Carbohydrates, Nucleotides, and Lipids

Major concepts:

1. What is the characteristic structure of a carbohydrate? What is an anomeric carbon, and why is it important?

2. How are disaccharides and polysaccharides formed? How do similar disaccharides differ from each other?

3. What are the major components of a nucleotide? How do nucleotides differ?

4. What is the important characteristic that defines a lipid? What are the different classes of lipids?

5. How are phospholipids and sphingolipids alike? How are they different? How is their relationship to water related to their function?

6. How are triacylglycerols, steroids, and eicosanoids alike? How are they different?

Core knowledge:

1. What are the ring and chain forms of D-glucose? How are the structures of D-fructose different?

2. What is a reducing sugar? What is the non-reducing end of a disaccharide?

3. What is the structure of ribose? How is deoxyribose different? How can you identify adenine, guanine, cytosine, thymine, and uracil?

4. What are the characteristics of fatty acids? How is the structure of a triacylglycerol different from the structure of a phospholipid?

Carbohydrates

carbohydrate = carbon hydrated (C + H₂O)
monosaccharides = simple sugars = polyhydroxy aldehyde (aldose) or ketone (ketose)
disaccharides = two monosaccharides; polysaccharides = multiple monosaccharides

Simplest sugar = glyceraldehyde or dihydroxyacetone:

Nearly all sugars found in cells are D sugars, so we'll ignore L sugars.
The chiral C farthest from the carbonyl gives the D/L orientation
      based upon similarity to glyceraldehyde.

Glucose is a 6-carbon aldose.
1. Glucose in solution forms a hemiacetal ring structure (pyranose)
2. Ring structure results from the attack of C5-OH on the anomeric C (C=O).
     a. produces anomers = molecules that differ only in orientation of anomeric C–OH
     b. change is spontaneous (no catalyst required)
     c. α-anomer has anomeric –OH directed down; β-anomer has anomeric–OH directed up
 

Fructose is a 6-carbon ketose.
1. Fructose in solution forms a hemiketal ring structure (furanose):

2. Locate the anomeric carbon, the –OH that attacks, and the α and β rings.

Derivatives of sugars = modified sugars
A. amine derivatives: –OH is replaced by –NH₂
B. phosphorylation: –OH is replaced by –O–PO₃^2-, frequently represented by
C. reduced sugars: C=O is reduced to CH–OH = sugar alcohol
     or CH–OH is reduced to CH₂ = deoxy-sugar

Oxidation of a reducing sugar
C=O is oxidized to COOH (accompanied by reduction of something else)

Glycoside bond = acetal formation by dehydration reaction between two –OH
glycoside bond is named by orientation of the anomeric C and
      by the numbers of the carbons involved going from non-reducing → reducing end

Examples of disaccharides:
                        
maltose   = Glc (α–1→4) Glc                              cellobiose = Glc (β–1→4) Glc

                         
lactose = Gal (β–1→4) Glc                                 sucrose = Glc (α–1→ β–2) Fru

Polysaccharides: large, indefinite weight macromolecules
starch: Glc linked by α–(1→4) bonds = amylose; with some 1→6 branches = amylopectin
glycogen: Glc linked by α–(1→4) bonds + significant α–(1→6) branches
cellulose: Glc linked by β–(1→4) bonds, unbranched; forms fibers (extended chains)

Nucleic Acids

Nucleotide structure
     A. nitrogenous base
         a. purine (double ring)
               adenine (A) or guanine (G)
                   shown anti , as in DNA
         b. pyrimidine (single ring)
               cytosine (C) , uracil (U) , or thymine (T)
     B. sugar: ribose (C's numbered 1′ → 5′) or, in DNA, deoxyribose at 2′ C
            or , always β
     C. phosphate bonded to 5′ C

ATP     c-AMP

Lipids

Defining characteristic of lipids: don't relate well to water (nonpolar).

Types of lipids
fatty acids
complex lipids (triacylglycerols, phospholipids, sphingolipids)
steroids
eicosanoids

Fatty acids = long chain hydrocarbons with a carboxyl group at C1
1. Structure
     a. usually an even number of C's
     b. types
          saturated (all C's connected by single bonds) = 16: 0
          unsaturated: double bonds are almost always cis
2. Chemical characteristics
     a. increasing length = decreasing solubility in water, increasing melting point
     b.. decreasing saturation (increasing the number of double bonds) = decreasing T_M.
3. Function = components of other lipids

Triacylglycerols (neutral fats)
1. Structure = glycerol + three different fatty acids (saturated or not) joined by ester bonds
2. Chemical characteristics
     a. nonpolar, so not water-soluble
     b. less dense than water
3. Function = energy storage and insulation

Phospholipids
1. Structure   = glycerol + 2 fatty acids + phosphate + alcohol bonded to phosphate
     a. fatty acids may vary in length and saturation
     b. phosphate may be attached to a variety of alcohols, including serine, glycerol, sugar
2. Chemical characteristics: amphipathic
     a. fatty acid chains are nonpolar (hydrophobic) = phospholipid tails ( )
     b. phosphate – alcohol are polar/charged (hydrophilic) = phospholipid head
3. Function = membrane structure – composition varies with species, type of cell, etc.

Sphingolipids
1. Structure = sphingosine + fatty acid + phosphate + alcohol bonded to phosphate
     sphingosine:
     fatty acid has an amide bond to sphingosine
     phosphate is added to the lower C–OH
2. Chemical characteristics like phospholipids
3. Function = membrane structure

Steroids = cholesterol derivatives
1. Structure:
2. Chemical characteristics: great variety of structures, mostly nonpolar
3. Functions = membrane structure, hormones, bile acids

Eicosanoids = paracrine hormones derived from arachidonic acid = C 20:4 (Δ 5, 8, 11, 14 )
types of eicosanoids
     prostaglandins have a C₅ ring; exist in several different types (PGE, PGF)
         many functions in inflammation, reproduction, blood flow, etc.
     thromboxanes are involved in clot formation
     leukotrienes = signals to cells, especially regulating smooth muscle contraction

top of page