Isocitric acid plant

Enzymes usually function stereospedfically. In chiral substrates, they only accept one of the enantiomers, and the reaction products are usually also sterically uniform. Aconitate hydratase (aconitase) catalyzes the conversion of citric acid into the constitution isomer isocitric acid (see p.l36). Although citric acid is not chiral, aconitase only forms one of the four possible isomeric forms of isocitric acid (2i ,3S-isocitric acid). The intermediate of the reaction, the unsaturated tricarboxylic acid aconitate, only occurs in the cis form in the reaction. The trans form of aconitate is found as a constituent of certain plants. 

Glyoxylic acid is either derived from isocitric acid (Fig. 89) or, in plants, from glycolic acid formed in photosynthesis. The oxidation of glycolic acid to glyoxylic acid is also catalyzed by glycolate oxidase ... 

Verduin, . Diffusion through multiperforated septa. In Photosynthesis in plants. Franck, ., Loomis, W.E. (eds.), pp.95-112. Ames, Iowa Iowa State College Press 1949 Vickery, H.B. The behaviour of isocitric acid in excised leaves of Bryophyllum calycinum during culture in alternating light and darkness. Plant Physiol. 27,9-17 (1952)... 

Aconitic acid involved in the TCA and glyoxylate cycles and the acid commonly occurring in nature has the c/5-configuration. The trans- om x has also been isolated from some plant materials - for example, sugarcane Saccharum offi-cinarum) juice (17), tomato (Lycopersicon esculentum) (56) or moss (Bryophyta) (34). However, some of the occurrences might be artifacts of the isolation procedures, for an interconversion between two isomers of aconitic acid has been reported (14, 81). In both cycles, cw-aconitic acid is formed upon dehydration of citric acid catalyzed by aconitase (aconitate hydratase) which also catalyzes the rehydration of cw-aconitate to isocitric acid. 

Of the four stereoisomers, erythro-Ti - and Ls-, and threo-T> - and Lg-isodtric acid, it is the t/j/ieo-Dg-isomer that occurs in nature. The formation of isocitric acid from c/s-aconitic acid is catalyzed by aconitase. This acid is also accumulated in Crassulacean plants as a result of dark CO2 fixation. 

This is an enzyme occurring in many animal and plant tissues that accelerates the conversion of citric acids (1) into aconitic acid, and (2) then into isocitric acid. 

The glyoxylate cycle is active in the germinating seeds of some plants and in certain microorganisms that can live on acetate as the sole carbon source. In plants, the pathway takes place in glyoxysomes in seedlings. It involves several citric acid cycle enzymes and two additional enzymes isocitrate lyase and malate synthase. 

Topic LI). Thus, animals cannot convert fatty acids into glucose. In contrast, plants have two additional enzymes, isocitrate lyase and malate synthase, that enable them to convert the carbon atoms of acetyl CoA into oxaloacetate. This is accomplished via the glyoxylate pathway, a route involving enzymes of both the mitochondrion and the glyoxysome, a specialized membranous plant organelle. 

Figure 17.23 The glyoxylate pathway. The glyoxylate cycle allows plants and some microorganisms to grow on acetate because the cycle bypasses the decarboxylation steps of the citric acid cycle. The reactions of this cycle are the same as those of the citric acid cycle except for the ones catalyzed by isocitrate lyase and malate synthase, which are boxed in blue.

It is important to note that animals are unable to effect the net synthesis o/glu-cose from fatty acids. Specifically, acetyl Go A cannot be converted into pyruvate or oxaloacetate in animals. Recall that the reaction that generates acetyl CoA from pyruvate is irreversible (p. 477). The two carbon atoms of the acetyl group of acetyl CoA enter the citric acid cycle, but two carbon atoms leave the cycle in the decarboxylations catalyzed by isocitrate dehydrogenase and a-ke-toglutarate dehydrogenase. Consequently, oxaloacetate is regenerated, hut it is not formed de novo when the acetyl unit of acetyl CoA is oxidized by the citric acid cycle. In contrast, plants have two additional enzymes enabling them to convert the carbon atoms of acetyl CoA into oxaloacetate (Section 18.4.). 

Fluoroacetic acid has been identifled as the toxic component of the South African plant gijblaar (Dichapetalum cymosum) [34]. Its mechanism of action is based on inhibition of the citric acid cycle, the main source of metabolic energy in all animals [35]. In this cyde, fluoroacetate can replace acetate as a substrate of aconi-tase, an enzyme complex which usually forms dtrate by addition of acetate to a-oxoglutarate. The resulting fluorocitrate is binds tightly to the enzyme, but cannot be further converted to ds-aconitate and isocitrate [36], thus inhibiting aconitase. 

Plants and some bacteria contain two enzymes (isocitrate lyase and malate synthase) that enable them to synthesize sugars by using the glyoxylate cycle, a variant form of the citric acid cycle. Notice in Figure 14.20 that the glyoxylate cycle uses some of the same enzymes as the citric acid cycle, but that the steps in which decarboxylations occur are bypassed. One of the intermediates in the bypass is glyoxylate, which gives the cycle its name. 

This enzyme, which is present in plants and bacteria, allows for net syntheis of glucose using a two-carbon source, such as acetyl-CoA from breakdown of fatty acids. Because animals do not have isocitrate lyase, they cannot synthesize glucose in net amounts from fat. 

In plants and in some bacteria, but not in animals, acetyl-GoA can serve as the starting material for the biosynthesis of carbohydrates. Animals can convert carbohydrates to fats, but not fats to carbohydrates. (Acetyl-GoA is produced in the catabolism of fatty acids.) Two enzymes are responsible for the ability of plants and bacteria to produce glucose from fatty acids. Isocitrate lyase cleaves isocitrate, producing glyoxylate and succinate. Malate synthase catalyzes the reaction of glyoxylate with acetyl-GoA to produce malate. 

IsocHricacid HOOC-CH2-CH(COOH)-CHOH-COOH, a monohydroxy tricarboxylic acid, an isomer of citric acid, which is widely distributed in the plant kingdom and occurs in free form especially in plants of the stone-crop family (Crassulaceae), and in fruits. The salts of I. a., isocitrates, are important metaboli-cally as intermediates in the Tricarboxylic acid cycle (see), where they are formed from citrate by the enzyme aconitase, then oxidized to 2-oxoglutarate. In the Glyoxylate cycle (see), isocitrate is cleaved to succinate and glyoxylate. 

Isocitrate lyase Cytoplasm Isocitrate Glyoxylate, Dicarboxylic acids Phospho- enol- p)rruvate Found only in bacteria and plants... 

Fig. 3.1. Autoradiogram of two dimensional paper chromatogram of 60 minute dark fixation by Bryophyllum calycinum. 1, alanine 2, glutamine 3, asparagine 4, glycine 5, serine 6, glutamate 7, aspartate 8, citrate 9, isocitrate 10, malate 11, fumarate 12, succinate. The main labeled compound is malate. Other metabolically related organic and amino acids also appear. The only products after a 6-s exposure are malic and aspartic acids (data of P.Saltman et al., Plant Physiol. 32, 197-200, 1957, by permission)...

Tricarboxylic acid, known as aconitic acid (prop-1-ene-1,2,3-tricarboxylic acid, also known as achilleic or citridinic acid), occurs in two geometric isomers, the (Z)- and the (E)-isomers (8-64). (Z)-Aconitate is widespread as an intermediate produced in the isomerisation of citrate to D-isocitrate (catalysed by aconitase) in the citric acid cycle. About 5% aconitic acid is found in molasses from cane sugar production, where the (E)-isomer prevails, as it is formed by isomerisation of (Z)-aconitic acid at elevated temperatures and low pH. The amount of (Z)-aconitic acid in the growing cane is low, because it is used in the citric acid cycle and not stored in the plant. Decarboxylation of aconitic acid at elevated temperatures yields itaconic acid. 

---