Flux control and excess capacity in the enzymes of glycolysis and their relationship to flight metabolism inDrosophila melanogaster

Abstract

An important question in evolutionary and physiological genetics is how the control of flux-base phenotypes is distributed across the enzymes in a pathway. This control is often related to enzymespecific levels of activity that are reported to be in excess of that required for demand. In glycolysis, metabolic control is frequently considered vested in classical regulatory enzymes, each strongly displaced from equilibrium. Yet the contribution of individual steps to control is unclear. To assess enzyme-specific control in the glycolytic pathway, we used P-element excision-derived mutagenesis in Drosophila melanogaster to generate full and partial knockouts of seven metabolic genes and to measure tethered flight performance. For most enzymes, we find that reduction to half of the normal activity has no measurable impact on wing beat frequency. The enzymes catalyzing near-equilibrium reactions, phosphoglucose isomerase, phosphoglucomutase, and triosephosphate isomerase fail to show any decline in flight performance even when activity levels are reduced to 17% or less. At reduced activities, the classic regulatory enzymes, hexokinase and glycogen phosphorylase, show significant drops in flight performance and are nearer to saturation. Our results show that flight performance is canalized or robust to the activity variation found in natural populations. Furthermore, enzymes catalyzing near-equilibrium reactions show strong genetic dominance down to low levels of activity. This implies considerable excess enzyme capacity for these enzymes.I dentifying the causes of the differing patterns of molecular evolution among genes is a compelling problem in biology. In Drosophila and yeast, genome-wide studies have emphasized gene-specific variables, such as codon bias, expression level, dispensability, and connectedness, to establish correlations with the levels of purifying selection (1-7). One complex but unexplored cause of variation is the relative difference among enzymes that functional variation imparts to phenotype. When the phenotypic variation reflects the flux rate through a pathway, this relationship between enzyme activity and phenotype is defined by the level of flux control. For enzyme steps with little flux control, activity differences will have no impact on phenotypic variation and will have consequently no effect on phenotype-associated fitness. For this reason, the relative level of control at an enzyme step is expected to be a predictor of molecular evolution for a gene. The goal of this study is to evaluate the control of flight performance associated with activity variation for a number of enzymes of the glycolytic pathway.Flux control is expected to differ among enzymes as a result of pathway context, equilibrium status, and the presence or absence of steady-state conditions (8). The classical view of metabolic flux control proposes that unique steps, such as those under allosteric control and strongly displaced from equilibrium, control the pathway flux. These steps represent the textbook regulatory e...

Genetic perturbation of key central metabolic genes extends lifespan in Drosophila and affects response to dietary restriction

“…The P-element insertions used to generate the excision allele pairs were: Idh, Idh EP3729 ; Mdh2, Mdh2 EY01940 ; Gdh, Gdh KG00965 ; Men, Men BG02790 ; Hex-A (20817) [EP(X)352]; Hex-C, CG8079 KG09154 . Hex-A, Idh and Gpdh alleles have been described previously [44,46,47]. In each gene, we have an excision allele with impaired activity and an excision allele with normal activity.…”

“…In each gene, we have an excision allele with impaired activity and an excision allele with normal activity. Enzyme activities were assayed as in earlier work [44,[46][47][48][49] and are shown in units of change in optical density per minute (DOD). The Men 9D allele is a perfect excision.…”

Molecular population genetics and selection in the glycolytic pathway

“…There is a growing appreciation that this has resulted in geographical selection for many traits, especially those associated with the ability to withstand the stresses associated with overwintering in temperate populations (Schmidt et al, 2005;Schmidt and Paaby, 2008;Reaume and Sokolowski, 2006;Mitrovski and Hoffmann, 2001;Hoffmann et al, 2001;Umina et al, 2005). From an experimental standpoint, the genetic tools of D. melanogaster are unparalleled and methods are available to permanently knockout or modify activity in many genes (Bellen et al, 2004;Venken and Bellen, 2005) and to control metabolic function (Merritt et al, 2005;Merritt et al, 2006;Eanes et al, 2006;Eanes et al, 2008;Merritt et al, 2009). In addition, D. melanogaster has been a focal model in the study of energy partitioning and aging, where differential expression of the energyproducing and energy-consuming pathways has come under intense scrutiny as a response to dietary restriction (Gershman et al, 2007;Baker and Thummel, 2007).…”

Coincidence of P-Insertion Sites and Breakpoints of Deletions Induced by Activating P Elements in Drosophila

“…The imprecise excision is frequently accompanied by loss of genomic sequences immediately flanking the starting P element, which became a tool to produce null mutations for genes whose 59 regulatory sequences were tagged with P insertions. In genetic screens to recover deficiencies from P imprecise excision, phenotypes associated with deletions extending into the flanking genes (Tsubota and Schedl 1986;Salz et al 1987), molecular detections of flanking genomic rearrangements (Zhang and Spradling 1993), or a combination of phenotypic selection and molecular detection (Mellerick et al 1992;Eanes et al 2006;Xu et al 2006;Hao et al 2007), were applied. Here, we characterized seven deletions generated by activating a nearby P element.…”