monosaccharide stereochemistry

Like all biological molecules, monosaccharides have a specific stereochemistry. Just as all amino acids are L-amino acids, all monosaccharides are D-sugars with respect to the base sugar (glyceraldehyde or dihydroxyacetone).

In my biochemistry class, we were required to be able to reproduce the Fischer projections of the major aldehyde and ketone sugars, with the proper stereochemistry. It's important to notice that carbon-1 in aldehyde sugars is ALWAYS HC=O, and carbon 1 in ketone sugars is ALWAYS CH₂OH. Likewise, the terminal carbon in the chain is ALWAYS CH₂OH. Also, as you're building sugars and going from 3 to 4 to 5 to 6 carbons, each addition will fall just after carbon 1. Thus, you just move the rest of the molecule down and stick in another carbon; therefore, the bottom two carbons ALWAYS look exactly the same. So, without further adieu...

Aldehyde Sugars

                                 HC=O
                                 HC-OH
                                H2C-OH
                            glyceraldehyde
                         /                  \
              HC=O                               HC=O
              HC-OH                            HO-CH
              HC-OH                              HC-OH
             H2C-OH                             H2C-OH
            erythrose                           threose
           /         \                        /        \
     HC=O               HC=O              HC=O           HC=O
     HC-OH            HO-CH               HC-OH        HO-CH
     HC-OH              HC-OH           HO-CH          HO-CH
     HC-OH              HC-OH             HC-OH          HC-OH
    H2C-OH             H2C-OH            H2C-OH         H2C-OH
    ribose           arabinose           xylose         lyxose 
   /      \           /      \          /      \       /      \
 HC=O     HC=O     HC=O     HC=O     HC=O     HC=O     HC=O    HC=O
 HC-OH  HO-CH      HC-OH  HO-CH      HC-OH  HO-CH      HC-OH HO-CH
 HC-OH    HC-OH  HO-CH    HO-CH      HC-OH    HC-OH  HO-CH   HO-CH
 HC-OH    HC-OH    HC-OH    HC-OH  HO-CH    HO-CH    HO-CH   HO-CH
 HC-OH    HC-OH    HC-OH    HC-OH    HC-OH   HC-OH     HC-OH   HC-OH
H2C-OH   H2C-OH   H2C-OH   H2C-OH   H2C-OH   H2C-OH   H2C-OH  H2C-OH
allose      |     glucose     |     gulose      |   galactose   |
         altrose           mannose           idose           talose

You'll notice that there's a lot of 6-carbon sugars, and you may think, "Oh, my God! That's impossible to remember!" Never fear. There's an easy trick. Take your right hand, and hold it sideways, with your thumb sticking up. Your thumb is carbon 1. Your four fingers are carbons 2, 3, 4, and 5. Carbon 6 would be below your hand, but it always looks the same way, anyway, so it doesn't much matter. Your pinky finger (carbon 5) will always be folded in towards your wrist (toward the right). The other three fingers are either pointing out (left) or in (right) depending on the structure. So, for instance, altrose would be with just your index finger sticking out, and mannose would be with your index and middle fingers sticking out. See how easy it is?

Ketone Sugars

           dihydroxyacetone
                 |
               H2C-OH
                 C=O
                HC-OH
               H2C-OH
            erythrulose
          /             \
     H2C-OH              H2C-OH
       C=O                 C=O
      HC-OH             HO-CH
      HC-OH               HC-OH
     H2C-OH              H2C-OH
    ribulose            xylulose
    /      \           /        \
H2C-OH    H2C-OH    H2C-OH    H2C-OH
  C=O       C=O       C=O       C=O
 HC-OH   HO-CH       HC-OH   HO-CH
 HC-OH     HC-OH   HO-CH     HO-CH
 HC-OH     HC-OH     HC-OH     HC-OH
H2C-OH    H2C-OH    H2C-OH    H2C-OH
psicose  fructose   sorbose  tagatose

And that, my friends, is your lesson on monosaccharide stereochemistry. Remember, if all else fails, you can always remember the structure for glucose. Just give your professor the finger.