Loading....
Coupon Accepted Successfully!

Biochemistry

Open Flashcards

Carbohydrates

Question
21 out of 40
 

After overnight fasting, level of glucose transporters are decrease in. (AIIMS May 2009)



A Brain cell

B Adipocytes

C Hepatocytes

D RBC

Ans. C Hepatocytes Ref: Harper 28th edition, Page 171

Facilitated glucose transport in humans

a. Glucose transporters in humans come in five subtypes called GLUT1(gene name SLC2A1; solute carrier family 2), through GLUT5.

b. They are located in the plasma membrane of cells and have glucose binding sites outside and inside the membrane.

c. The relative affinities of these two binding sites may differ. Different cells express different glucose transport genes.

Types of GLUT

A. GLUT 1 and GLUT 3

a. These two types are found in all cells (except liver, intestinal epithelia and pancreatic beta cells) and are responsible for a basic glucose uptake.

b. The Michaelis-Menten constant Km = 1mM and is slightly lower than the lower range of healthy blood glucose concentration of 4 - 8mM.

c. Thus, glucose is efficiently absorbed by cells even during fasting and starving conditions.

d. Liver, kidney and intestinal cells can synthesize glucose and release it into the blood circulation, a process known as gluconeogenesis (see GLUT2).

B. GLUT 2:

a. A low affinity glucose transporter that is typically expressed in liver, intestinal epithelia and beta cells (insulin secretion) of the pancreas.

b. Its Km = 15-20mM is several fold higher than average blood glucose levels of 4 - 8mM glucose.

c. As result glucose entry into liver cells (and beta cells) is normally slow, but proportional to the glucose level in blood, while GLUT1/3 systems are saturated and promote a steady glucose influx into cells (e.g. in neurons).

d. In the small intestine, glucose absorption into mucosal epithelial cells occurs via a Na/glucose symporter.

e. Glucose is pumped into the cells using the energy of the Na gradient (flux coupling) where both substrates accumulate.

f. Glucose subsequently diffuses via GLUT2 into the extracellular fluid reaching the portal vein blood circulation.

g. Intracellular Na+ is kept low by the Na-K-ATPase on the same basal membrane as GLUT2.

h. The Na/glucose transporter is the primary uptake system of intestinal glucose and galactose during fasting conditions, when sugar levels are low.

i. During and after a meal, GLUT2 is also found in the apical membrane supporting the efficient glucose uptake from the intestinal lumen.

C. GLUT 4

a. Is found in adipocytes and skeletal muscle cells and has an affinity of Km = 5mM, right at blood serum levels of glucose (4-8mM).

b. This receptor is upregulated by insulin. High glucose levels, which will saturate the muscle transporter (but not the liver/pancreas type), cause the secretion of insulin.

c. Insulin activates the glut4 gene and more transporter proteins are synthesized and incorporated into the muscle cell membrane, increasing the capacity for glucose transport in this system (increase of Jmax; see below).

D. GLUT 5:

a. Has a preference for fructose, the monosaccharide found in fruit sugar together with glucose.

b. GLUT 5 is found in the apical membrane of intestinal enterocytes allowing the diffusion driven absorption of fructose into the enterocytes.

c. Fructose is thought to leave the enterocytes on the basolateral side via the GLUT2 transporter or may be metabolically converted to glucose before leaving the enterocytes.

Carbohydrates Flashcard List

40 flashcards
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
12)
13)
14)
15)
16)
17)
18)
19)
20)
21)
22)
23)
24)
25)
26)
27)
28)
29)
30)
31)
32)
33)
34)
35)
36)
37)
38)
39)
40)