Comparison of Sensitive and Desensitized Forms of Maize Homoserine Dehydrogenase

Dicamelli, Cynthia A.; Bryan, John K.

doi:10.1104/pp.65.2.176

Cited by 16 publications

(9 citation statements)

References 13 publications

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“…A BLAST search of available databases reveals that multiple plants, including Medicago truncatula (barrel clover; 85%), Populus trichocarpa (California poplar; 76%), Ricinus communis (castor; 75%), Vitis vinifera (grape; 70%), A. thaliana (69%), Z. mays (maize; 66%), Sorghum bicolor (66%), and Oryza sativa (rice; 66%), contain sequences sharing high identity with GmHSD that are in many cases annotated as either unknown or hypothetical proteins or even incorrectly as AK. This suggests that monofunctional HSD in plants are more prevalent than previously described (22)(23).…”

Section: Discussioncontrasting

confidence: 50%

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Threonine-insensitive Homoserine Dehydrogenase from Soybean

Schroeder

Zhu

Yanamadala

et al. 2010

Journal of Biological Chemistry

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Aspartate kinase (AK) and homoserine dehydrogenase (HSD) function as key regulatory enzymes at branch points in the aspartate amino acid pathway and are feedback-inhibited by threonine. In plants the biochemical features of AK and bifunctional AK-HSD enzymes have been characterized, but the molecular properties of the monofunctional HSD remain unexamined. To investigate the role of HSD, we have cloned the cDNA and gene encoding the monofunctional HSD (GmHSD) from soybean. Using heterologously expressed and purified GmHSD, initial velocity and product inhibition studies support an ordered bi bi kinetic mechanism in which nicotinamide cofactor binds first and leaves last in the reaction sequence. Threonine inhibition of GmHSD occurs at concentrations (K i ‫؍‬ 160 -240 mM) more than 1000-fold above physiological levels. This is in contrast to the two AK-HSD isoforms in soybean that are sensitive to threonine inhibition (K i ϳ150 M). In addition, GmHSD is not inhibited by other aspartate-derived amino acids. The ratio of threonine-resistant to threonine-sensitive HSD activity in soybean tissues varies and likely reflects different demands for amino acid biosynthesis. This is the first cloning and detailed biochemical characterization of a monofunctional feedback-insensitive HSD from any plant. Threonine-resistant HSD offers a useful biotechnology tool for manipulating the aspartate amino acid pathway to increase threonine and methionine production in plants for improved nutritional content.

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Section: Discussioncontrasting

confidence: 50%

“…In both bacterial and plant AK-HSD, the linker region between each enzyme domain is responsible for the regulatory effect of threonine (13,21). Interestingly, plants also appear to use monofunctional HSD, as the activity of a threonine-resistant HSD in maize extracts was reported (22,23), but no molecular details described.…”

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Threonine-insensitive Homoserine Dehydrogenase from Soybean

Schroeder

Zhu

Yanamadala

et al. 2010

Journal of Biological Chemistry

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“…However, under most assay conditions, II/III would not be totally inhibited and the amount of I would be overestimated. Evaluation of the composition of insensitive enzyme preparations is further complicated by the fact that class II/ III can be desensitized under a variety of conditions (3,7). In this report, we describe a simple procedure which permits quantitative estimates of the amounts and properties of both isozymes in crude preparations isolated from maize.…”

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confidence: 99%

“…Information gained from several studies suggests that most plants are characterized by the presence of two basic types of the enzyme, a low mol wt threonine-resistant form (class I, mol wt 70-80K) and a larger threonine-sensitive form which can exist as a dimer (mol wt 150-190K) or as a tetramer (mol wt 300-400K). The latter type of enzyme has been designated as class II/III to indicate the interconvertibility of forms which differ in mol wt (7). Repeated attempts to dissociate II/III into a form possessing the physical and regulatory characteristics of class I have failed.…”

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Quantitative Estimates of the Distribution of Homoserine Dehydrogenase Isozymes in Maize Tissues

Bryan¹,

Lochner²

1981

Plant Physiol.

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The low molecular weight threonine-resistant (class I) and the higher molecular weight threonine-sensitive (class II/III) isozymes of homoserine dehydrogenase (EC 1.1.1.3) isolated from Zea mays L. were shown to differ in stability during incubations in the presence of urea. Class II/III was inactivated by urea in a time-and concentration-dependent manner, with complete inactivation occurring within 24 hours at 5°C in 4.0 M urea. Under identical conditions, neither the activity nor the properties of class I were affected. Therefore, it was possible to estimate the amounts and properties of both maize isozymes in crude mixtures by measurements of enzyme activity before and after treatment with urea.The relative amounts of the two isozymes proved to be tissue-specific.

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“…However, the enzyme has been studied from several different plant species (1,2,4,9,(19)(20)(21)(22). HSDH has been localized in the chloroplast fractions of maize (4) and pea leaves (24,25).…”

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Immunological Characterization of in Vitro Forms of Homoserine Dehydrogenase from Carrot Suspension Cultures

Turano¹,

Jordan²,

Matthews³

1990

Plant Physiol.

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Multiple forms of homoserine dehydrogenase (HSDH) from carrot (Daucus carota L.) have been identified. One form of HSDH (T-form) has a relative molecular weight of 240,000 and is strongly inhibited by threonine. Another form (K-form) has a relative molecular weight of 180,000 and is insensitive to inhibition by threonine. The interconversion of these two forms is dependent upon the presence or absence of threonine and potassium. Polyacrylamide electrophoretic gels stained for HSDH activity and protein, paralleled with Western blot analysis, verified the interconversion of the T-and K-forms in 5 millimolar threonine and 100 millimolar potassium, respectively. Carrot HSDH also aggregates to form higher molecular weight complexes of 240,000 up to 720,000 Mr. Polyclonal antibody from mouse was raised against the T-form (240,000 Mr) of carrot HSDH. Specificity of the mouse antisera to carrot HSDH was verified by immunoprecipitation and Western blot analysis. The T-form, K-form, and all of the higher molecular aggregates of carrot HSDH cross-reacted with the anti-HSDH antiserum. The antiserum also cross-reacted with soybean HSDH, but did not cross-react with either of the two HSDH forms found in Escherichia coi. A model for the in vivo regulation of threonine biosynthesis in the chloroplast is presented. The model is based on the interconversion of the HSDH forms by potassium and threonine.HSDH2 (EC 1.1.1.3) is a key branch-point enzyme in the biosynthetic pathway of the aspartate family of amino acids. In plants and bacteria the essential amino acids lysine, threonine, isoleucine and methionine are synthesized by this pathway (Fig. 1)

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Comparison of Sensitive and Desensitized Forms of Maize Homoserine Dehydrogenase

Cited by 16 publications

References 13 publications

Threonine-insensitive Homoserine Dehydrogenase from Soybean

Threonine-insensitive Homoserine Dehydrogenase from Soybean

Quantitative Estimates of the Distribution of Homoserine Dehydrogenase Isozymes in Maize Tissues

Immunological Characterization of in Vitro Forms of Homoserine Dehydrogenase from Carrot Suspension Cultures

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