Organic Chemistry

PROBLEM 8

a. Draw the structures of three different sphingomyelins.

b. Draw the structure of a galactocerebroside.

PROBLEM 9

The membrane phospholipids in animals such as deer and elk have a higher degree of un-

saturation in cells closer to the hoof than in cells closer to the body. Explain how this trait

can be important for survival.

010 CHAPTER 26 Lipids

MULTIPLE SCLEROSIS

AND THE MYELIN SHEATH

The myelin sheath is a lipid-rich material that is

wrapped around the axons of nerve cells. Composed largely of

sphingomyelins and cerebrosides, the sheath functions so as to

increase the velocity of nerve impulses. Multiple sclerosis is a

disease characterized by loss of the myelin sheath, a consequent

slowing of nerve impulses, and eventual paralysis.

PROBLEM 6

Membranes contain proteins. Integral membrane proteins extend partly or completely

through the membrane, whereas peripheral membrane proteins are found on the inner or

outer surfaces of the membrane. What is the likely difference in the overall amino acid

composition of integral and peripheral membrane proteins?

PROBLEM 7

A colony of bacteria accustomed to an environment at 25°C was moved to an identical en-

vironment, at 35°C. The higher temperature increased the fluidity of the bacterial mem-

branes. What could the bacteria do to regain their original membrane fluidity?

Sphingolipids

Sphingolipidsare also found in membranes. They are the major lipid components in

the myelin sheaths of nerve fibers. Sphingolipids contain sphingosine instead of glyc-

erol. In sphingolipids, the amino group of sphingosine is bonded to the acyl group of

a fatty acid. Both asymmetric carbons in sphingosine have the Sconfiguration.

Two of the most common kinds of sphingolipids are sphingomyelins and

cerebrosides. In sphingomyelins, the primary OH group of sphingosine is bonded

to phosphocholine or phosphoethanolamine, similar to the bonding in lecithins and

cephalins. In cerebrosides, the primary OH group of sphingosine is bonded to a

sugar residue through a linkage (Section 22.13). Sphingomyelins are

phospholipids because they contain a phosphate group. Cerebrosides, on the other

hand, are not phospholipids.

a sphingomyelin

a glucocerebroside

CH

CH OH

CH(CH 2 ) 12 CH 3

CH 2 O P OCH 2 CH 2 NCH 3

CH NH C

O

O

O− CH 3

CH 3

R

CH

CH OH

CH(CH 2 ) 12 CH 3

CH 2

O

CH NH C

O

R

HO

HO

CH 2 OH

H H

H

OH

H H

+ O

3-D Molecule: b-glycosidic

Sphingosine

sphingosine

CH

CH OH

CH(CH 2 ) 12 CH 3

CH 2 OH

CH NH 2

s configuration