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Understanding Proteins and Protein Related Disorders
Proteins are called as the “building blocks of body” which has significant role in all cellular functions contributing to the overall function of the body. Proteins are the key structures in all biochemical and metabolic pathways of a human body. The dominance of proteins in cell functions is highly commendable, which lead to the development of various fields like proteomics and protein biotechnology enabling the complete understanding and study of proteins under one roof.

The basic units of the proteins are called as amino acids. One or more amino acids linked by peptide bond contribute to the structure of protein. The amino acid sequence is specific for each protein and the sequence of amino acids is controlled by gene sequence. There are twenty different amino acids and they are Glycine, Alanine, Leucine, Isoleucine, histidine, methionine, phenylalanine, cysteine, glutamine, glutamic acid, asparagines, aspartic acid, proline, serine, threonine, tryptophan, tyrosine, valine, arginine and lysine. The various combination of these amino acids result in a specific type of protein. Thus any alteration or absence of even a single amino acid in an amino acid sequence results in protein dysfunction.

Proteins are classified based on their structure, function and composition. Proteins are grouped under primary, secondary, tertiary and quaternary types based on their structure. The factors like solubility and shape classifies proteins as fibrous, globular and intermediate proteins. Some examples of fibrous proteins are elastin, collagen and keratin which are insoluble in water. Globulin and insulin are globular proteins, soluble in water. Intermediate proteins depict the nature of fibrous proteins in shape and solubility like globular proteins. Fibrinogen is a classic example of an intermediate protein.

Based on the protein composition, they are grouped into simple and conjugated proteins. The simple proteins are the ones which are made up of only amino acids whereas the conjugated proteins represent the presence of proteins along with other molecules. The albumins, globulins, histones and scleroproteins are simple proteins. The lipoproteins, glycoprotein, nucleoprotein, chromoprotein phosphoprotein, flavoprotein and metaloprotein are all conjugated proteins. Based on their function proteins are grouped under categories like enzyme, storage, transport, hormonal, receptor, contractile, defensive and genetic proteins.

Most of the proteins are synthesized in the human body by transcription of DNA into mRNA (messenger RNA) and translation of mRNA into amino acids (proteins) with the help of tRNA (transfer RNA). Some proteins are derived from the dietary supplements like meat, egg, milk, vegetables, pulses and nuts. The deficiency or dysfunction of the bodily synthesized protein and deficiency of dietary protein causes various diseases.

Since all the synthesis and sequence of amino acids are controlled by genes, any mutation in the gene causes either dysfunction of the protein or absence of the protein. As a result various diseases and syndromes develop. Marfan Syndrome (dysfunction of fibrillin protein due to mutation of FBN1 gene), Huntington disease (dysfunction of huntingtin protein resulting in excess glutamine), Hereditary hemochromatosis associated with the HFE protein, Alzheimer’s disease are some of the diseases identified as a result of protein dysfunction due to gene abnormality or mutation. Transmissable spongiform encephalopathies like mad cow disease, Creutzfeld Jakob disease (prion disease), cancer, cystic fibrosis are diseases due to misfolding of proteins or absence of correctly folded proteins. Phenylketonuria is a condition in children whose ability to degrade the amino acid phenylalanine is disturbed.

Kwashiorkor, Marasmus and Marasmic- Kwashiorkor (grouped under Protein-Energy malnutrition) diseases are the mainly discussed dietary protein deficiency related disorders. To combat these disorders, required quantity of protein intake through diet is suggested based on the severity of the disease. Also excess intake of protein poses problems like ketosis, kidney and liver related disorder and so on. The other diseases like hypertension, diabetes which imposes stress on kidney resulting in kidney failure is associated with a condition called proteinuria where the essential proteins passes through kidney and appears in urine.

The protein related disorders due to dietary intake is well managed. If the disease is due to malnutrition of protein, then the patient is given protein rich diet or if it is due to excess intake of protein then the patient is restricted of protein intake. Whereas the treatment of genetic related protein disorders or syndromes is quite challenging and the subject of gene therapy should be understood and considered for treatments.
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The word metabolism refers to all the biochemical reactions and pathways that take place inside the living organism. The enzymes play a critical role in a lot of metabolic processes and any alteration in their function, due to any genetic mutation, can result in malfunctioning pathways. Genetic defect that hinders the functioning of any of the key metabolic pathways is referred to as an inborn error of metabolism or congenital if it is present from birth.

Maple Syrup Urine Disease or MSUD is a harmful metabolic disorder and the individual with it has an altered mode of breaking down the three amino acids, namely isoleucine, leucine and valine. The three amino acids are usually broken down by a complex of six proteins that act collectively called as BCKD (that is Branched Chain Alpha- Keto Acid Dehydrogenase). Individuals who possess the MUSD mutation have a deficiency in any one of the six proteins that form the BCKD complex and therefore are not able to break down the three amino acids isoleucine, leucine and valine. This results in the elevated levels of accumulation of the three amino acids in the blood causing the speedy degeneration of brain cells and eventually death if untreated. In majority of the cases, the defect is a result of mutation in a gene that is present on the chromosome 19.Children who have the maple syrup urine disease are also not able to metabolize these amino acids. The build up of the amino acids and certain bi- products causes neurologic disorders such as seizures and mental disability. The accumulations cause body fluids, such as urine, to smell like unique sweet odour as a maple syrup.

Tyrosinemia is a metabolic error as a result of deficiency of an enzyme that is essential for the metabolism of the amino acid tyrosine. The common form of this metabolic disorder affects the vital organs such as the kidneys and the liver. Children having tyrosinemia are incapable of metabolizing tyrosine completely. Accumulation of the bi- products of the amino acid tyrosine results in a wide variety of symptoms. The disorder Tyrosinemia is inherited in an autosomal recessive manner and it can be detected in the newborns with the aid of screening tests.
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Homocystinuria is another metabolic disorder and is associated with the metabolism of the amino acid methionine. It is caused by the deficiency of the enzyme Cystathionine-β-synthase (or CBS) that is essential to metabolize homocysteine. This disorder causes a variety of symptoms that includes decreased vision and CNS (central nervous system) abnormalities. The disorder is inherited as autosomal recessive. Homocystinuria exemplify the hereditary metabolic defect described by the build-up of homocysteine in the blood serum and its elevated excretion in the urine. Newborns show up to be normal and the early symptoms are unclear.

Alkaptonuria is a rare metabolic condition due to the defect in the metabolism of the amino acids phenylalanine and tyrosine. It is an inherited disorder in which an individual’s urine changes from dark brown to black colour when it is exposed to air. The disorder results due to the faulty enzyme homogentisate 1,2- dioxygenase which is involved in the breakdown of the amino acid tyrosine. As a consequence homogentistic acid and alkapton (oxide of homogentistic acid) build-up in the blood and get expelled in the urine in huge amounts. The presence of homogentistic acid in excess causes harm to the cartilage and also causes kidney stones. Defect in the HGD gene results in alkaptonuria.

Phenylketonuria (PKU) is one of the most typical metabolic disorders of protein. The disorder is due to the non functional enzyme called phenylalanine hydroxylase (PAH). This enzyme is essential in the metabolism of phenylalanine to tyrosine. Therefore, as a result, the amino acid phenylalanine builds-up in the blood and gets converted to a phenylketone (phenylpyruvate) which gets excreted in the urine. The disorder is inherited in an autosomal recessive manner. If the disorder is untreated it results in the symptoms such as Eczema, Lethargy, Hyperactivity, Seizures and Intellectual disability. The newborn screening for the disorder is usually performed in most of the nations.
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Understanding Proteins and Protein Related Disorders00