The International Journal of Biochemistry & Cell Biology
Molecules in focusTransketolase: Observations in alcohol-related brain damage research
Introduction
In mammals, transketolase connects the pentose phosphate pathway (PPP) to glycolysis, feeding excess sugar phosphates into the main carbohydrate metabolic pathways. The PPP is important for generating reducing equivalents, which is essential for energy transduction and for generating ribose for nucleic acid synthesis. Transketolase is a thiamin TPP-dependent enzyme. Thiamin, or vitamin B1, is an essential nutrient required by all tissues, including the brain. In its active form, thiamin pyrophosphate (TPP), it is an important co-enzyme for several enzymes including transketolase. TPP catalyzes the metabolism of carbohydrates and branched-chain amino acids. Chemically, thiamin consists of substituted pyrimidine and thiazole rings linked by a methylene bridge and the synthesis of TPP from free thiamin requires magnesium, ATP and the enzyme, thiamin pyrophosphokinase. Thiamin deficiency leads to reduced activities of transketolase and other TPP-dependent enzymes with consequent impairment of cellular energy metabolism that can affect the viability of brain cells (Martin et al., 2003, Sechi and Serra, 2007).
The major manifestations of thiamin deficiency in humans involve the cardiovascular (wet beriberi) and nervous systems. The most devastating neurological disorder caused by thiamin deficiency is the Wernicke-Korsakoff's Syndrome (WKS). The WKS is a potentially fatal neurological disorder that is seen most commonly in alcoholics (Martin et al., 2003). However, if it is diagnosed, treatment with thiamine generally results in a dramatic clinical improvement. Abnormal transketolase protein expression and/or activity have been implicated in a number of diseases including WKS (Butterworth et al., 1993), yet the precise mechanism by which this enzyme is involved in the pathophysiology of this disorder remains controversial.
Section snippets
Structure
In most sources where transketolase has been purified, it is a homodimer of approximately 70 kDa (see Fig. 1). Transketolase is comprised of two identical subunits containing 3 alpha/beta domains with two catalytic sites formed at the interface between these subunits (Schenk et al., 1998). The diphosphate moiety of TPP is bound to the enzyme at the carboxyl end of the parallel beta-sheet of the N-terminal domain and interacts with the protein through a Ca2+ ion (Lindqvist et al., 1992). The
Gene and protein expression
Transketolase sequences from a variety of eukaryotic and prokaryotic sources (Abedinia et al., 1992, Fletcher et al., 1992) show that the enzyme has been evolutionarily conserved. Using a 10-kb genomic clone in fluorescence in situ hybridization, the human transketolase enzyme was mapped to chromosome 3p14 at a site distal to FRA3B, thus localizing at 3p14.3 (Lapsys et al., 1992). Human transketolase cDNA was isolated and it was concluded that the gene is present in a single copy in the haploid
Biological function
Transketolase is a key enzyme in the non-oxidative branch of the PPP (also called the phosphogluconate pathway or hexose monophosphate shunt; see Fig. 2 for transketolase reaction). The functions of this pathway are to provide pentose phosphate for nucleotide synthesis and to supply reduced NADP for various synthetic pathways (Lonsdale, 2006). This pathway is also essential for the maintenance and synthesis of myelin. Transketolase catalyzes the reversible transfer of a two-carbon ketol unit
Diagnostic application
Like other TPP-dependent enzymes, the activity of transketolase is decreased in conditions of thiamin deficiency and the measurement of transketolase activity in erythrocytes is routinely used as a measure of thiamin nutrition in commercial laboratories. Apart from the baseline enzyme activity (which may be normal even in deficiency states), acceleration of enzyme activity after the addition of TPP may be diagnostic of thiamin deficiency (0–15% normal, 15–25% deficiency, >25% severe deficiency (
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