Function
GLUT3 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 approximately 500 amino acids in length and 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[2]. 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[3]. In humans, GLUT3 is found predominantly in brain tissue, highly and specifically expressed by neurons, and has some expression in peripheral tissues. For this reason GLUT3 is commonly known as the “neuronal glucose transporter”[3][4]. GLUT3 has a more restricted expression pathway, which represents specialized functions for the protein[5]. 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[6].
Structure
GLUT3() is a transport protein consisting of 481 amino acids and weighing 52,520 Daltons in its asymmetrical unit[7]. This protein is an alpha-helical protein consisting of two chains, two different ligands and water[7]. The structure was determined by X-Ray diffraction and was measured at a resolution of 2.65 Angstroms[8]. GLUT3 consists of 12 transmembrane segments (TMs) folded “into the N-terminal and C-terminal domains, each comprising ‘3+3’ inverted repeats”[9] These TMs consist of four 3 repeated sections. Here is a figure by Deng, D., et al. showing these repeated transmembrane segments[9]. The protein consists of two different ligands, Y01 and 37X[10]. 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[11]. To view 37X in 3D use JSmol. 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[12]. To view Y01 in 3D use JSmol. 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[9]. The alpha and beta d glucose were coordinated in a by amino acids N315, E378, Q159, W368, Q280, Q281, N286. These are located on TM8 and TM10a and TM10b[9]. A figure of this glucose coordination by Deng, D., et al. is available here. 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. A figure of this maltose coordination by Deng, D., et al. is available here. To get a better view of the structure of the protein use FirstGlance.
This is 5c65 shown with . This is 5c65 shown as a of the protein.
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 bind[1]. 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 side. Trans acceleration or accelerated transport occurs when unidirectional uptake of sugar is stimulated by the presence of intracellular sugar[13]. 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 available[14]. 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 it is not known if all class 1 glut transporters undergo a trans-acceleration model[14]. The alternating access model explains the mechanism for class 1 glut transporters that are symmetrical and follows three steps[15]. 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 mechanism 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 state[16]. Trans-acceleration is only observed in a minority of class 1 glut transporters[17]. GLUT3 has been proven to be dependent on trans-acceleration. This method was discovered when hexose was found to be moving against its concentration gradient[1]. 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[13]. While other scientists argue that hexose could move from outward to inward without glucose binding[18].
Disease in Humans
Type 2 Diabetes
Higher glucose concentration, as seen in diabetics, influences GLUT expression in leukocytes. Patients with type 2 diabetes have decreased GLUT3 in granulocytes, lymphocytes, and monocytes. In addition, the level of transcripts that encode GLUT3 are reduced in diabetic patients. Decreased expression of GLUT3 and other GLUT isoforms could possibly impair immune function and increase susceptibility to infection in type 2 diabetes[2].
Alzheimer's Disease
Alzheimer’s disease shows levels of impaired glucose uptake and metabolism, which leads to neurodegeneration via down-regulation of many other factors in the brain. GLUT3 is responsible for transporting glucose from extracellular space to neuronal tissue, specifically dendrites and axons. Decreased levels of GLUT3 in Alzheimer brain shows a positive correlation to decreased levels of N-acetylglucosamine, which is a derivative of glucose. 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 revealed that there was a 25-30% decrease in GLUT3 levels in the Alzheimer brain[6].
Huntington’s Disease
Huntington’s disease leads to decreased expression of GLUT3 in the plasma membrane. Increasing the expression of GLUT3 in a Huntington’s disease brain can delay the onset of the disease[19]. Rab11 is a protein that is involved with the regulation of transporter trafficking. It helps in the regulation of glucose transporters particularly the GLUT3 transporter. Its regulation is impaired by Huntington’s disease, which leads to the decreased cell surface expression of GLUT3 in the brain. The exact mechanism of Huntington’s disease is still unknown to this day[20].
3D structure of sugar transporters
See ABC transporter
