# what is a sugar unit?

How a sugar unit is defined? I am trying to understand the structure of calicheamicins, and it is noted that They consist of an enediyne headpiece and a modified carbohydrate tail consisting of four sugar units and an unusually substituted aromatic ring. I can understand in the picture where sugar unit is referred, but I would like a general definition. Thank you

• The sugar unit features a linear chain of carbon carbon bonds. Distinct units are usually connected by glycosidic bonds (which may be hydrolyzed without breaking carbon carbon bonds). May 11 '20 at 11:36
• A sugar unit is a monosaccharide, the Wikipedia entry is a good introduction en.wikipedia.org/wiki/Monosaccharide May 11 '20 at 13:53

Calicheamicins are natural products, which were isolated from the fermentation broth of Micromonospora echinospora subsp. calichensis. These antibiotics exhibited significant activity against Gram-positive and Gram-negative bacteria in vitro. Calicheamicin $$\gamma_1$$ (Ref.1) demonstrated antitumor activity against P388 leukemia and B16 melanoma in vitro (Ref.2).

The structure of Calicheamicin $$\gamma_1$$ contains biologically active part of bicycloring system, which is described in the following diagram:

The rest of the structure contains four modified sugar units and one aromatic nucleus. The sugar 1 unit is a 6-deoxy-4-aminohexose unit (that's why it is called a modified sugar). Its anomeric oxygen (hemiacetyl $$\ce{OH}$$) bound to the $$\ce{C}$$9 position of the endiyne unit in $$\beta$$-linkage. The sugar 2 unit is a 2,6-dideoxy-4-thiohexose unit, hence it is also a modified sugar. This linkage is unusual among other sugar linkages in natural origin. The inkage is a hydroxyl amine bond ($$\ce{-NH-O-}$$) without eliminating $$\beta$$-$$\ce{OH}$$ of 2nd sugar unit. According to proposed biosynthetic pathway, these two sugars have been added to endiyne unit in first and second steps of the pathway:

The third step of the biosynthesis is addition of the aromatic nucleus (5-iodo-4-hydroxy-2,3-methoxy-6-methylbenzoate unit) to 4-thial group of sugar 2 as a thioester bond. The forth and fifth additions of other sugar units can be alternative. Nonetheless, one sugar (a 6-deoxy-3-O-methylhexose unit) attached to the free 4-phenolic $$\ce{OH}$$ group of aromatic nucleus in $$\alpha$$-linkage. The last sugar unit is attached with its $$\alpha$$-$$\ce{OH}$$ of the anomeric center to the free 2-$$\ce{OH}$$ group of the first sugar in $$2 \rightarrow 1$$ linkage. The unusual about this sugar is it is a pentose (a 2-deoxy-3-O-methyl-4-N-ethylaninopentose unit) instead of hexose among other three.

You can find a good description of mechanism of action of Calicheamicin $$\gamma_1$$ in Ref.3.

1. May D. Lee, Theresa S. Dunne, Marshall M. Siegel, Conway C. Chang, George O. Morton, Donald B. Borders, “Calichemicins, a Novel Family of Antitumor Antibiotics. 1. Chemistry and Partial Structure of Calichemicin $$\gamma_1$$,” J. Am. Chem. Soc. 1987, 109(11), 3464-3466 (https://doi.org/10.1021/ja00245a050).
• Sounds like it it is calicheamicin $\gamma_1$ with a linker. I didn't know biochemical manipulation of the natural product. Yet, it won't hurt if you ask the question. May 12 '20 at 9:41