The oxidative phase of the pentose phosphate pathway is a major source of NADPH in cells, and in cells without mitochondria, it is the only source. However there are several other lesser-known mechanisms of generating NADPH, all of which depend on the presence of mitochondria. The key enzymes in these processes are: NADP-linked malic enzyme, NADP-linked isocitrate dehydrogenase, and nicotinamide nucleotide transhydrogenase. The isocitrate dehydrogenase mechanism appears to be the major source of NADPH in fat and possibly also liver cells. Also in mitochondria, NADH kinase produces NADPH and ADP using NADH and ATP as substrate. Ribulose-1,5-bisphosphate (RuBP) is an organic substance that is involved in photosynthesis. The anion is a double phosphate ester of the ketose (ketone-containing sugar) called ribulose. Salts of this species can be isolated, but its crucial biological function involves this colourless anion in solution. To simplify the presentation, the image in the above table depicts the acid form of this anion. Role in photosynthesis
The Calvin cycle showing the role of ribulose-1,5-bisphosphate.The enzyme ribulose bisphosphate carboxylase oxygenase (RuBisCO) catalyzes the reaction between RuBP with carbon dioxide. The product is the highly unstable 6-carbon intermediate known as 3-keto-2-carboxyarabinitol 1,5-bisphosphate. This six-carbon intermediate decays virtually instantaneously into two molecules of glycerate 3-phosphate (G3P) (see figure). Less commonly (e.g. at high temperatures) RuBisCO catalyzes RuBP with oxygen (O2) in a process called photorespiration, a process that occurs at high temperatures in "C3 plants." In the Calvin Cycle, RuBP is a product of the phosphorylation of ribulose-5-phosphate by ATP. Carboxylation in chemistry is a chemical reaction in which a carboxylic acid group is introduced in a substrate. The opposite reaction is decarboxylation.  Carboxylation in organic chemistry
In organic chemistry many different protocols exist for carboxylation. One general approach is by reaction of nucleophiles with dry ice (solid carbon dioxide) or formic acid Carboxylation in biochemistry
Further information: Carboxy-lyases
Carboxylation in biochemistry is a posttranslational modification of glutamate residues, to γ-carboxyglutamate, in proteins. It occurs primarily in proteins involved in the blood clotting cascade, specifically factors II, VII, IX, and X, protein C, and protein S, and also in some bone proteins. This modification is required for these proteins to function. Carboxylation occurs in the liver and is performed...