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Abiogenesis enigma: Protein’s origin

Proteins

As you might know, proteins are one of the major “building blocks” of cells; there’s up to 10.000 different types of proteins, all manufactured inside each cell. Abiogenesis theorists  obviously supports the view that these molecules have arisen “by chance”, in a prebiotic world, billion years ago, however, to date, they have absolutely no clue about it, as we can read from this article:

“Proteins are the most complex chemicals synthesized in nature and must fold into complicated three-dimensional structures to become active. This poses a particular challenge in explaining their evolution from non-living matter. So far, efforts to understand protein evolution have focused on domains, independently folding units from which modern proteins are formed. Domains however are themselves too complex to have evolved de novo in an abiotic environment. We think that domains arose from the fusion of shorter, non-folding peptides, which evolved as cofactors supporting a primitive, RNA-based life form (the ‘RNA world’).” 1

So, why is it so complicated to explain its origin? Despite the often repeated innuendo that life and all of its components has “assuredly” originated through natural means, the clear failure of scientists to solve this puzzle can be easily explained by some truths about proteins, its synthesis, structure and so on. After that, no one can reasonably take its abiogenetic origin as logically granted. These truths also explain without shadow of doubt the intriguing fact that absolutely no single protein (even the lesser one, composed of only 8 amino acids) has ever been observed to appear anywhere in the world, outside the cells and high-tech labs, of course!

What’s a protein?

“Proteins are large biological molecules consisting of one or more chains of amino acids. Proteins perform a vast array of functions within living organisms, including catalyzing metabolic reactionsreplicating DNAresponding to stimuli, and transporting molecules from one location to another. Proteins differ from one another primarily in their sequence of amino acids, which is dictated by the nucleotide sequence of their genes, and which usually results in folding of the protein into a specific three-dimensional structure that determines its activity.

A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in a protein is defined by the sequence of a gene, which is encoded in the genetic code. In general, the genetic code specifies 20 standard amino acids;” 2

Talking about amino acids, we’d like to recall another crucial problem for abiogenesis: The absence of self-occurring homochiral mixtures. As it has been told in a previous article, the laws of thermodynamics obliges the occurrence of racemic mixtures, ever:

“The left and right handed forms have identical free energy (G), so the free energy difference (ΔG) is zero. The equilibrium constant for any reaction (K) is the equilibrium ratio of the concentration of products to reactants. The relationship between these quantities at any Kelvin temperature (T) is given by the standard equation:

K = exp (–ΔG/RT)

where R is the universal gas constant (= Avogadro’s number x Boltzmann’s constant k) = 8.314 J/K.mol.

For the reaction of changing left-handed to right-handed amino acids (L → R), or the reverse (R → L), ΔG = 0, so K = 1. That is, the reaction reaches equilibrium when the concentrations of R and L are equal; that is, a racemate is produced.”

Therefore, any abiogenetic theorist has this astounding problem to deal with from the very beginning; without homochiral monomers, we can have zero possibility of a ‘magic’ protein self-assembling…

 

Protein synthesis

 

It’s quite uncanny that intelligent people with advanced knowledge on the subject might attempt to conceive hypothesis of such molecules originating spontaneously, in the wild and morbid inorganic environment, because for cells to build proteins, an intricate, complex and laborious process must take place!

 

 

First, genetic information is needed:

“Proteins are assembled from amino acids using information encoded in genes. Each protein has its own unique amino acid sequence that is specified by the nucleotide sequence of the gene encoding this protein. The genetic code is a set of three-nucleotide sets called codons and each three-nucleotide combination designates an amino acid (for example AUG (adenine-uracil-guanine) is the code for methionine).”

 

Many proteins use more that one of the 64 possible codons to be built. Moreover, that specific genetic code must be first translated, transcribed:

“Genes encoded in DNA are first transcribed into pre-messenger RNA (mRNA) by proteins such as RNA polymerase. Most organisms then process the pre-mRNA (also known as a primary transcript) using various forms of Post-transcriptional modification to form the mature mRNA, which is then used as a template for protein synthesis by the ribosome.”

Oh great, a bit complicated, isn’t it? Please, read the Wikipedia article referring to the messenger RNA, for further comprehension of what it is, its manufacturing, composition, etc; all of which adds up more complexity for the protein origin’s explanation.

 

 

The process of synthesizing a protein from an mRNA template is known as translation. The mRNA is loaded onto the ribosome and is read three nucleotides at a time by matching each codon to its base pairing anticodon located on a transfer RNA molecule, which carries the amino acid corresponding to the codon it recognizes. The enzyme aminoacyl tRNA synthetase “charges” the tRNA molecules with the correct amino acids. The growing polypeptide is often termed the nascent chain. Proteins are always biosynthesized from N-terminus to C-terminus.[6]

The size of a synthesized protein can be measured by the number of amino acids it contains and by its total molecular mass, which is normally reported in units of daltons (synonymous with atomic mass units), or the derivative unit kilodalton (kDa). Yeast proteins are on average 466 amino acids long and 53 kDa in mass.[5] The largest known proteins are the titins, a component of the muscle sarcomere, with a molecular mass of almost 3,000 kDa and a total length of almost 27,000 amino acids.[8]

 

Phew! How complicated! You may ask now: are we finally done? And I reply you: Huh, nope! Now that the ribosome, together with the rRNA and more than 50 other proteins, has finally finished the process, a protein is formed. However, it is always found in a  random coil shape. So what? This shape is mostly useless for its usage on organism, as we can read:

Each protein exists as an unfolded polypeptide or random coil when translated from a sequence of mRNA to a linear chain of amino acids. This polypeptide lacks any stable (long-lasting) three-dimensional structure (the left hand side of the neighbouring figure). 3

In that randomly coiled shape, the protein is highly unstable, breakable, useless for cell building, so, for proper biological use and better stability, the protein folding process must take place. This 3D-shape is known as the native state.

The correct three-dimensional structure is essential to function, although some parts of functional proteins may remain unfolded.[4] Failure to fold into native structure generally produces inactive proteins, but in some instances misfolded proteins have modified or toxic functionality. Several neurodegenerative and other diseases are believed to result from the accumulation of amyloid fibrils formed by misfolded proteins.[5] Many allergies are caused by incorrect folding of some proteins, for the immune system does not produce antibodies for certain protein structures.[6]

Another importance of the protein folding is:

 

Minimizing the number of hydrophobic side-chains exposed to water is an important driving force behind the folding process.[9] Formation of intramolecular hydrogen bonds provides another important contribution to protein stability.[10] 

 

And how does the folding occurs?

 

 

The amino-acid sequence of a protein determines its native conformation.[7] A protein molecule folds spontaneously during or after biosynthesis. While these macromolecules may be regarded as “folding themselves“, the process also depends on the solvent (water or lipid bilayer),[8] the concentration of salts, the pH, the temperature, the possible presence of cofactors and of molecular chaperones.

The process of folding often begins co-translationally, so that the N-terminus of the protein begins to fold while the C-terminal portion of the protein is still beingsynthesized by the ribosome. Specialized proteins called chaperones assist in the folding of other proteins.

Although most globular proteins are able to assume their native state unassisted, chaperone-assisted folding is often necessary in the crowded intracellular environment to prevent aggregation; chaperones are also used to prevent misfolding and aggregation that may occur as a consequence of exposure to heat or other changes in the cellular environment.

There are two models of protein folding that are currently being confirmed: The first: The diffusion collision model, in which a nucleus is formed, then the secondary structure is formed, and finally these secondary structures are collided together and pack tightly together. The second: The nucleation-condensation model, in which the secondary and tertiary structures of the protein are made at the same time. Recent studies have shown that some proteins show characteristics of both of these folding models.

The essential fact of folding, however, remains that the amino acid sequence of each protein contains the information that specifies both the native structure and the pathway to attain that state. Folding is a spontaneous process independent of energy inputs from nucleoside triphosphates. The passage of the folded state is mainly guided by hydrophobic interactions, formation of intramolecular hydrogen bonds, and van der Waals forces, and it is opposed by conformational entropy.

Only after the folding process, we have an useful, stable protein, with a properly designed shape with its up to four layers, so that the molecule can perform its biological function.

But, remember, many conditions and external factors can destroy proteins, such as hydrolysis (it’s a slow, but ceaseless process, because proteins are metastable, hydrophobic) and others:

Under some conditions proteins will not fold into their biochemically functional forms. Temperatures above or below the range that cells tend to live in will cause thermally unstableproteins to unfold or “denature” (this is why boiling makes an egg white turn opaque). High concentrations of solutes, extremes of pH, mechanical forces, and the presence of chemical denaturants can do the same.

A fully denatured protein lacks both tertiary and secondary structurel. Under certain conditions some proteins can refold; however, in many cases, denaturation is irreversible.[15] Cells sometimes protect their proteins against the denaturing influence of heat with enzymes known as chaperones or heat shock proteins, which assist other proteins both in folding and in remaining folded. Some proteins never fold in cells at all except with the assistance of chaperone molecules, which either isolate individual proteins so that their folding is not interrupted by interactions with other proteins or help to unfold misfolded proteins, giving them a second chance to refold properly. This function is crucial to prevent the risk of precipitation into insoluble amorphous aggregates.

 

For a further an in-depth study about different factors capable of disrupting proteins, read the following articles:

https://en.wikipedia.org/wiki/Protein_biosynthesis

http://creation.com/native-folds-in-polypeptide-chains-1 (a series of 6 parts)

 

To conclude our observation, it’s impossible not to be sceptic of any theoretic proposition that claims self-caused origin of proteins, because it turns out that science unveiled tons of facts that easily prevent any possibility of such proposed scenario:

 

-Absence of homochiral monomers forming in the environment;

-Necessity of genetic specific information;

-Need for an highly controlled ambient, with proper Ph level, temperature, absence of mechanical forces that may easily damage, disrupt the protein, toxins, etc; 

-Need for specific methods to protect the protein against hydrolysis, oxidation;

-Necessity of having 50 other types of protein already manufactured to help on the protein synthesis;

 

The question raises: how in the world could such a specific set of conditions be found in a prebiotic Earth? Such condition can only be barely found in a first-class laboratory, driven by qualified and experienced scientists!

You might as well enjoy watching this short video talking about protein synthesis:

 

References

1. <http://www.eb.tuebingen.mpg.de/research/departments/protein-evolution/protein-evolution.html>

2. <https://en.wikipedia.org/wiki/Protein>

3. <https://en.wikipedia.org/wiki/Protein_folding>

3 responses to “Abiogenesis enigma: Protein’s origin

  1. Lorraine August 5, 2013 at 14:24

    Greetings! Very useful advice within this post! It is the
    little changes that make the biggest changes. Many thanks for sharing!

  2. alysdexia July 25, 2013 at 22:13

    “Therefore, any abiogenetic theorist has this astounding problem to deal with from the very beginning; without homochiral monomers, we can have zero possibility of a ‘magic’ protein self-assembling…”

    You don’t know that. The other halves stay inert.

    You should ask other bloggers to answer your challenges.

    • adonizedek July 25, 2013 at 22:28

      That’s what the scientific experiments have demonstrated with crystal clear sureness! What can we do other than sticking to the evidences? That’s how the science works..

      Read my other article: “Oh homochirality”, which sheds more light in that homochirality issue..

      Be welcome, anyway, thanks for commenting!

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