Function
is one of fourteen facilitative sugar transporters which use the glucose diffusion gradient to move across various plasma membranes to display various specificities, kinetics and tissue expression profiles[1]. Glucose transporters are about 500 amino acids in length and are a part of a growing superfamily of integral membrane glycoproteins that have 12 transmembrane (TM) helices. The transmembrane regions presumably create channels through which glucose can move (Kipmen-Korgun et al. 2009). GLUT3 is categorized as a Class I transporter due to its protein sequence and structural similarity to other glucose transporters grouped in Class I [1]. GLUT3 displays the highest affinity for glucose of all of the Class I glucose transporters and has a transport capacity five times greater than that of GLUT1 and GLUT4 (Simpson et al., 2008). In humans, GLUT3 is found predominantly in brain tissue, highly and specifically expressed by neurons, and has some expression in peripheral tissues (Maher et al., 1994; Simpson et al. 2008). It is commonly known as the “neuronal glucose transporter” because of it is most commonly found in the brain, specifically seen mostly in the axons and dendrites of neurons (Simpson et al. 2008). GLUT3 has a more restricted expression pathway, which represents specialized functions for the protein (Xu et al., 2015). GLUT3 has been found to play an important role in gestational development and maintaining the brain's structure. Defects in GLUT3 can cause fetal death as well as neurodegeneration, which can lead to diseases like Alzheimer’s (Liu et al. and Zu et al. 2008).
Structure
GLUT3() is a transport protein consisting of 481 amino acids and weighing 52,520 Daltons in its asymmetrical unit. This protein is an alpha-helical protein consisting of two chains, two different ligands and water. The structure was determined by X-Ray diffraction and was measured at a resolution of 2.65 Angstroms. GLUT3 consists of 12 transmembrane segments (TMs) folded “into the N-terminal and C-terminal domains, each comprising ‘3+3’ inverted repeats” These TMs consist of four 3 repeated sections (figure 1 part d). The protein consists of two different ligands, Y01(green) and 37X(green). Octyl Glucose Neopentyl Glycol () has a chemical formula of C27H52O12 and a molecular weight of 569 Da. There are six 37X (501-506a) bound to chain A of 5c65. These ligands are kept in place by hydrogen bonds to arginine, proline, and serine and by van der Waals forces. Chain B has three 37X ligands attached to it (501-503b). These are attached through hydrogen bonds by arginine, proline, and serine as well as by van der Waals forces. Cholesterol hemisuccinate () has a chemical formula of C31H50O4 and has a molecular weight of 487 Da. One Y01 is attached to chain a and another Y01 is attached to chain b. GLUT3 was also identified and analyzed in a complex with alpha & beta d-glucose. This model was reported with a resolution of 1.5 Å and was in an open-occluded state. The alpha and beta d glucose were coordinated by amino acids N315, E378, Q159, W368, Q280, Q281, N286. These are located on TM8 and TM10a and TM10b. GLUT3 structure was also determined when bound to maltose in an outward-open and an outward-occluded conformation. This was measure to a resolution of 2.6 Å and 2.4 Å respectively. (include Figure 3 part a). To get a better view of the structure of the protein use FirstGlance (link).
Disease in Humans
Alzheimer’s disease, shows levels of impaired glucose uptake and metabolism, which leads to the downgrade of many other factors in the brain. GLUT3 is responsible for transporting glucose from extracellular space to neuronal tissue (i.e. dendrites and axons). Decreased levels of GLUT3 in Alzheimer brain shows a positive correlation to decreased levels of N-acetylglucosamine. The impaired presence of GLUT3 leads to hyperphosphorylation of the Tau protein, which normally stabilizes neuronal microtubules. Lastly there is a reduction in the transcription for factor hypoxia-inducible factor 1, which plays a role in glucose metabolism in the brain. The comparison between a normal healthy brain and an Alzheimer brain relieved that there was a 25-30% decrease in GLUT3 levels in the Alzheimer brain.
Possible Mechanism
Multiple mechanistic theories have been proposed for facilitated glucose transporters. The simple carrier model was the earliest theory proposed by Widdas and contains four steps. First, the empty carrier opens to the cis side of the membrane for glucose to bind1. Then the substrate binding carrier translocates to the trans side of the membrane where it then releases glucose on that side. Last the empty carrier switches to the cis side. Multiple mechanistic theories, including the simple carrier model were proposed but all attempted to explain two key components of GLUT transporters, the asymmetry of the transport affinities and the trans-acceleration that occurs in the presence of hexose on the trans side2. After considerable research, two popular models remain for class 1 glut transporters.
The two-site/fixed site transporter theory explains the asymmetry by having both substrate binding sites simultaneously available3. After glucose is bound, hexoses exchange between sites and speed the binding process. Although this method explains the asymmetry and the kinetics of class 1 glut transporters, not all class 1 glut transporters contain asymmetric glucose transporters3.
The alternating access model follows three steps4. The transporter has a cavity for small substrates, and contains a substrate binding site. The transporter also has two different configurational openings to one cell membrane or the other. This mechanisms differs from the two-site/fixed site transporter theory by assuming there is only one binding site available at a time, leading to four different conformation states. An empty outward open state, an occluded transporter state, a inward open state and finally another occluded state5.
Trans-acceleration is only observed in a minority of class 1 glut transporters6. GLUT3 has been proven to be dependent on trans-acceleration. This method was discovered when hexose was found to be moving against its concentration gradient1. This movement is argued to support both the two-site transporter theory and the alternating access model. Geminate exchange, named by Naftalin et al, explains this movement with the idea that hexose could exchange freely between two binding sites within the carrier. While other scientist argue that hexose could move from outward to inward without glucose binding7.
Relevance
Structural highlights
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