Читать книгу Solar-to-Chemical Conversion - Группа авторов - Страница 36

Capture of CO₂ and reduction to products such as CO, HCOOC, H₂CO, CH₃OH, or CH₄ are a principal target for artificial photosynthesis in the quest for solar fuels, as discussed in detail elsewhere in this book. A strictly biomimetic approach does not seem ideal in this case compared to photocatalytic and (photo)electrochemical reduction of CO₂. However, there are still lessons to be learned from biology, and here we will briefly cover the CO₂ fixation process in natural photosynthesis, which is carried out by the enzyme RuBisCO (ribulose‐1,5‐bisphosphate carboxylase/oxygenase) [4]. RuBisCO is considered the most abundant protein on earth [347, 348] and uses CO₂ to convert the five‐carbon molecule ribulose‐1,5‐bisphosphate (RuBP) to two three‐carbon 3‐phosphoglycerate (3PGA) molecules, one of which incorporates the CO₂‐derived carbon atom. This carboxylation reaction provides the substrate for subsequent reactions in the Calvin–Benson cycle that phosphorylate and reduce 3PGA using ATP and NADPH to produce glyceraldehyde 3‐phosphate (G3P), the precursor molecule of glucose and other carbohydrates. In most photosynthetic organisms RuBisCO is present as a complex composed of eight copies of large (L) proteins and eight copies of small (S) proteins, L₈S₈ [349]. The active form of the enzyme is generated by carbamylation of an active site lysine residue [350] via reaction with CO₂ and subsequent binding of Mg²⁺ at the carbamate. This is the form that binds RuBP, at the Mg²⁺ ion [351]. The carboxylation reaction is thought to proceed by initial creation of the enediol form of RuBP, which reacts with CO₂ and is hydrated before C—C bond cleavage and release of the two 3PGA molecules (Figure 3.13).

Figure 3.13 Reaction mechanism of RuBisCO proposed by Taylor and Andersson.

Source: Taylor and Andersson [351].

RuBisCO evolved at a time when the atmosphere of our planet was much richer in CO₂ and did not contain much O₂. The oxygenation of the atmosphere posed a serious challenge for RuBisCO because O₂ is a competitive substrate to CO₂. Binding of O₂ by RuBisCO leads to an alternative reaction pathway that results in oxygenation of RuBP. This is an unproductive pathway (photorespiration) that leads to creation of 2‐phosphoglycolate (2PGA) and eventual loss of previously fixed CO₂. Although several adaptations at the cellular or metabolic level exist in biology to deal with this problem, evolution has not come up with a “solution” at the molecular level, i.e. with restriction of oxygenase activity by adaptation of the enzyme itself. The inability of RuBisCO to discriminate strongly between CO₂ and O₂ is considered to be the reason for the utilization of very large quantities of the enzyme by photosynthetic organisms and is viewed as the primary reason for the low overall efficiency of natural photosynthesis [352–355].