**3. The family of glucose transport facilitators**

## **3.1. Synonyms**

GLUT1-14, Gene Symbols: *SLC2A1-14*, solute carrier family 2A1-14

#### **3.2. Summary**

Glucose transporters are uniporters that facilitate the diffusion of their respective substrates (e.g. glucose) across cellular membranes along a concentration gradient [2, 4]. The protein family comprises 14 isoforms that share common structural features such as 12 transmembrane domains, N-and C-termini facing the cytoplasm of the cell and an N-glycosylation site within either the first or fifth extracellular loop. Based on their sequence homology (14-63% identity) three classes can be distinguished: Class I includes the 'classic' glucose transporters GLUT1-4 and GLUT14, the Class II members are GLUT5, 7, 9, 11, and the Class III transporters comprise GLUT6, 8, 10, 12 and the proton driven myo-inositol transporter HMIT (or GLUT13). Despite their structural similarities, the different isoforms are characterized by tissue specific expres‐ sion and distinct characteristics such as alternative splicing and (sub) cellular localization. With respect to their substrate specificities, the protein family includes transporters of glucose (GLUT1-4, 8, 14), fructose (GLUT5, 7, 11), polyol (GLUT12), myo-inositol (GLUT13), and urea (GLUT9) transporters.

More in-depth phylogenetic analysis of the GLUT sequences implied a different clustering of GLUT6 and 8, GLUT10 and GLUT12, and HMIT as more distant family members implying the existence of 5 subclasses based on sequence identities but in addition functional properties. Based on these analysis, the evolutionary more ancient forms HMIT, GLUT10, 12, 6, and 8 are postulated to mediate substrate transport which is linked to electrochemical gradients as it has been described for HMIT and GLUT12 [25].

For GLUT1, the topology and relative orientation of the 12 transmembrane helices with the outward-facing substrate-binding sites have been been proposed by two models. More than 50% of the complete polypeptide sequence has been analyzed by cysteine scanning mutagen‐ esis using the substituted cysteine accessibility method (SCAM) allowing a detailed prediction of the exofacial substrate-binding site and the folding of the human GLUT1. A three-dimen‐ sional model for GLUT1 has been developed based on structural information from crystallized members of the major facilitator superfamily, glycerol-3-phosphate transporter and lactose permease. Binding of glucose, forskolin, and phloretin was predicted in close proximity to the exofacial vestibule in this model. While a second binding site for forskolin and phloretin was predicted at the intracellular portion of GLUT1, cytochalasin B has been docked only at one

Mammalian Sugar Transporters http://dx.doi.org/10.5772/58325 19

Isoforms of Class I GLUTs are well characterized transporters with GLUT1 being the first

GLUT1, also known as the HepG2 or erythrocyte sugar transporter, is highly abundant in erythrocyte membranes making up 3-5% of the total erythrocyte proteins. The high amount of GLUT1 in red blood cells allowed the generation and characterization of an antibody that was used for the molecular cloning of GLUT1 from a hepatoma cDNA expression library in 1985 [26]. Although not present in hepatocytes, however, GLUT1 represents the most ubiquitously expressed isoform. The transporter is already found throughout early mammalian embryo development from the oocyte to the blastocyst and is present at high levels in endothelial and epithelial-like barriers of the brain, the eye, peripheral nerves, the placenta and especially in certain tumor cell lines and tissues, although not in hepatocytes. Also, GLUT1 is highly

When assessed in *Xenopus laevis* oocytes, GLUT1 transports glucose with a Km of ~3 mM. Under equilibrium exchange conditions GLUT1 has a Km of 20-21 mM for 3-O-methylglucose and 5 mM for 2-DG. Other hexoses transported by GLUT1 are galactose, mannose and glucosamine. When expressed in *S. cerevisae* rat GLUT1 showed a Km for D-glucose of 3.4 mM and transport was inhibited by cytochalasin B (IC50=0.44µM), HgCl2 (IC50=3.5µM), phloretin (IC50=49 µM)

Mutations in the GLUT1 gene are causative for an autosomal-dominant disorder that is characterized by infantile seizures, developmental delay, acquired microcephaly, and ataxia and infantile seizures which is assumed to be caused by the decreased rate of glucose transport

particular endofacial position of the protein.

**4. Class I family members**

**4.1. GLUT1 (***Slc2a1***)**

and phlorizin (IC50=355 µM).

*4.1.1. GLUT1 physiology*

isoform cloned and described in 1985.

expressed in most routinely used laboratory cell lines.

*Human Physiology-the GLUT1 deficiency syndrome (OMIM #606777)*
