Gene name  and REFSEQ mRNAs Protein localization Biological function Relevance in the pathway of insulin resistance (IR) Reference Hexokinase 2 (HK2 or HKII) NM_000189.4 Enzyme localized in various compartments (cytoplasm, membrane, mitochondrial outer membrane) Hexokinases phosphorylate glucose to produce glucose 6-phosphate, thus committing glucose to the glycolytic pathway. The expression of this gene is insulin responsive. Non-insulin dependent diabetes mellitus (NIDDM) patients are characterized by a reduced activity and a reduced gene expression of HK2 in muscle, which may be secondary to the metabolic perturbations. [21,22] Glucokinase (hexokinase 4).” (GCK) 3 alternative transcripts: NM_000162.3 NM_033507.1 NM_033508.1 Enzyme localized in various compartments (cytosol, nucleoplasm, cytoplasm, nucleus). Alternative splicing of this gene results in three tissue-specific forms of glucokinase, one found in pancreatic islet beta cells and two found in liver. Hexokinases phosphorylate glucose to produce glucose-6-phosphate, the first step in most glucose metabolism pathways. This enzyme regulates carbohydrate metabolism by acting as a glucose sensor. It is not inhibited by its product glucose-6-phosphate but remains active while glucose is abundant. Mutations in this gene have been associated with: NIDDM, Maturity onset diabetes of the young, type 2 (MODY2), persistent hyperinsulinemic hypoglycemia of infancy (PHHI). [23,24] Pyruvate dehydrogenase lipoamide kinase isozyme 4 (PDK4) NM_002612.3 Enzyme localized in various compartments (cytoplasm, mitochondrial inner membrane, mitochondrial matrix). It plays an important role in maintaining normal blood glucose levels under starvation, and is involved in the insulin signaling cascade. PDK4 plays a crucial role in glucose utilization and lipid metabolism by regulating the pyruvate dehydrogenase complex (PDC) and is an emerging therapeutic target for type 2 diabetes. [25,26] Pyruvate dehydrogenase kinase, isozyme 2 (PDK2) 4 alternative transcripts: NM_001199898.1 NM_001199899.1 NM_001199900.1 NM_002611.4 Enzyme localized in various compartments (cytoplasm, mitochondrial matrix). This gene encodes a member of the pyruvate dehydrogenase kinase family. The encoded protein phosphorylates pyruvate dehydrogenase, down-regulating the activity of the mitochondrial pyruvate dehydrogenase complex. Plays an important role in maintaining normal blood glucose levels under starvation, and is involved in the insulin signaling cascade. Overexpression of this gene may play a role in both cancer and diabetes. Plays a role in cell proliferation by regulating carbohydrate and fatty acid metabolism. [27] Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) Two transcript variants: NM_001256799.1 NM_002046.4 Localized in various compartments (condensed chromosome, condensin complex, condensin core heterodimer, cytoplasm, etc). GAPDH is a key enzyme in glycolysis that catalyzes the first step of the pathway by converting D-glyceraldehyde 3-phosphate (G3P) into 3-phospho-D-glyceroyl phosphate. GAPDH activates the major pathways implicated in diabetic complications, including advanced glycation end products (AGEs), protein kinase C, and hexosamine pathway. [28] Protein kinase C (PKC) isoforms NM_002737.2 Localized in various compartments (cytoplasm, membrane raft, mitochondrion, nucleus, protein complex, synaptosome). PKC is a family of serine/threonine kinases that consist of 12 isoforms. Theses enzymes are involved in the control of the function of other proteins. Activation of diacylglycerol (DAG)-protein kinase C (PKC) pathway enhances polyol pathway, increases oxidative stress, and the accumulation of advanced glycation end products; all proposed as potential cellular mechanisms by which hyperglycemia induces many vascular abnormalities in renal, retinal, and cardiovascular tissues in diabetic animals and patients. [29,30] Glycogen synthase (UDP-glucose-glycogen  glucosyltransferase) (GYS1) Two transcript variants: NM_001161587.1 NM_002103.4 Localized in key tissues as skeletal muscle and adipose tissue; as in various compartments as mitochondrion and cytoplasm. This enzyme converts excess glucose residues one by one into a polymeric chain for storage as glycogen. It is a key enzyme in glycogenesis. Glycogen synthase plays an important biological role in regulating glycogen/glucose levels. Patients with type 2 diabetes normally exhibit low glycogen storage levels because of impairments in insulin-stimulated glycogen synthesis and suppression of glycogenolysis. Insulin stimulates glycogen synthase by inhibiting glycogen synthase kinases or/and activating protein phosphatase 1 (PP1) among other mechanisms. [31] Glycogen synthase kinase (GSK-3) Two  isoforms in mammals, GSK-3α and GSK-3β NM_001146156.1 NM_002093.3 Protein is localized predominantly in the cytoplasm but is also found in the nucleus. Its subcellular localization is changed in response to stimuli. Serine/threonine kinase with important roles in the regulation of glycogen synthesis, protein synthesis, gene transcription, and cell differentiation in various cell types. Constitutively active protein kinase that acts as a negative regulator in the hormonal control of glucose homeostasis. Deregulation of the GSK-3 has been implicated in the development of type 2 diabetes mellitus. GSK-3 protein expression and kinase activity are elevated in diabetes, while selective GSK-3 inhibitors have shown promise as modulators of glucose metabolism and insulin sensitivity. [32-34] Glycogen phosphorylase. (PYGL) Two alternative transcripts: NM_001163940.1 NM_002863.4 Protein is localizing in various compartments (cytoplasm, soluble fraction). Phosphorylase is an important allosteric enzyme in carbohydrate metabolism. PYGL breaks up glycogen into glucose subunits. This enzyme participates in the glycolysis pathways, Insulin signaling pathway, and sucrose metabolism. Glycogen phosphorylase activity is critical for normal skeletal muscle function. Mutations in liver PYGL inhibit the conversion of glycogen to glucose and results in moderate hypoglycemia. The inhibition of PYGL has been proposed as one method for treating type 2-diabetes. Inhibiting the release of glucose from the liver glycogen supply appears to be a valid approach. [35-37] Protein phosphatase 1, regulatory (inhibitor) subunit 3A. (PPP1R3A) NM_002711.3 Proteins are localized in various compartments (cytoplasm, membrane, integral to membrane). Participate in insulin signaling pathway, carbohydrate metabolic process and glycogen metabolic process. This gene has been associated to insulin resistance and type 2-diabetes. [38] Glucagon-like peptide 1 receptor. (GLP1R) NM_002062.3 Protein is localized in various compartments (integral to membrane, plasma membrane). The protein encoded by this gene is a member of the glucagon receptor family of G protein-coupled receptors GLP1R is known to be expressed in pancreatic beta cells. Activated GLP1R stimulates the adenylyl cyclase pathway which results in increased insulin synthesis and release of insulin. Consequently GLP1R has been suggested as a potential target for the treatment of diabetes [39,40] Aldolase B, fructose-bisphosphate (ALDOB) NM_000035.3 Protein is localized in various compartments (cytosol, lysosome, microsome and others). Fructose-1,6-bisphosphate aldolase is a tetrameric glycolytic enzyme that catalyzes the reversible conversion of fructose-1,6-biphosphate to glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. ALDOB is involved in cellular response to extracellular stimulus; cellular responses to insulin, glycolysis. It has been associated with type2-diabetes. [41,42] Phosphofructoki-nase (PFK). 4 alternative transcripts in the liver: NM_000289.5 NM_001166686.1 NM_001166687.1 NM_001166688.1 Protein is localized in various compartments (cytoplasm, membrane, cytosol, soluble fraction). PFK is the most important enzyme in glycolysis. Three phosphofructokinase isozymes exist in humans: muscle, liver and platelet. These isozymes function as subunits of the mammalian tetramer phosphofructokinase, which catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate. A deficiency of phosphofructokinase can be inherited due to the genetic disorder glycogenosis type VII Tarui's disease. Research has shown that this disease can lead to insulin resistance and reduced insulin secretion by beta cells. [43,44]

Table 1: List of genes involved in glycolysis, oxidative metabolism, and mitochondrial function associated with progressive insulin resistance and diabetes.