## 1 Introduction  
Globally, the petrochemical industry is one of the main sources of current energy, which involves fossil fuels. The uncontrolled usages of these fossil fuels over the years led to the inequities in the carbon cycle, as the consumption of these fuels accumulated additional carbon to the atmosphere. With the mounting global population, the dependency on these fuels is in turn increasing, leading to their over-usage. This created catastrophic changes in nature, leading to global warming, and raised questions on energy security [1]. One way to handle this imminent demand of energy as well as the climatic concerns is to focus more on renewable sources of energy and develop efficient technologies to ensure maximum utilization. According to the International Energy Agency, to avoid further increases in world-wide temperatures, the carbon dioxide (CO\({}_{2}\)) emissions must be reduced. It also stated that by 2060, 70% of CO\({}_{2}\) emission (with respect to 2017) should be reduced to control the global warming issue in future [2]. To achieve this target, along with measures to reduce the current CO\({}_{2}\) levels in the atmosphere, there should be emphasis on sectors which are creating the CO\({}_{2}\) emissions, like the automobile sector. Blending fossil fuels with biofuels and designing machinery that runs with biofuel alone can act as key factors for achieving this target. Consequently, these steps will eventually reduce the harmful effects of fossil fuels on the global environment. One such almost-commercial biofuel is bio-ethanol. Nevertheless, there are few drawbacks, like that it produces less energy (30% less) and vapour pressure when compared to gasoline, its toxicity and corrosiveness, etc. Ethanol is hygroscopic and blends with gasoline before it can be used, a limiting factor for its use as an alternate fuel. Moreover, as stated above, for using these biofuel blends, automobile engines need to be modified accordingly. For instance, automobiles which use 10% and 5% ethanol blends (E10 and E5) must follow the regulations like ASTM D5798 and EN228 in North America and Europe, respectively [3]. With these drawbacks, there is a need for more dense energy sources similar to current fossil fuels, unlike low-density ethanol. To resolve the present density, storage, and transportation issues, new biofuels like iso-butanol, n-butanol, isoprenoid, and fatty-acid-derived biofuels came into existence. These biofuels possess relatively similar energy content when compared to fossil fuels like diesel, petrol, gasoline, and jet fuels [4]. Even though many microorganisms possess inbuilt metabolic pathways to produce these compounds, for them the requirements in the system are much less and hence they produce very minimal quantities, which are insufficient to meet the commercial needs. There are several factors like metabolic flux, transcriptional regulators, inhibitors, cofactor imbalances, etc., which influence these minimal quantities. Most of these tasks can be handled by metabolic engineering approaches [3; 5; 6; 7]. Metabolic engineering is a developing area of research which reconstructs the cellular pathways for the enhanced production of metabolites of interest. It has been applied to develop cell factories for scheming new metabolic frameworks for producing compounds that act as advanced biofuels in the required host [7]. The metabolic engineering approaches used to enhance the production of the target metabolites are heterologous gene expression, constructing orthogonal pathways, engineering co-factors, blocking the inhibitors and other competitor pathways, and overexpressing the required enzymes [8]. As there is a huge amount of omics-related data generated on hosts like _Saccharomyces cerevisiae_ and _Escherichia coli_, these are most often chosen for metabolic engineering. As the genome editing approaches are established for these hosts, it becomes easy for engineering the genes associated with the target metabolites.