Vitamin E Biosynthetic Pathway in Plants

Vitamin E biosynthetic pathway in plants was elucidated over 30 years ago. The tocopherols are exclusively synthesized in photosynthetic organisms including higher plants; significant amounts are found in all green tissues but predominantly occur in seeds [114]. In plants, tocopherol biosynthesis takes place in the plastid, and the enzymes are associated with the plastidial envelop [40, 41]. Figure 18.2 shows the vitamin E biosynthetic pathway in plants [111].

Homogentisic acid (HGA) and phytyldiphosphate (PDP), derived from cytosolic aromatic amino acid metabolism and plastidic deoxyxylulose 5-phosphate pathway, respectively (phytyl-PP for tocopherols and GGDP for tocotrienols) [42], serve as precursors for vitamin E biosynthesis.

The first step involves the production of the aromatic head group, HGA, from p-hydroxyphenylpyruvic acid (HPP) by the enzyme p-hydroxyphenylpyruvic acid dioxygenase (HPPD) [26,28,43-46]. The following committed reaction, the preny-lation of HGA with PDP, resulting in the formation of 2-methyl-6-phytyl-benzoquinol (MPBQ), is catalyzed by the homogentisate phytyltransferase (HPT), encoded by slr1736 in Synechocystis sp. PCC 6803 (Syn-vte2), or in Arabidopsis by the vitamin E2 gene (At-VTE2). Some plant species, such as oil palm, corn, and tobacco, accumulate substantial amounts of tocotrienols. These plants may harbor a VTE2 isoform exhibiting an altered substrate specificity, with preference toward geranylgeranyl diphosphate (GGDP) rather than PDP [47]. This results in the preny-lation of HGA with GGDP to form 2-methyl-6-geranylgeranylbenzoquinol (MG-GBQ), a tocotrienol precursor.

In Arabidopsis the metabolic fate of the VTE2 reaction product, MPBQ or MG-GBQ, is determined by the relative enzymatic activities of the tocopherol cyclase (TC) and the MPBQ (MGGBQ) methyltransferase (MPBQ MT) encoded by the VTE1 and VTE3 loci, respectively. After methylation by MPBQ MT, TC converts MPBQ and DMPBQ to 5- and y-tocopherol, respectively (and also cyclizes the corresponding geranylgeranylated intermediates to form 5- and y-tocotrienols).

The VTE3 reaction product, 2,3-dimethyl-5-benzoquinol, serves as a substrate for VTE1 to form y-tocopherol. This enzyme also utilizes MPBQ as substrate, resulting in the formation of 5-tocopherol. However, the link between the phenotype and tocopherol biosynthesis was not established at that time and continues to be elusive.

Shi ki mate pathway

Tyrosine

4- Hydroxyphenylpyra vate HPPD Homogentisate

Non-mevalonate pathway Glyceraldehyde-3-P + pyruvate

DXP synthase l-Deoxy-D-xylulose-5-P

GGDP reductase

-Phytyl-DP Geranylgeranyl-DP

MPBQMT

MPBQ

Cyclase

HGGT

DMPBQ

Homogentisate

S-tocopherol 1

y- tocopherol 1

5- tocotrienol y- tocotrienol

ß- tocopherol a- tocopherol a- tocotrienol

Fig. 18.2 Vitamin E biosynthetic pathway in plants (from Ref. [103] with permission of the authors), a R1 = R2 = CH3, known as a-tocopherol, is designated a-tocopherol or 5,7,8-trimethyltocol; R1 = CH3; R2 = H.known as ß-tocopherol, is designated ß-tocopherol or 5,8-dimethyltocol; R1 = H; R2 = CH3. known as y-tocopherol, is designated y-tocopherol or 7.8-dimethyltocol; R = R = H. known as S-tocopherol, is designated S-tocopherol or 8-methyltocol. b R1 = R2 = H. 2-methyl-2-(4.8.12-tiimethyltrideca-3.7.11-trienyl)chroman-6-ol. is designated tocotrienol; R1 = R2 = CH3, formerly known as £1 or £2-tocopherol, is designated 5.7.8-tiimethyltocotrienol or a-tocotrienol. The name tocochromanol-3 is also used; R1 = CH3; R2 = H. formerly known as e-tocopherol. is designated 5,8-dimethyltocotrienol or ß-tocotrienol; R1 = H; R2 = CH3, formerly known as y-tocopherol, is designated 7.8-dimethyltocotrienol or y-tocotrienol. The name plastochromanol-3 is also used; R1 = R2 = H is designated 8-methyltocotrienol or S-tocotrienol

ß- tocopherol a- tocopherol a- tocotrienol

Fig. 18.2 Vitamin E biosynthetic pathway in plants (from Ref. [103] with permission of the authors), a R1 = R2 = CH3, known as a-tocopherol, is designated a-tocopherol or 5,7,8-trimethyltocol; R1 = CH3; R2 = H.known as ß-tocopherol, is designated ß-tocopherol or 5,8-dimethyltocol; R1 = H; R2 = CH3. known as y-tocopherol, is designated y-tocopherol or 7.8-dimethyltocol; R = R = H. known as S-tocopherol, is designated S-tocopherol or 8-methyltocol. b R1 = R2 = H. 2-methyl-2-(4.8.12-tiimethyltrideca-3.7.11-trienyl)chroman-6-ol. is designated tocotrienol; R1 = R2 = CH3, formerly known as £1 or £2-tocopherol, is designated 5.7.8-tiimethyltocotrienol or a-tocotrienol. The name tocochromanol-3 is also used; R1 = CH3; R2 = H. formerly known as e-tocopherol. is designated 5,8-dimethyltocotrienol or ß-tocotrienol; R1 = H; R2 = CH3, formerly known as y-tocopherol, is designated 7.8-dimethyltocotrienol or y-tocotrienol. The name plastochromanol-3 is also used; R1 = R2 = H is designated 8-methyltocotrienol or S-tocotrienol

The final reaction to convert 5- and y-tocopherol to P- and a-tocopherol is catalyzed by the y-methyltransferase (y-TMT), encoded by VTE4, and 5- and y-tocotrienols to P- and a-tocotrienols [17, 34,40, 48, 49].

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