Lecture 11-Transport across biological membranes II (March 27) PDF

LECTURE 11 W24 Passive Transport (Facilitated Diffusion) 1. The glucose transporter of erythrocytes- GLUT 1 Blood [glucose] is maintained at ~4.55mM Glucose enter erythrocytes via passive transport by GLUT1 Facilitated Diffusion k1, k-1, k2, k-2, k3, k-3, k4, k-4 = Rate – Sout = “Substrate” constant...

LECTURE 11 W24 Passive Transport (Facilitated Diffusion) 1. The glucose transporter of erythrocytes- GLUT 1 Blood [glucose] is maintained at ~4.55mM Glucose enter erythrocytes via passive transport by GLUT1 Facilitated Diffusion k1, k-1, k2, k-2, k3, k-3, k4, k-4 = Rate – Sout = “Substrate” constants governing each step in – Sin = “Product” the forward and reverse directions – Transporter (T) = “Enzyme” T1 and T2 are outward and inward facing conformations of the transporter Kinetics of transport is described similarly to enzyme kinetics Similar to enzyme catalysis: measure initial velocity, v0, of substrate transport into the cell at various external substrate concentrations, [S]out Since v0 is measure at t=0, [S]in=0 and there is no reverse reaction plot v0 vs. [S]out → Michaelis-Menten-type hyperbolic curve At high [S] v0 approaches Vmax V max[ S ] out vo = K t + [ S ] out Vmax = maximum velocity Kt = substrate concentration at ½Vmax (like KM) Double Reciprocal plot of the Michaelis Menten Curve for transport 1/V0 on Y-axis 1/[S]out on X-axis X and Y intercept values used to determine Vmax and KM Kt ▪ Y int = 1/V0 ▪ X int = -1/Kt Kinetics of glucose transport into erythrocytes Kt for GLUT1 is measured to be about 3 mM At fasting blood [glucose] of 4.5-5 mM, GLUT1 is about half saturated and works at a rate close to ½ Vmax The transport process is readily reversible but under cellular conditions, GLUT1 works to transport glucose into the cell as cytoplasmic [glucose] is kept low by the immediate metabolism of the glucose in the cell. What is the immediate metabolic fate of glucose that enter the cell? GLUT1 shows the hallmarks of passive transport 1. High rate of diffusion down a concentration gradient Movement of glucose stops once Sin = Sout. Cannot accumulate glucose in the cell higher than [glucose]out 2. Saturable Rate of transport does not increase appreciably after a certain [Glucose]Out has been exceeded (reaches Vmax) 3. Stereospecific Specific for D glucose (Kt = 3 mM), Kt for D-mannose, DGalactose and L-Gluocose 20, 30 and 3000 mM respectively. (Similar to enzymes, susceptible to competitive & chemical inhibition)