ACTIVE TRANSPORT

I. ATP-DRIVEN

1. (Na⁺-K⁺)-ATPase of Plasma Membrane [= (Na⁺-K⁺) Pump] (Fig. 10-21)

· Pumps Na⁺ out of and K⁺ into the cell.

· Concomitant hydrolysis of intracellular ATP.

· 3 Na⁺(in) + 2 K⁺(out) + ATP + H₂O ® 3 Na⁺(out) + 2 K⁺(in) + ADP + Pi

· Electrogenic antiport. Generates an electrochemical potential gradient. Responsible for electrical excitability of nerve cells.

· Maintain a low internal [Na⁺] to prevent too much H₂O rushing in osmotically.

Mechanism of Transport

· The enzyme has two major conformational states: E₁ and E₂.

· E₁ has an inward-facing, high affinity Na⁺-binding site that is weak in K⁺ binding.

· E₁ can be activated by ATP phosphorylation (ONLY when Na⁺ is bound).

· Activated E₁-P undergoes conformational change to E₂-P.

· E₂-P has an outward-facing, high affinity K⁺ binding site that is weak in Na⁺ binding.

· E₂-P undergoes hydrolysis of the bound Pi ONLY when K⁺ is bound.

· Dephosphorylated E₂:K⁺ undergoes conformational change to E₁:K⁺.

· K⁺ is release inside of cell to complete the cycle.

· The location and orientation of the metal binding site in E₁ are different from that in E₂. But neither E₁ nor E₂ undergoes transverse movement across the membrane.

2. Ca²⁺-ATPase (Ca²⁺ Pump) (Fig. 10-22)

· Pumps 2 Ca²⁺ out of cytosol to extracellular space at the expense of the hydrolysis of one ATP.

· Electrogenic (uniport considering that it is transporting only one type of molecules; or symport if emphasizing that "two" molecules are being transported in the same direction at the same time).

· Enzyme has two conformational states: E and E-P.

· E has inward-facing tight sites for 2 Ca²⁺.

· E:2Ca²⁺ can be phosphorylated by ATP.

· The phosphorylation results in a conformational change to form E-P:2Ca²⁺.

· The E-P has outward-facing sites that are weak in Ca²⁺ binding.

· After release of 2 Ca²⁺ to the extracellular space, the phosphate is hydrolyzed off from E-P to complete the cycle.

II. Ion Gradient-Driven Active Transport

Utilizes the energy stored in the form of an electrochemical potential gradient.

1. Na⁺-Glucose Symport (Fig. 10-24)

· Glucose is actively concentrated in brush border cells of the intestinal epithelium.

· Na⁺-Glucose symport driven by Na⁺ gradient.

· Glucose is then transported to the circulatory system by a passive-mediated glucose uniport.

· The intracellular Na⁺ concentration is kept low by the (Na⁺-K⁺)-ATPase.

· The ultimate energy source is ATP hydrolysis.

· Glucose enhances Na⁺ resorptoin ==> enhances water resorption. Therefore, glucose is fed to individuals suffering from salt and water losses due to diarrhea.

2. Lactose Permease (= Galactoside Permease) (Fig. 10-25)

· Utilizes the H⁺ gradient across the bacterial cell membrane to cotransport H⁺ and lactose.

· Two conformational states: E-1 and E-2.

· E-1 has a low-affinity lactose binding site, facing the interior of the cell.

· E-2 has a high-affinity lactose binding site facing the exterior of the cell.

· E-2 must bind both lactose and H⁺ in order to change to E-1.

· E-1 must be freed from lactose and H⁺ in order to change to E-2.