Introduction to the World of Proteins
The shape, structure and function of the human body is one of the Nature’s marvels. The fertilisation of an egg by a sperm to the growth of a whole human body involves numerous steps of growth and differentiation. When we breathe, we feel a sense of oxygen flowing through our lungs and racing in our blood vessels, to be delivered to all our tissues. While we flex our muscles, we can feel them first tightening and then relaxing. The molecules involved are proteins, haemoglobin which transports oxygen, collagen which provides the strength to our bones and extracellular tissue and actin, myosin and several others which help in muscle contraction (Fig. 1). It is noteworthy that among the biomolecules you have studied, proteins have the maximum diversity in function. The key to this enormous diversity is the unique structure of proteins. Although all proteins are made up of 20 different amino acids the sizes and sequence combinations and variations of each protein leads to millions of unique 3-D structures and thereby functions. Scientists have been striving to relate protein structure with function and hence the first step would be to determine 3-D structure of a protein.
Fig. 1. Proteins having multiple roles
Even more amazing is the structure and data processing abilities of the human brain. We tend to marvel at the incredible speed and processing functions of the super computers little realising the creativity of the human being who invented them. The speed and correlation of sensory stimulation is unique to the human brain which can grasp the diversity of sensory inputs and convert them to learning and memory for later application. What are these proteins which enable these functions and why are some brain related diseases like Alzeimers occurring, in which certain proteins show abnormal structure and behaviour?
A number of human diseases are due to the deficiency or abnormal structure of proteins. The lack of a particular subunit, alpha or beta of the oxygen carrying protein haemoglobin results in Thalassaemia, a devastating disease in which an infant cannot grow without repeated transfusions. If the beta chain is present but with a substituent in one of the amino acid residues another debilitating condition called Sickle Cell Anaemia results which is endemic to certain parts of Africa. The absence of an enzyme- Adenosine deaminase results in the birth of a severely immunocompromised baby who cannot last infancy (SCID). More recently it has been discovered that certain "rogue proteins" whose structure has been altered can result in diseases such as the Mad cow disease wherein the disease itself appears to be propagated by infectious proteins called "prions". Clearly proteins need to be understood in detailed terms.
The completion of the Human Genome Sequence has revealed about 35,000 genes. However the actual number of proteins encoded by these genes may be many more due to posttranscriptional modifications. Different cells have specialised proteins for their unique functions in addition to the housekeeping proteins required for metabolism and generation of ATP. Sometimes these proteins are secreted to the outside like the proteolytic enzymes from the pancreas or hormones from ductless glands like the pituitary. We are yet to identify all the proteins required for a body to function and this presents a challenge to the future biotechnologists. One of the outcomes is the merging field of protein structure and function- proteomics.