Moonlighting Proteins: The Case of the Hexokinases

Abstract

Moonlighting proteins are defined as proteins with two or more functions that are unrelated and independent to each other, so that inactivation of one of them should not affect the second one and vice versa. Intriguingly, all the glycolytic enzymes are described as moonlighting proteins in some organisms. Hexokinase (HXK) is a critical enzyme in the glycolytic pathway and displays a wide range of functions in different organisms such as fungi, parasites, mammals, and plants. This review discusses HXKs moonlighting functions in depth since they have a profound impact on the responses to nutritional, environmental, and disease challenges. HXKs’ activities can be as diverse as performing metabolic activities, as a gene repressor complexing with other proteins, as protein kinase, as immune receptor and regulating processes like autophagy, programmed cell death or immune system responses. However, most of those functions are particular for some organisms while the most common moonlighting HXK function in several kingdoms is being a glucose sensor. In this review, we also analyze how different regulation mechanisms cause HXK to change its subcellular localization, oligomeric or conformational state, the response to substrate and product concentration, and its interactions with membrane, proteins, or RNA, all of which might impact the HXK moonlighting functions.

Loss of hexokinase 1 sensitizes ovarian cancer to high-dose metformin

“…Despite glycolysis being an essential and highly conserved metabolic pathway, all the glycolytic enzymes also have nonenzymatic moonlighting functions. These include the long-known structural functions of the lactate dehydrogenase B4 and α-enolase [ 61 , 62 ], the RNA-enzyme-metabolite functions of the HKs, and other glycolytic enzymes [ 63 , 64 ], yet unclear function of HKs in the cell nucleus [ 65 , 66 ], immune receptor function of HK2 [ 67 ], apoptosis inhibition by HK2 [ 68 , 69 ], and the glucose-sensing function of HKs [ 70 ]. The mechanism of the glucose-sensing function of HKs remains unclear and we speculate that the glucose-sensing function might be responsible for the observed differences between the cells with catalytically dead HK1 (“lack of glucose signal” present) and without any HK1 (“lack of glucose signal” absent).…”

Elucidation of the Translation Initiation Factor Interaction Network of Haloferax volcanii Reveals Coupling of Transcription and Translation in Haloarchaea

“…A classic example is enolase, which is part of the glycolytic pathway, but which is also involved in RNA degradation ( Henderson and Martin, 2013 ). However, the number of known moonlighting proteins has increased drastically in recent years ( Jeffery, 2020 ; Liu and Jeffery, 2020 ; Singh and Bhalla, 2020 ; Beaufay et al, 2021 ; Rodríguez-Saavedra et al, 2021 ; Turek and Irving, 2021 ). An indication that the archaeal proteins might have two different functions is the recent observation that the eukaryotic eIF2Bα still has the ability to bind sugar phosphates with high affinity, e.g., fructose-6-phosphate has a K D of 9.4μM ( Hao et al, 2021 ).…”

Voltage-Dependent Anion-Selective Channels and Other Mitochondrial Membrane Proteins Form Diverse Complexes in Beetroots Subjected to Flood-Induced Programmed Cell Death

“…Hexokinase associates with VDAC, inhibiting apoptosis by preventing the release of cyt c in mammalian mitochondria ( Azoulay-Zohar et al, 2004 ; Abu-Hamad et al, 2008 ; Shoshan-Barmatz et al, 2008 ). There is also evidence indicating that HK bound to VDAC inhibits the formation of MPTP in plant mitochondria ( Godbole et al, 2013 ; Rodríguez Saavedra et al, 2021 ). A functional association between HK and VDAC has been reported in beetroots, where it was observed that the ATP produced by oxidative phosphorylation in the mitochondrial matrix is preferentially channeled toward the HK bound to VDAC ( Alcántar-Aguirre et al, 2013 ).…”

“…Among the antiapoptotic VDAC-binding proteins found in both animals and plants is hexokinase (HK). This enzyme, associated with VDAC, not only catalyzes the phosphorylation of glucose using preferentially the ATP produced by respiring mitochondria ( Pedersen, 2008 ; Alcántar-Aguirre et al, 2013 ) but also inhibits PCD ( Godbole et al, 2013 , reviewed in Rodríguez Saavedra et al, 2021 ). In contrast, it has been proposed that VDAC associates with some mitochondrial inner membrane proteins, such as the adenine nucleotide and/or the phosphate translocators, to form MPTP ( Takahashi and Tateda, 2013 ; Hurst et al, 2017 ).…”