1
|
Rodrigues JR, Couto A, Cabezas A, Pinto RM, Ribeiro JM, Canales J, Costas MJ, Cameselle JC. Bifunctional homodimeric triokinase/FMN cyclase: contribution of protein domains to the activities of the human enzyme and molecular dynamics simulation of domain movements. J Biol Chem 2014; 289:10620-10636. [PMID: 24569995 DOI: 10.1074/jbc.m113.525626] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
Mammalian triokinase, which phosphorylates exogenous dihydroxyacetone and fructose-derived glyceraldehyde, is neither molecularly identified nor firmly associated to an encoding gene. Human FMN cyclase, which splits FAD and other ribonucleoside diphosphate-X compounds to ribonucleoside monophosphate and cyclic X-phosphodiester, is identical to a DAK-encoded dihydroxyacetone kinase. This bifunctional protein was identified as triokinase. It was modeled as a homodimer of two-domain (K and L) subunits. Active centers lie between K1 and L2 or K2 and L1: dihydroxyacetone binds K and ATP binds L in different subunits too distant (≈ 14 Å) for phosphoryl transfer. FAD docked to the ATP site with ribityl 4'-OH in a possible near-attack conformation for cyclase activity. Reciprocal inhibition between kinase and cyclase reactants confirmed substrate site locations. The differential roles of protein domains were supported by their individual expression: K was inactive, and L displayed cyclase but not kinase activity. The importance of domain mobility for the kinase activity of dimeric triokinase was highlighted by molecular dynamics simulations: ATP approached dihydroxyacetone at distances below 5 Å in near-attack conformation. Based upon structure, docking, and molecular dynamics simulations, relevant residues were mutated to alanine, and kcat and Km were assayed whenever kinase and/or cyclase activity was conserved. The results supported the roles of Thr(112) (hydrogen bonding of ATP adenine to K in the closed active center), His(221) (covalent anchoring of dihydroxyacetone to K), Asp(401) and Asp(403) (metal coordination to L), and Asp(556) (hydrogen bonding of ATP or FAD ribose to L domain). Interestingly, the His(221) point mutant acted specifically as a cyclase without kinase activity.
Collapse
Affiliation(s)
- Joaquim Rui Rodrigues
- Grupo de Enzimología, Departamento de Bioquímica y Biología Molecular y Genética, Facultad de Medicina, Universidad de Extremadura, E-06006 Badajoz, Spain; Escola Superior de Tecnologia e Gestão, Instituto Politécnico de Leiria, P-2411-901 Leiria, Portugal
| | - Ana Couto
- Grupo de Enzimología, Departamento de Bioquímica y Biología Molecular y Genética, Facultad de Medicina, Universidad de Extremadura, E-06006 Badajoz, Spain
| | - Alicia Cabezas
- Grupo de Enzimología, Departamento de Bioquímica y Biología Molecular y Genética, Facultad de Medicina, Universidad de Extremadura, E-06006 Badajoz, Spain
| | - Rosa María Pinto
- Grupo de Enzimología, Departamento de Bioquímica y Biología Molecular y Genética, Facultad de Medicina, Universidad de Extremadura, E-06006 Badajoz, Spain
| | - João Meireles Ribeiro
- Grupo de Enzimología, Departamento de Bioquímica y Biología Molecular y Genética, Facultad de Medicina, Universidad de Extremadura, E-06006 Badajoz, Spain
| | - José Canales
- Grupo de Enzimología, Departamento de Bioquímica y Biología Molecular y Genética, Facultad de Medicina, Universidad de Extremadura, E-06006 Badajoz, Spain
| | - María Jesús Costas
- Grupo de Enzimología, Departamento de Bioquímica y Biología Molecular y Genética, Facultad de Medicina, Universidad de Extremadura, E-06006 Badajoz, Spain
| | - José Carlos Cameselle
- Grupo de Enzimología, Departamento de Bioquímica y Biología Molecular y Genética, Facultad de Medicina, Universidad de Extremadura, E-06006 Badajoz, Spain.
| |
Collapse
|
2
|
Hijmans BS, Grefhorst A, Oosterveer MH, Groen AK. Zonation of glucose and fatty acid metabolism in the liver: mechanism and metabolic consequences. Biochimie 2013; 96:121-9. [PMID: 23792151 DOI: 10.1016/j.biochi.2013.06.007] [Citation(s) in RCA: 114] [Impact Index Per Article: 10.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2013] [Accepted: 06/12/2013] [Indexed: 12/13/2022]
Abstract
The liver is generally considered as a relatively homogeneous organ containing four different cell types. It is however well-known that the liver is not homogeneous and consists of clearly demarcated metabolic zones. Hepatocytes from different zones show phenotypical heterogeneity in metabolic features, leading to zonation of metabolic processes across the liver acinus. Zonation of processes involved in glucose and fatty acid metabolism is rather flexible and therefore prone to change under (patho)physiological conditions. Hepatic zonation appears to play an important role in the segregation of the different metabolic pathways in the liver. As a consequence, perturbations in metabolic zonation may be a part of metabolic liver diseases. The metabolic syndrome is characterized by the inability of insulin to adequately suppress hepatic gluconeogenesis, leading to hyperglycemia, hyperinsulinemia and eventually to type II diabetes. As insulin promotes lipogenesis through the transcription factor sterol regulatory element binding protein (SREBP)-1c, one would expect that lipogenesis should also be impaired in insulin-resistant states. However, in the metabolic syndrome hepatic de novo lipogenesis is increased, leading to hyperlipidemia and hepatosteatosis, primarily in the pericentral zone. These observations suggest the co-existence of insulin resistant glucose metabolism and insulin sensitive lipid metabolism in the metabolic syndrome. Here we provide a theoretical framework to explain this so-called 'insulin signaling paradox' in the context of metabolic zonation of the liver.
Collapse
Affiliation(s)
- Brenda S Hijmans
- Departments of Pediatrics and Laboratory Medicine, University of Groningen, University Medical Center Groningen, The Netherlands.
| | | | | | | |
Collapse
|
3
|
Rippe JM, Angelopoulos TJ. Sucrose, high-fructose corn syrup, and fructose, their metabolism and potential health effects: what do we really know? Adv Nutr 2013; 4:236-45. [PMID: 23493540 PMCID: PMC3649104 DOI: 10.3945/an.112.002824] [Citation(s) in RCA: 119] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
Both controversy and confusion exist concerning fructose, sucrose, and high-fructose corn syrup (HFCS) with respect to their metabolism and health effects. These concerns have often been fueled by speculation based on limited data or animal studies. In retrospect, recent controversies arose when a scientific commentary was published suggesting a possible unique link between HFCS consumption and obesity. Since then, a broad scientific consensus has emerged that there are no metabolic or endocrine response differences between HFCS and sucrose related to obesity or any other adverse health outcome. This equivalence is not surprising given that both of these sugars contain approximately equal amounts of fructose and glucose, contain the same number of calories, possess the same level of sweetness, and are absorbed identically through the gastrointestinal tract. Research comparing pure fructose with pure glucose, although interesting from a scientific point of view, has limited application to human nutrition given that neither is consumed to an appreciable degree in isolation in the human diet. Whether there is a link between fructose, HFCS, or sucrose and increased risk of heart disease, metabolic syndrome, or fatty infiltration of the liver or muscle remains in dispute with different studies using different methodologies arriving at different conclusions. Further randomized clinical trials are needed to resolve many of these issues. The purpose of this review is to summarize current knowledge about the metabolism, endocrine responses, and potential health effects of sucrose, HFCS, and fructose.
Collapse
Affiliation(s)
- James M. Rippe
- University of Central Florida Medical School, Orlando,FL and Rippe Lifestyle Institute, Shrewsbury, MA; and,To whom correspondence should be addressed. E-mail:
| | - Theodore J. Angelopoulos
- Laboratory of Applied Physiology, Department of Health Professions, University of Central Florida, Orlando, FL
| |
Collapse
|
4
|
Burns SP, Murphy HC, Iles RA, Cohen RD. Lactate supply as a determinant of the distribution of intracellular pH within the hepatic lobule. Biochem J 2001; 358:569-71. [PMID: 11535120 PMCID: PMC1222093 DOI: 10.1042/0264-6021:3580569] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
When isolated livers from starved rats are perfused with lactate at constant perfusate pH and P(co(2)), there is a marked gradient of cell pH (pH(i)) along the length of the lobular radius, with periportal cells being substantially more alkaline than perivenous cells. In the present studies, the perivenous 21% of the lobular volume was destroyed by retrograde digitonin perfusion, and antegrade perfusion restored. pH(i) was determined by (31)P-NMR. The remaining periportal cells, the site of gluconeogenesis from lactate, had a substantially higher mean pH(i) (7.42) than did the intact liver (7.23). When lactate was removed from the perfusate, mean pH(i) decreased to 7.25. The corresponding concentration of cell bicarbonate fell with a half-time of approximately 5 min. When lactate was re-introduced mean pH(i) rose to 7.34. We conclude that a major contributor to periportal alkalinity under these conditions is proton consumption during gluconeogenesis from lactate ions.
Collapse
Affiliation(s)
- S P Burns
- Department of Diabetes and Metabolic Medicine, St Bartholomew's and The Royal London School of Medicine and Dentistry, Queen Mary and Westfield College, University of London, Whitechapel Road, London E1 1BB, UK.
| | | | | | | |
Collapse
|