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Glucose, fructose and mannose share the formula C₆H₁₂O₆, yet they are different sugars, and an enzyme can tell them apart. Explaining both facts took one of the great careers in chemistry. This post covers what Emil Fischer found, works through the stereoisomer count that his sugar work rests on, and ends with what you can learn from how he worked.
Who was Emil Fischer?
According to his Nobel Prize facts page, Fischer was born on 9 October 1852 in Euskirchen and died on 15 July 1919 in Berlin. He received the 1902 Nobel Prize in Chemistry, unshared, "in recognition of the extraordinary services he has rendered by his work on sugar and purine syntheses."
His Nobel biography adds a story worth knowing. His father wanted him in the family lumber business. After an unsuccessful trial, the father reportedly said his son was too stupid to be a businessman and had better be a student. Fischer took his PhD at Strasbourg in 1874 under Adolf von Baeyer, and early in his career discovered phenylhydrazine, a reagent he would use for years to identify sugars.
How did Fischer work out the structures of sugars?
The same biography summarises the sugar work. Fischer used phenylhydrazine to turn sugars into crystalline derivatives called osazones. Because glucose, fructose and mannose give a common osazone, he could link them. Between 1891 and 1894 he established the stereochemical configuration of all the known sugars and predicted the possible isomers, using the van 't Hoff and Le Bel theory of the asymmetric carbon atom. In 1890 he synthesised glucose, fructose and mannose starting from glycerol.
Your NCERT Class 12 Biomolecules chapter acknowledges this directly: after describing the reactions that show glucose is a straight six-carbon chain with an aldehyde group and five –OH groups, it says "the exact spatial arrangement of different —OH groups was given by Fischer".
Worked example: how many aldohexoses are there?
Open-chain glucose is an aldohexose: CHO–CHOH–CHOH–CHOH–CHOH–CH₂OH.
Find the chiral carbons. C1 is the aldehyde carbon and C6 is CH₂OH, so neither is chiral. C2, C3, C4 and C5 each carry four different groups. That is 4 chiral centres.
Each centre can have two arrangements, so the maximum number of stereoisomers is 2⁴ = 16.
These form 8 pairs of mirror images: 8 D-sugars and 8 L-sugars. D-glucose is one of the 8 D-aldohexoses.
This is the kind of counting Fischer needed to know which sugars could exist before matching each one to a real compound. For the exam version of these reactions and structures, see our post on biomolecules in chemistry.
What is the lock and key model?
In 1894 Fischer published Einfluss der Configuration auf die Wirkung der Enzyme ("The influence of configuration on the action of enzymes") in the Berichte der deutschen chemischen Gesellschaft. Asking why an enzyme acts on some sugar compounds but not on others that differ only in configuration, he compared the enzyme and the molecule it acts on to a lock and a key that must fit each other.
A centenary article by Frieder Lichtenthaler in Angewandte Chemie traces how Fischer reached the analogy. It also points out that Fischer, who distrusted theoretical speculation, did not push the metaphor further. In Lichtenthaler's translation, Fischer wrote that the hypothesis "can only be thoroughly tested, when we are able to isolate the enzymes in a pure state and thus investigate their configuration."
The NCERT Class 11 Biomolecules chapter gives the modern version: the substrate must bind the enzyme at its active site, in a cleft or pocket, forming a short-lived enzyme-substrate complex before the product is released. Our post on enzymes for NEET biology covers the kinetics and inhibition that follow from it.
Common mistakes students make here
"Same formula means same compound." Glucose, mannose and fructose all have the formula C₆H₁₂O₆. Structure and configuration decide identity.
Counting C1 or C6 as chiral. The aldehyde carbon and the CH₂OH carbon do not have four different groups in the open chain.
Treating lock and key as the full story. Fischer himself called it a hypothesis to be tested. Learn the active-site description in NCERT and use the analogy only as a picture.
If the idea of an active site still feels vague, the Fermi Concept Library has every concept in physics, chemistry, biology and mathematics with its own lesson, so you can fill that one gap and return to your chapter.
What can a student learn from Fischer's way of working?
His Nobel biography credits his "insistence on experimental proof of hypotheses". He built a tool, phenylhydrazine, that let him compare sugars reliably, then used it for years. And he labelled his best-known idea as a hypothesis until it could be tested. Try the same with your own guesses: write down what would prove you wrong.
Today, draw open-chain D-glucose, circle each chiral carbon and check that you get four.
