More light on the amino acid proline
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Due to its imino character and ring structure, proline alters the protein chain
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— Photo: S. Siva Saravanan
SAVIOUR: Salivary proline-rich proteins protect the mouth and teeth.
THE TAMIL proverb in tribute to the elephant states that this majestic pachyderm is worth a thousand sovereigns, alive or dead (Yanai Irundalum Ayiram Pon, Irandalum Ayiram Pon).
One could as well paraphrase this sentiment in tribute to proline. It is worth a great lot to the body, whether it occurs in the free form or strung together with other amino acids in a peptide or protein chain.
Beneficial properties
Proline molecules tend to aggregate together in water, and these assemblies offer some beneficial properties to proteins. They stabilise the native, functional shapes and structures of proteins, and prevent proteins from misfolding and clumping together.
Such protein clumps can cause havoc in cells, particularly in organs such as the brain or central nervous system, resulting in some neurological disorders.
This loss of a hydrogen atom in the proline nitrogen has interesting consequence to the structure and shape of protein molecules, which contain proline in their polymeric chain.
Thanks to its imino character and its ring structure, proline tends to play spoilsport in the protein chain. It prevents the protein from taking up the classical alpha-helical spiral shape (that muscle proteins are famous for), or the beta-pleated sheet form (that proteins of silk tend to adopt). Rather than allow the protein chain to take up a compact form, it tends to unwind and expose the chain to the environment. It is thus called a helix-breaker or a structure destabiliser.
Great functional value
Even the unshapely, randomly coiled noodle forms that proline-containing proteins tend to take are of great functional value.
Dr. Renee Borges of the Indian Institute of Science Bangalore drew my attention to a recent review (T. Shimada, Journal of Chemical Ecology (2006; 32, 1149-63) entitled `salivary proteins as a defence against dietary tannins', which highlights the role of what are called proline-rich proteins or PRPs.
Tannins are the brownish coloured, metallic tasting, sour-bitter components that are abundant in many plants. The common real-life example is encountered while brewing tea.
It tastes fine when brewed for a minute or two. Boil or brew too long — you get the awful tasting scum. That is a collection of tannins. Besides awful taste, they have the other nasty habit of binding to a variety of proteins and enzymes; reducing digestion and acting as toxins that affect the kidney and liver as well.
What then can a plant-eating animal like goats, cows and elephants do? Evolution has made sure that they have countermeasures against dietary tannins.
These come both in pre-ingestive (before eating) and post-ingestive ways. One of course is the thankfully awful taste, which is a deterrant. In addition, there are special enzymes and proteins that occur in saliva. These bind to the tannins and remove them by precipitation or breaking the tannins down.
A common feature of these proteins is that they are rich in proline, as well as three other amino acids, namely glycine, glutamine and glutamic acid. Together, they make up 70-90 per cent of the chain. Why are these four used to string up the PRP chains? The answer lies in their property of preventing the protein chain from taking up any ordered, compact or regular shape.
The resultant wet noodle shape allows many regions of the protein chain to be exposed and available to bind to the tannins and disable or destroy them.
Salivary PRPs are important in one other manner. They protect the mouth and our teeth. Teeth are exposed to a lot of variable, often stressful conditions — leading to wear and tear.
How nice would it be if teeth too can be frequently replaced as skin or hair is! It is here that salivary PRPs help us.
Stabilise, protect, repair
Our saliva is rich in calcium salts, which help stabilise, protect and repair tooth surface enamel in a balanced equilibrium. This equilibrium is maintained by the presence of acidic PRPs. In addition, we also need to lubricate food and tooth surfaces— both during the processing of food and reducing tooth wear. Some of the major lubricating molecules in the saliva are basic PRPs.
The story is not over yet. Recall that we animals came on earth as late entrants. Preceding us by millions of years are the insects.
Over these millennia of evolution, ancestral genes have been modified, chiselled and remodelled.
An outstanding example
One outstanding example is the evolution of immunity genes. While we have an enormous combinatorial library of them, thanks to these millennia in time, insects have had to do with a primitive model zero variety of molecules that offer protection against viral and bacterial attack.
Analysis of the antibacterial molecules in a variety of insects reveals that they belong to a class called short, proline-rich peptides.
They are not proteins, but shorter chains containing only 20-40 amino acids joined together. When we study the sequence of these chains, we find the motif proline-arguinine-proline repeated in them. These peptides penetrate the bacterial wall/membranes and disable two vital biochemical steps common to many bacteria.
Here again, the structure-breaking tendency of proline in the chain seems to be a decisive factor. Not for nothing is it a cyclic imino acid!
D. BALASUBRAMANIAN
dbala@lvpei.org
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