Chemistry. How Heavy Metals Affect Protein Folding

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To make proteins, ‘machines’ known as ribosome chemically string together amino acids into long, linear chains. Like shoelaces, these chains loop about each other in a variety of ways (i.e., they fold). But, as with a shoelace, only one of these many ways allows the proteins to function properly. Yet lack of function is not always the worst scenario. For just as a hopelessly knotted shoelace could be worse than one that won’t stay tied, too much of a misfolded protein could be worse than too little of a normally folded one. This is because misfolded proteins can actually poison the cells around it (Anfinsen, 1972).

As a group, heavy metals include both those essential for normal biological functioning (e.g. Cu and Zn), and non-essential metals (e.g. Cd, Hg, and Pb). Both essential and non-essential metals can be present at concentrations that disturb normal biological functions, and which evoke cellular stress responses. The cellular targets for metal toxicity include tissues of the kidney, liver, heart, immune response and nervous system. Intriguingly, manipulations of specific metals, their reservoirs, and the cellular stress response can have therapeutic effects on certain diseases (Mitani, et al. 1993)

Amino acids are proteins primary structures. The primary structure of a protein moves to work on it to form its unique structure. This process is known as folding. They will fold spontaneously during or after synthesis. Folding does not happen by the act of amino acids only but also when other conditions are at a correct level, these are temperature, concentration and chaperone activity. (Lee & Tsai, 2005).

The amino acid sequence of each protein contains unique information that work to produce individually unique structures and pathways as too specific or particular protein. Amino acid sequences don’t always fold similarly because of different conditions they occur in. Environmental conditions affect the process as similar proteins fold differently based on where they are found. Nucleoside triphosphates produce energy for the independent process.

Hydrophobic interactions, provide the basis of attachment for this molecules and intramolecular hydrogen bonds. Protein folding may involve covalent bonding in the form of disulfide bridges formed between two cysteine residues or the formation of metal clusters. Before settling into their more energetically favorable native conformation, molecules may pass through an intermediate state. Heavy metals attach to the desulphated bridges causing misfolding (Anfinsen, 1972).

The protein is widely expressed in highly homologous forms, suggesting that the roles played by metallothionein are essential to a variety of biological processes, and that the protein has a long evolutionary lineage. Metals have long been known to evoke changes in the immune response. These changes are dependent both on the specific metal and metal valency, as well as on the genetic makeup and physiological status of the exposed individual.

Although this small stress response protein (Metallothionein) is not a member of heat shock protein family, it serves many roles in both normal and stressed cells, acting as a reservoir of essential heavy metals (e.g., Cu+2 and Zn+2), as a scavenger for both heavy metal toxicants (e.g., Hg+2, Cd+2) and free radicals, and as a regulator of transcription factor activity. Metallothionein is induced by a range of different agents, including heavy metals, reactive oxygen species, glucocorticoids, acute phase cytokines and interferon, and by endotoxin. Traditionally considered an intracellular protein, recent work has suggested important roles for metallothionein both in intracellular compartments and as an extracellular agent (Pace et al. 1996).

The changes induced in leukocyte behavior can result in immunosuppression and a consequent increase in susceptibility to infectious disease. These changes can also result in an autoreactive immune response that subsequently damages self tissues.

Focused is on the mechanisms of metal-mediated immune modulation that are influenced by basal levels of metallothionein and by those levels of metallothionein that are induced by inflammation, autoimmune disease and toxic metal exposure. We have also explored the potential opportunities this immunomodulation provides both for the management of disease that results from toxic metal exposures, and for the management of diseases that are accompanied by metallothionein synthesis as a consequence of cellular stresses that are associated with infection, inflammation or other biological stressors (Borghesi, et al. 1996).

Livers are used to detect heavy metal concentration as it is found to be of the highest metal accumulation point compared to other tissues. The total concentrations of six heavy metals: Zn, Cu, Cd, Pb, Cr and Ni are detected by Flame AAS. If we consider Cu and Zn concentrations to be high, we analyze their sub cellular distribution. Fractions of cytosol, nucleus, microsome, mitochondria and plasma membrane from the livers are obtained by high speed and ultra-centrifugation. Their Cu and Zn contents are also determined by Flame AAS which found they are highest in cytosolic fractions. proteomic approach to study the copper binding proteins. Separation of cytosolic fraction by gel-filtration chromatography reveals the presence copper (Mitani, et al. 1993).

Bibliography

Anfinsen C (1972). “The formation and stabilization of protein structure”. 737-49.

Borghesi, L. A., et al. 1996. Interactions of metallothionein with murine lymphocytes: plasma membrane binding and proliferation. 129-40.

Lee S, Tsai F (2005). “Molecular chaperones in protein quality control”. 259-65.

Martin, J. 2000. Group II chaperones as mediators of cytosolic protein folding. 309-24.

Mitani, K., et al. 1993. The role of inorganic metals and metalloporphyrins in “The induction of haem oxygenase and heat-shock proteins 70 in human hepatoma cells.”819-25.

Pace C, Shirley B, McNutt M, Gajiwala K (1996). “Forces contributing to the conformational stability of proteins”. 75-83.

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