His great name

Advanced Biochemistry Dr. Moradi

ACLY BY:MS Mahnaz Azadpoor

1. Introduction  ATP citrate lyase (ACLY) is a cytosolic enzyme responsible for the synthesis of acetyl-CoA.  It catalyzes the formation of acetyl-CoA and oxaloacetate from citrate and CoA with a simultaneous hydrolysis of ATP to ADP and phosphate

2. Tissue distribution and subcellular localization  ACLY is mainly a cytosolic enzyme.  ACLY is most abundantly expressed in the liver and white adipose tissue  it exhibits low expression levels in brain, heart, small intestine and muscles  ACLY is also expressed and active in pancreatic beta cells

3. Crystal structure  ACLY protein is a homotetramer of four identical subunits.  Each polypeptide chain contains 1101 amino-acid residues

4. Enzymatic properties 4.1. Reaction  Citrat+ATP Acetyl-coA+oxaloacetate+ADP+Pi

 4.2. Assays  Indirect  Direct  4.3. Inhibitors Citrate analogs are one of the most widely studied ACLY inhibitors. These included the (+) and (-)-2,2-difluorocitrate, both of which demonstrated activity against rat-liver ACLY. Benzenesulfonamides

 hydroxycitrate  a naturally citrate analog  Radicicol  a naturally occurring antifungal macrolide noncompetitively inhibits rat liver ACLY

5. Regulation of ACLY expression and activity  ACLY expression is mainly regulated by the transcription factor SREBP-1 (sterol regulatory element binding protein-1)  PI3K/Akt pathway stimulates ACLY activity predominantly through phosphorylation of ACLY rather than transcriptional up-regulation  ACLY activity is also regulated by some other pathways :  nucleoside diphosphate kinase  cyclic AMP dependent protein kinase

6. Pathways served by ACLY

7. Physiological roles associated with ACLY

7.1. Fetal growth and development  Beigneux et al : ACLY expression in mice was found to be much higher in developing brain in comparison to the adult brain 7.2. Tumor cell growth and survival  ACLY expression and activity has been reported to be up-regulated in lung, prostate, bladder, breast, liver, stomach, and colon tumors

7.3. Metabolic disorders  ACLY knockdown or inhibition leads to a decrease in glucose-induced insulin secretion. This indicated potential involvement of ACLY in diabetes pathophysiology.  over-expression of ACLY is observed in nonalcoholic fatty liver disease which leads to accumulation of triglycerides and hepatic steatosis  ACLY is also shown to be involved in citrate metabolism. Increased ACLY activity has been observed in rats with hypocitraturia

 The study compared the level of ATP citrate lyase mRNA and enzyme activity in pancreatic islets of humans and rats and the INS-1 832/13 cell line to levels in liver, a lipid synthesizing organ, and also kidney

 The mRNA level was much higher in human islets and rat islets than in liver and kidney of the same genus and the enzyme activity was 8-fold and 12-fold higher in islets of humans and rats, respectively, compared to liver of the same genus.  These data support other evidence that indicates ATP citrate lyase is important for the pyruvate citrate shuttle and lipid synthesis in insulin secretion.

 In 1975 Berne, reported that the enzyme activity of ATP citrate lyase is considerably higher in pancreatic islets of the obese New Zealand (NZO) mouse than in the liver and kidneys of this mouse.  The observation that the activity of ATP citrate lyase is very high in the islet is important because : 1) the enzyme is a component of the pyruvate citrate shuttle 2) Confi rming a high level of this enzyme could support other evidence that the pancreatic islet is a lipid synthesizing tissue and that lipid remodeling is necessary for normal insulin secretion

Results:  Relative mRNA levels:  Table 1 shows that the level of ATP citrate lyase (Acly) mRNA in human or rat pancreatic islets was much higher than in liver of the same genus.  Enzyme activity:  Table 2 shows that the enzyme activity of ATP citrate lyase in rat pancreatic islets was 12- fold higher than rat liver and 17-fold higher than in rat kidney.

Relative mRNA levels:

Enzyme activity:

Conclusion:  The high level of ATP citrate lyase mRNA and enzyme activity in pancreatic islets supports the idea that the enzyme is important for insulin secretion.  the enzyme being involved in the pyruvate citrate shuttle, which uses oxaloacetate formed from mitochondrially-derived citrate in the ATP citrate lyase reaction for transporting redox equivalents of NAD(P)(H) between the mitochondrial matrix and the cytosol….

References:  1) Horm Metab Res. Author manuscript; available in PMC 2014 February 13.  2) Cancer Res 2012;72: Published OnlineFirst July 10,  3) Melanie Chypre a, Nousheen Zaidi a,b, ⇑, Karine Smans a Department of Oncology, Janssen Research and Development, A Division of Janssen Pharmaceutica NV, Turnhoutseweg 30, 2340 Beerse, Belgium b Laboratory of Lipid Metabolism and Cancer, Department of Oncology, Faculty of Medicine, K.U.Leuven, Leuven, Belgium

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