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




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 Structure of Calicheamicin g1](https://i.stack.imgur.com/lW8lG.jpg)](https://i.stack.imgur.com/lW8lG.jpg)


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 Biosynthetic Path of Calicheamicin](https://i.stack.imgur.com/Atty2.jpg)](https://i.stack.imgur.com/Atty2.jpg)


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.


I hope this is what you want to know about this molecule.


**References:**


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>).
2. W. M. Maiese, M. P. Lechevalier, H. A. Lechevalier, J. Korshalla, N. Kuck, A.Fantini, M.J. Wildey, J. Thomas, M. Greenstein, “Calicheamicins, a novel family of antitumor antibiotics: taxonomy, fermentation and biological properties,” *The Journal of Antibiotics* **1989**, *42(4)*, 558-563 (DOI: 10.7164/antibiotics.42.558).
3. Miklós Simonyi, ”The Concept of Chiral Conformers and its Significance in Molecular Pharmacology,” In *Advances in Drug Research: Volume 30*; Bernard Testa, Urs A. Meyer, Eds.; Academic Press: San Diego, CA, 1997, pp. 73-111 (ISBN: 0-12-013330-X).


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