Some structural properties of plant serine:glyoxylate aminotransferase?

Abstract

The structural properties of photorespiratory serine:glyoxylate aminotransferases (SGAT, EC 2.6.1.45) from maize (Zea mays L.) and wheat (Triticum aestivum L.) leaves were examined. By means of molecular sieving on Zorbax SE-250 column and filtration through centrifugal filters it was shown that dimers of wheat enzyme (molecular mass of about 90 kDa) dissociate into component monomers (molecular mass of about 45 kDa) upon decrease in pH value (from 9.1 or 7.0 to 6.5). At pH 9.1 a 50-fold decrease of ionic strength elicited a similar effect. Under the same conditions homodimers of the maize enzyme (molecular mass similar to that of the wheat enzyme) remained stable. Immunoblot analysis with polyclonal antiserum against wheat seedling SGAT on leaf homogenates or highly purified preparations of both enzymes showed that the immunogenic portions of the wheat enzyme are divergent from those of the maize enzyme. The sequence of 136 amino acids of the maize enzyme and 78 amino acids of the wheat enzyme was established by tandem mass spectrometry with time of flight analyzer. The two enzymes likely share similarity in tertiary and quaternary structures as well as high level of hydrophobicity on their molecular surfaces. They likely differ in the mechanism of transport from the site of biosynthesis to peroxisomes as well as in some aspects of secondary structure.

Properties of serine:glyoxylate aminotransferase purified from <italic>Arabidopsis thaliana</italic> leaves

“…The mean hydropathy indexes of the maize, wheat, and Arabidopsis SGAT amino acid sequences calculated according to the method of Kyte and Doolittle were 0.024, 0.020, and 0.023, respectively, each distinctly above the average value (−0.4) reported by those authors for 84 fully sequenced soluble enzymes [29]. This result confirmed the high hydrophobicity of these three aminotransferases [30] and explained their stability in acetone. A comparison of the molecular mass of Arabidopsis SGAT determined under denaturing conditions (39.8 kDa), the results of molecular filtration of the active enzyme (82.4 kDa), the molecular mass obtained from mass spectrometry (42.0 kDa), and the reported molecular mass of SGATs from other plants [5,11,12,15] indicates that these aminotransferases are all dimers.…”

Regulatory mechanisms underlying C4 photosynthesis

“…SGT converts serine to hydroxypyruvate and thus could be expected to accumulate in peroxisomes of BS cells. However, glyoxylate aminotransferase proteins are usually multifunctional, and thus SGT may be required for alanine and aspartate metabolism when alanine is used as the amino‐group donor (Truszkiewicz & Paszkowski, 2005). This nonphotorespiratory activity may demand the expression of SGT in both BS and ME cells.…”