HORT640 - Metabolic Plant Physiology
Branched chain amino acid and lysine biosynthesis
Lysine biosynthesis and catabolism
Key to enzymes (Bryan, 1980):
1. Dihydrodipicolinate synthase [EC 184.108.40.206]
2. Dihydrodipicolinate reductase [EC 220.127.116.11]
3. Tetrahydrodipicolinate N-acetyltransferase [EC 18.104.22.168]
Key to enzymes (Bryan, 1980):
4. Acyldiaminopimelate aminotransferase [EC 22.214.171.124]
5. Acyldiaminopimelate deacylase [EC 126.96.36.199]
6. Diaminopimelate epimerase [EC 188.8.131.52]
7. Diaminopimelate decarboxylase [EC 184.108.40.206]
Dihydrodipicolinate synthase (DHPS) [EC 220.127.116.11] catalyzes the first reaction that is unique to lysine biosynthesis; the condensation of aspartate semialdehyde with pyruvate to form 2,3-dihydrodipicolinate. Of the aspartate family amino acid biosynthetic enzymes DHPS is the most sensitive to inhibition by lysine. It is ~10-fold more sensitive to inhibition by lysine than the lysine-sensitive AK (Galili, 1995).
Many of the enzymes involved in lysine and threonine synthesis have been localized to plastids (Galili, 1995). The lysine content in the seeds of canola and soybean plants has been increased by circumventing the normal feedback regulation of two enzymes of the biosynthetic pathway, aspartokinase (AK) and dihydrodipicolinic acid synthase (DHDPS) (Falco et al, 1995). Lysine-feedback-insensitive bacterial DHDPS and AK enzymes encoded by the Corynebacterium dapA gene and a mutant E. coli lysC gene, respectively, were linked to a chloroplast transit peptide and expressed from a seed-specific promoter in transgenic canola and soybean seeds (Falco et al, 1995). Expression of Corynebacterium DHDPS resulted in more than a 100-fold increase in the accumulation of free lysine in the seeds of canola, doubling total seed lysine content. Expression of Corynebacterium DHDPS plus lysine-insensitive E. coli AK in soybean caused several hundred-fold increases in free lysine and a 5-fold increase in total seed lysine content (Falco et al, 1995).
The accumulation of free lysine in tobacco seeds stimulates lysine-ketoglutarate reductase (LKR) [EC 18.104.22.168], an enzyme that acts in lysine catabolism (lysine conversion to saccharopine); activation of this enzyme requires Ca2+ and protein phosphorylation (Karchi et al, 1995). This pathway may be specific to seeds inasmuch as leaves can accumulate large amounts of lysine without detectable LKR activity (Shaul and Galili, 1992).
The first enzyme of the lysine degradation pathway in maize, lysine-ketoglutarate reductase, condenses lysine and 2-oxoglutarate into saccharopine using NADPH as a cofactor, whereas the second enzyme, saccharopine dehydrogenase (SDH) [EC 22.214.171.124], converts saccharopine to alpha-aminoadipic-delta-semialdehyde and glutamic acid using NAD(P)+ as a cofactor. In maize endosperm the two activities are located in two functionally independent domains of a bifunctional polypeptide (Goncalves-Butruille et al, 1996). Arabidopsis contains bifunctional LKR/SDH and monofunctional SDH isoenzymes which may be derived from a single gene (Tang et al, 1997). Aminoadipic semialdehyde dehydrogenase [EC 126.96.36.199] catalyzes the reduction of aminoadipic semialdehyde to alpha-aminoadipic acid. Accumulation of alpha-aminoadipic acid (in canola) and saccharopine (in soybean) is associated with elevated seed lysine content in transgenic plants expressing lysine-feedback-insensitive bacterial DHDPS and AK enzymes (Falco et al, 1995).
A lysine and histidine specific amino acid transporter (LHT1) has been identified in Arabidopsis (Chen and Bush, 1997). This transporter appears to be different from other plant amino acid transporters identified thus far.
Bryan JK 1980 Aspartate family and branched-chain amino acids. In (BJ Miflin ed) "The Biochemistry of Plants", Vol. 5, Academic Press, New York, pp. 403-452.
Chen L, Bush DR 1997 LHT1, a lysine- and histidine-specific amino acid transporter in Arabidopsis. Plant Physiol. 115: 1127-1134.
Falco SC, Guida T, Locke M, Mauvais J, Sanders C, Ward RT, Webber P 1995 Transgenic canola and soybean seeds with increased lysine. Biotechnology (N.Y.) 13: 577-582.
Galili G 1995 Regulation of lysine and threonine synthesis. Plant Cell 7: 899-906.
Goncalves-Butruille M, Szajner P, Torigoi E, Leite A, Arruda P 1996 Purification and characterization of the bifunctional lysine-ketoglutarate reductase-saccharopine dehydrogenase from maize. Plant Physiol. 110: 765-771.
Karchi H, Miron D, Ben-Yaacov S, Galili G 1995 The lysine-dependent stimulation of lysine catabolism in tobacco seed requires calcium and protein phosphorylation. Plant Cell 7: 1963-1970.
Shaul O, Galili G 1992 Increased lysine synthesis in transgenic tobacco plants expressing a bacterial dihydropicolinate synthase in their chloroplasts. Plant J. 2: 203-209.
Tang G, Miron D, Zhu-Shimoni JX, Galili G 1997 Regulation of lysine catabolism through lysine-ketoglutarate reductase and saccharopine dehydrogenase in Arabidopsis. Plant Cell 9: 1305-1316.
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