchanged

Reversibly Non-Competitive Inhibition

• A non-competitive inhibitor binds to a site other than the active site, decreasing Vmax
• KM is not significantly affected

Reversibly Un-competitive Inhibition

• An un-competitive inhibitor binds to the enzyme-substrate complex, which lowers both Vmax and Km

Types of Enzyme Inhibition by Lineweaver-Burk Plot

• The Lineweaver-Burk plot provides a visual representation of how different types of inhibition affect enzymatic activity

Irreversibly Inhibition

• This type of enzyme inhibitor binds strongly to the active site, forming a covalent bond with the enzyme.
• An example, Aspirin, works by binding to the active site of cyclooxygenase (COX)

Factors Affecting Enzyme's Activity

• Various factors affect enzyme activity, such as substrate concentration, pH, temperature, and inhibitors

Rate-determining Step

• The slowest step in a metabolic pathway is the rate-limiting step

Product Inhibition

• The product of an enzymatic reaction can inhibit the overall reaction rate

Zymogen

• Zymogen is an inactive precursor of an enzyme; also called a pro-enzyme
• Conversion of active enzyme from its zymogen involves specific proteolytic cleavage
• Examples include pepsinogen and trypsinogen

Premature Activation of Zymogen

• Premature activation can occur in acute pancreatitis, leading to tissue damage