Table 2.

Enzyme numbers and cofactor equivalents for isobutyraldehyde production from C₁ feedstocks

Substrate	C1-Pathway	Intermediate	Enzyme number	ATP cost	NAD(P)H equivalents	Refs.
CO2	WLP	acetyl-CoA	12	2	11	[47]
CO2	rGlyP	pyruvate	11	4	11	[42, 48]
CO2	rTCA cycle	acetyl-CoA	13	4	11	[49, 50]
CO2 HCO3−	DC/HB cycle	acetyl-CoA	18	6	11	[51]
CO2 HCO3−	HP/HB cycle	acetyl-CoA	20	8	11	[52, 53]
HCO3−	3-HP bicycle	pyruvate	22	10	11	[54, 55]
CO2	CBB cycle	GAP	21a	14a	11a	[56]
CO2	ASAP	pyruvate	13a	− 2a	11a	[13]

All calculations are based on converting CO₂ equivalents to isobutyraldehyde via pyruvate. NAD(P)H equivalents refer to the reducing power generated from NADH, NADPH, ferredoxin, H_2, or H₂O.

WLP: Wood–Ljungdahl pathway, rGlyP: reductive glycine pathway, rTCA cycle: reductive TCA cycle, DC/HB cycle: dicarboxylate/4-hydroxybutyrate cycle, HP/HB cycle: 3-hydroxypropionate/4-hydroxybutyrate cycle, 3-HP bicycle: 3-hydroxypropionate bicycle, CBB cycle: Calvin–Benson–Bassham cycle, ASAP: artificial starch anabolic pathway.

^aHere, we assume that the CBB cycle and ASAP are employed to convert CO₂ to glyceraldehyde-3-phosphate (GAP), then a few steps of the glycolysis pathway are utilized to convert GAP to pyruvate, which will be further used to produce isobutyraldehyde