What is unique about exotoxins that are not in an AB configuration?

Exotoxins that are not in an AB configuration have unique characteristics that set them apart from other types of toxins, particularly those that do follow the AB model. The correct choice highlights that certain exotoxins, such as the heat-stable toxin produced by some strains of E. coli, fall into this category. This toxin is notable for its ability to retain activity even when exposed to high temperatures, distinguishing it from many other toxins that may lose their functionality under similar conditions.

The heat-stable toxin from E. coli, specifically the STa toxin, acts by stimulating guanylate cyclase in intestinal cells, leading to increased fluid secretion and potentially contributing to diarrhea. This characteristic of stability under heat conditions is a significant point of differentiation among various exotoxins, offering insights into their pathogenic mechanisms and effects on host organisms.

Other options do not accurately reflect the nature of unique exotoxins not in an AB configuration. For instance, not all exotoxins are encoded by genes in the bacterial chromosome, nor is heat stability uniformly applicable to all exotoxins. Additionally, while bacterium-produced toxins are a hallmark of pathogenicity, stating that such toxins cannot be produced by bacteria is factually incorrect, as these are indeed a product of bacterial