Organic Chemistry

952 CHAPTER 22 Carbohydrates

Type A D-galactose PROTEIN

L-fucose

N-acetyl-D-glucosamine

D-galactose PROTEIN

L-fucose

N-acetyl-D-glucosamine

D-galactose PROTEIN

L-fucose

N-acetyl-D-glucosamine

Type B D-galactose

Type O

N-acetyl-D-galactosamine

Figure 22.5

Blood type is determined by the nature of the sugar on the surfaces of red blood cells.

Fucose is 6-deoxygalactose.

Many different types of proteins are glycoproteins. For example, structural proteins

such as collagen, proteins found in mucous secretions, immunoglobulins, follicle-stimu-

lating hormone and thyroid-stimulating hormone, interferon (an antiviral protein), and

blood plasma proteins are all glycoproteins. One of the functions of the polysaccharide

chain is to act as a receptor site on the cell surface in order to transmit signals from hor-

mones and other molecules across the cell membrane into the cell. The carbohydrates on

the surfaces of cells also serve as points of attachment for other cells, viruses, and toxins.

Carbohydrates on the surfaces of cells provide a way for cells to recognize one an-

other. The interaction between surface carbohydrates has been found to play a role in

many diverse activites, such as infection, the prevention of infection, fertilization, in-

flammatory diseases like rheumatoid arthritis and septic shock, and blood clotting. For

example, the goal of the HIV protease inhibitor drugs is to prevent HIV from recog-

nizing and penetrating cells. The fact that several known antibiotics contain amino

sugars suggests that they function by recognizing target cells. Carbohydrate interac-

tions also are involved in the regulation of cell growth, so changes in membrane gly-

coproteins are thought to be correlated with malignant transformations.

Blood type (A, B, AB, or O) is determined by the nature of the sugar bound to the

protein on the outer surfaces of red blood cells. Each type of blood is associated with

a different carbohydrate structure (Figure 22.5). Type AB blood has the carbohydrate

structure of both type A and type B.

Antibodiesare proteins that are synthesized by the body in response to a foreign

substance, called an antigen. Interaction with the antibody either causes the antigen to

precipitate or flags it for destruction by immune system cells. This is why, for exam-

ple, blood cannot be transferred from one person to another unless the carbohydrate

glycoproteins

O-protein

CH 2 OH

O

HO

OH

O

NH-protein

CH 2 OH

O

HO

OH

O

AU: OK as changed?