Sulfur Metabolism and Protein Quality of Soybean

Abstract

Soybean is an important source of protein in livestock production, and is of growing importance for human consumption. As a sole dietary protein source, soybean seed protein is deficient in the amino acids methionine, cysteine, and threonine. Increasing the amount of methionine in the amino acid profile of soybean meal would enhance its value for producers and consumers. Methionine contains S, and so its production is necessarily linked to sulfur metabolism within the soybean plant. Sulfur is taken up from the soil in the form of sulfate. During vegetative growth, developing leaves appear to be the predominate site of sulfate reduction and incorporation of reduced S into amino acids. During reproductive growth, developing pods and seeds seem to be the predominate location of sulfate reduction. Sulfate reduction and methionine synthesis are complex and highly regulated processes. Synthesis of storage proteins within the developing seed is sensitive to the amount of methionine present such that provision of extra methionine blocks synthesis of poor quality storage proteins. Sulfur deficiency on the other hand dramatically enhances accrual of poor quality seed storage proteins. It appears that the plant manufacturers higher qualty storage proteins as long as methionine is present in adequate quanitities relative to the non-S-amino acids. Accumulation of poor-quality seed storage proteins thus appears to be a function, at least to some degree, of rate of methionine synthesis within the seed. Efforts to enhance soybean seed protein quality may require enhanced rates of methionine synthesis within the seed to be successful.

Key Words:

• Sulfur metabolism
• methionine
• soybean
• protein quality