Biochemistry: Enzymes and Glucose Transporters

Enzymes and Biochemical Reactions

• Enzymes are catalysts that accelerate the rate of biochemical reactions by lowering the activation energy.
• Enzymes achieve this by bringing reactants into close proximity, stabilizing the charged transition state, and using general acid-base catalysis, covalent catalysis, and metal ions.

Glucose Transporters

• Hexokinase is found in other tissues, while glucokinase is found in the liver.
• GLUT II is a high-capacity, low-affinity transporter found in the liver, β cells of the pancreas, and kidneys.
• GLUT II is a "two-way" transporter that allows for both uptake and release of glucose.
• GLUT I is a high-affinity transporter found in RBC and GLUT III is a major CNS glucose transporter.

Michaelis-Menten Kinetics

• The initial rate of an enzyme-catalyzed reaction is determined by the substrate concentration.
• The Michaelis-Menten equation is v = Vmax * [S] / (KM + [S]), where v is the initial rate, Vmax is the maximum rate, KM is the Michaelis constant, and [S] is the substrate concentration.
• The Lineweaver-Burk plot (1/v vs 1/[S]) is used to estimate Vmax and KM.
• In the given example, Vmax ≈ 140 μmol/L min-1 and KM ≈ 160 μmol/L.

Enzyme Kinetics Calculations

• The turnover number (kcat) is calculated as kcat = Vmax / [E]T, where [E]T is the total enzyme concentration.
• In the given example, kcat = 135 μmol/L min-1 / 1.0 nmol/L = 135,000 min-1.
• To calculate kcat in units of s-1, divide by 60 (60 s = 1 min).
• kcat/KM has units of L/(μmol min) and represents the enzyme's efficiency.

Liver Alcohol Dehydrogenase (ADH)

• Liver ADH catalyzes the oxidation of ethanol to acetaldehyde.
• The active site of liver ADH contains a bound zinc atom, a serine side chain –OH, and a histidine nitrogen that participate in the reaction.
• The histidine pulls an H+ off the active-site serine, which pulls the H+ off of the substrate –OH group, leaving the oxygen with a negative charge that is stabilized by zinc.