Monthly Archives: May 2005

Hargobind Khorana

In a recent discussion with some friends the topic drifted to Indian Nobel Prize winners. Almost everyone seemed to know about C.V Raman, or Rabrindranath Tagore and about Subramanian Chandershekhar and also their contributions. But very few of us knew much about Dr Hargobind Khorana and his contributions towards Science. In today’s VERITAS we will try to correct this situation.

 

        Dr Hargobind Khorana was born in 1922 at Raipur, Punjab ( this is now in Pakistan). He was born in a poor family. His father was a “patwari”

in the village. A “patwari” is an agrigulture tax collector. This is a post that is still common in Punjab villages. He was the youngest in the family of 5 children. Hargobind Khorana attended D.A.V high school in Multan. Later he studied in Punjab University at Lahore and got the MSc degree. He got a scholarship to study in England. He joined the University of Liverpool and got the Phd degree in 1948. IN 1949 he returned to India. He tried desperately to search for a job but he could not find one. He returned to England and joined Cambridge.

 

        At Cambridge Hargobind Khorana became interested in proteins and nucleic acids(DNA and RNA). He moved to Canada in 1952 and joined the University of British Columbia. In 1968 Khorana received the Nobel Prize in Physiology and Medicine along with Nirenberg and Holley. They were awarded the Nobel Prize for their research which led to deciphering of  the Genetic Code and helped understand its function in protein synthesis.

 

        Lets understand this in more detail. In 1944 Avery showed that genes are made up of Nucleic acids. Both DNA and RNA are composed of 4 nucleotide bases. DNA is composed of adenine(A), Guanine(G), Cytosine(C) and Thymine(T). DNA is composed of strings of these bases. So a DNA strand may look like AATGGGTTACCC etc. Nirenberg was the first to show that each set of 3 bases like AAA or GGG etc forms an amino acid. He formed an artificial DNA by constructing a string of a single repeating base. So Nirenberg showed how DNA/RNA is composed of repeating bases. Khorana developed this work independently and decipered the codes for several known amino acids. He discovered for example that the amino acid Glycine was  GGA etc. He did this by constructing giant nucleic acids( giant DNA and RNA) with complicated base strings. Holley succeeded in preparing something known as transfer-RNA and determined its exact structure.

Transfer-RNA is a special type of nucleic acid. It has the ability of reading genetic code from somewhere else and transforming it to form a protein.

So Hargobind Khorana was one of the scientists who succeeded in understanding the alphabet which is used to create DNA and RNA. He also managed to create DNA and RNA in his laboratory. So Hargobind Khorana is one of the pioneers of Genetic Engineering.

        Hargobind Khorana is still doing active research. I went to his webpage in the “faculty and research” section of MIT’s chemistry department. He is currently doing research on molecules that act as receptors of light in the eye- Rhodopsin, 11-cis-retinal, all-trans retinal( See VERITAS: Light and Life Part II, May 12, 2005 for details on these molecules and the bio-chemical mechanism of sight).

 

Kanwar

 

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  Go, wondrous creature! mount where Science guides:                 

  Go, measure earth, weigh air, and state the tides:                 

  Instruct the planets in what orbs to run,                          

  Correct old time and regulate the Sun;                             

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Light and Life – Part 2

This VERITAS was written on Thursday, May 12, 2005

 

Friends,

       This is part II of the VERITAS series in which we try to understand the complex relationship between light and life. In today’s part we will try to understand how light from outside us can create the sensation of vision. We will not try to understand the eye in detail. Our aim today is to understand the moment when the physics of the photon(particle of light) causes a biological(or chemical) sensation in our eye.

       The innermost layer of the eye is retina. The retina contains two kinds of cells: the rods and the cones. The rods help us see in low light.
And the cones help us perceive colour. The rod and cone cells contain a chemical called rhodopsin. This chemical is photosensitive. Rhodopsin is composed of two chemicals: scotopsin and 11-cis-retinal. Retinal is also called retinine. It looks something like this:

<something>-C=C-C=C-C=C-C=C=O
               | |
               H H

       We have written <something> for the part that does not interest us. The part that interests us is a series of Carbon atoms(written as C)
with alternate double and single bonds. The double means that there is an extra electron there. We have represented a double bond as = and a single bond as – . When a photon of light strikes the above molecule the electron in the double bond gets bumped. So for a short time the double
bond gets converted to a single bond. This allows the Carbon atoms to rotate. The molecule gets converted to something like this:
                 H
                 |
<something>-C=C-C=C-C=C-C=C=O
               |
               H

       There is no difference except the structure. The C-H have rotated. AFter rotation the double bond gets formed again and the C-H are locked in this new position. All this takes about 10 pico seconds.

      http://www.elmhurst.edu/~chm/vchembook/images/533cistransretinal.gif

The main point is that when 11-cis-retinal absorbs light its structure changes to form all-trans retinal. This creates an electrical impulse which is transmitted to the brain.

       11-cis-retinal and all-trans retinal are isomers: different structures of the same molecule.

       So we see that the retinal is very important for the sensation of light. How do we produce retinal? We dont!!!! We have to eat it! Vitamin A
gets convetred to retinal. Actually vitamin A looks just like retinal we showed above. There is just a extra Hydrogen atom which is removed by the body. It is extremely important that you eat your Vitamin A. Otherwise you could become blind when the number of photons around you are less (Night Blindness). So eat your carrots!

Media_httpwwwelmhurst_cwgyj

        Now we have seen when retinal gets hit by a photon the double bonds get bumped and that creates an electric sensation of light which is
transmitted to the brain. Chemically we say that when 11-cis-retinal gets hit by a photon it gets converted to all-trans retinal. But what happens when all 11-cis-retinals get converted? It never happens. There is a reverse process too. Very soon the all-trans retinal gets converted back to 11-cis-retinal. SO there is never a lack of the 11-cis-retinal. This reconversion of all-trans retinal to 11-cis-retinal happens very fast and it is a spontaneous process. It happens because all-trans retinal is a unstable molecule and 11-cis-retinal is a stable one.

       Friends! We have understood it! We have understood the process of the sensation of light. But have we? We have not. We have only understood the process, or the “how”. We have not understood the “why” of it. We should we have a vision at all? Why only retinal and not some
other chemical? Why only photons? These are the questions that we may never find answers to.

       I often use the example of a group of apes. Take a clock and throw it to a set of apes on an island. The apes will be very curious about the clock. They will try to understand it. After years of effort they will realize that it has a pattern: The clock hands move round and round. Centuries later… after several generations they may have invented numbers and they will know that for 12 rotations of the long hand the small hand moves one step. But it will take them a millions of years to discover the true meaning of the clock. Because to understand the true meaning they will have to discover the concept of time! And till they discover that concept their knowledge is merely a collection of facts. Why did this happen? This happened because the apes were trying to understand something created by a superior intellect. They would have to reach a superior intellect before they can understand the true meaning. COllection of facts is easy. Understanding the meaning is difficult.

       We find ourselves in a similar position. We are trying to understand a Universe created by a “superior intellect”. And we are struggling to understand it. Our intelligence would have to reach that of the Creator himself before we really understand. Till then we will have to be content with “collecting mere facts”.

Kanwar

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 Go, wondrous creature! mount where Science guides:
 Go, measure earth, weigh air, and state the tides:
 Instruct the planets in what orbs to run,
 Correct old time and regulate the Sun;
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Veritas by Kanwarpreet Grewal is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.