Biological Polymers Have Different Structure; They Also Have Different Functions

Biological polymers have different structure; they also have different functions. Describe how the structures of different biological polymers are related to their functions.

(word count 1097)

Biological polymers are traditionally grouped into four categories: proteins, poly/disaccharides, polynucleotides and lipids, although the latter could be argued against. Understanding their structure and functions is one of the greatest achievements of science and a very active research domain.

Proteins are composed of amino-acids (AA), attached through a peptide bond, in a condensation reaction. Each AA has unique chemical properties and influences the protein’s shape, that impacts on the function. For example, chains containing many nonpolar AA tend to fold into the centre of the protein by hydrophobic exclusion (e.g. globular proteins).

Haemoglobin is a globular protein, found in red blood cells. Structurally is a tetramer, with four polypeptide chains: 2α and 2β. A haem group containing a central iron is attached to each chain, therefore haemoglobin can carry 4 oxygen molecules. This structure leads to the molecule shaping like a ‘globe’, with a hydrophobic centre and hydrophilic ‘circumference’, which makes it ‘soluble’. It acts as a two-way respiratory carrier, transporting oxygen to the tissues and facilitating the return transport of carbon dioxide (Marengo-Rowe, 2006).

Figure from http://www.mhhe.com/biosci/genbio/raven6b/graphics/raven06b/other/raven06b_03.pdf

However, genetic mutations leading to glutamic acid substituted by valine (Raphael,2005) change the shape of haemoglobin from globular to half-moon in sickle cell anaemia. These ‘mutant’ polymers aggregate, causing vascular occlusion and thrombosis (Bennett, 2006).

Keratin is a fibrous protein which consists of repetitive sequences of hydrophobic AA, thus insoluble. Unlike globular proteins that contain weak hydrogen bonds and are easily denatured, fibrous proteins have stronger bonds, such as covalent disulphidic bonds, alongside ionic and hydrogen bonds.

Figure from http://www.texascollaborative.org/hildasustaita/bonds.gif

There are two structures, α-helixes and β-sheets.

α-helixes, found in cytoskeleton and hair are twisted structures, rich in cysteine, with bonds that keep the helix stable Being a helix, this keratin is flexible.

β-sheets, found in horns and hooves, have a planar configuration. They are rich in glycine, alanine and serine, and its planar structure with covalent bonds give great rigidity.

Polysaccharides consist of monosaccharides joined by glycosidic bonds.

Starch contains 70-80% amylopectin and 20-30% amylose, both polymers of α-glucose (Cummings and Englyst,1995)

Amylose is a linear chain of α-D-glucose in 1-4 linkages with an overall spiral shape.

Amylopectin is highly branched with 1-4 linkages (in un-branched chains) and 1-6 linkages (as branch points). Branches occur at every twelve to thirty residue.

Amylose provides energy storage for plants and helps starch products thicken. When starch is heated in water, the hydrogen bonds are broken and water enters into the starch molecule and the entire molecule thickens and ramifies.

Cellulose is a linear chain of 1-4 linked β-glucose (Festucci-Buselli et al, 2007), unlike amylose which is 1-4 linked α-glucose. The molecules are arranged parallel to each other and joined together by hydrogen bonds in long, cable-like structures. This provides support and allows plants to stand up right and paper to hold its shape.

Because of its strength, it is impossible to digest in humans, but is important in aiding digestion, under the pseudonym “fiber”. Animals can, however hydrolyze cellulose with the help of bacterial enzymes.

Retrieved from http://chemwiki.ucdavis.edu/Core/Biological_Chemistry/Carbohydrates/Polysaccharides/Cellulose

Glycogen is a human’s way of maintaining glucose homeostasis. When blood glucose is high, glucose is stored as glycogen in the liver, which is easily mobilized when the blood glucose is low. This process has a fast turnover and this is because of its very branched structure. Moreover, skeletal muscles also store glucose as glycogen, which is later used during intensive physical exercise.

Retrieved from http://chemwiki.ucdavis.edu/Wikitexts/Sacramento_City_College/SCC%3A_Nutri_300_(Coppola)/04%3A_Carbohydrates/4.4%3A_The_Functions_of_Carbohydrates_in_the_Body

Structurally, glycogen is an α-glucose polymer with 1-4 linkages (in un-branched chains) and 1-6 linkages (as branch points). Branches occur at every tenth residue (Berg et al, 2002). It is similar to amylopectin, but the ramifications are much denser. Branching gives glycogen two main advantages. First, it is more soluble than its unbranched relative, amylopectin. And secondly, the greater number of free ends means that glycogen can be synthesised and degraded very quickly, in hours.

Trehalose is a disaccharide with two glucose α, α-1,1-glycosidic linked (Elbein et al, 2003). As the reducing end of a glucosyl residue is connected to the other, trehalose has no reducing power. It occurs naturally in microorganisms, plants and animals but it does not exist in mammals, where glucose exists instead.

Because it is chemically stable and has a high melting point, there is growing interest over its use in stabilizing proteins and life and opposing ageing.

Butterfly larvae survive temperatures of minus 40 degrees, without additional glycerol as antifreeze, because of it. Similarly, the water bear withstands the harshest of environments, by maintaining intact all the DNA, membranes and cells. It becomes very similar to a bacterial spore and once favourable conditions return, all it takes to revive the water bear is water. This will dilute the glassy trehalose and gently release the molecules from their suspended state (Ritter, 2012).

Polynucleotides, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), contain many nucleotides, that are formed by:

a carbohydrate (2-deoxyribose in DNA and ribose in RNA)

a phosphate group

a nitrogenous base – adenine (A), guanine (G), cytosine (C), and thymine (T) in DNA and adenine (A), guanine (G), cytosine (C), and uracil (U) in RNA

Nucleotides join through a condensation reaction between the phosphate group of one nucleotide and the hydroxyl group of another, forming a sugar-phosphate backbone with bases attached at every step of the ‘ladder’.

DNA occurs in every single life form and its main role is to store genetic information that encodes for protein synthesis.

In 1953, Watson and Crick proposed a double helix structure for DNA, with two polynucleotide chains that run 'antiparallel' to each other, and nitrogenous bases projecting inwards. A double bonds T, C treble bonds G through hydrogen bonds. Yet is enough to maintain the DNA shape, these bonds are easily broken by RNA polymerase for the transcription of mRNA.

In eukaryotic cells, DNA coils around histone proteins and packs tightly into a chromosome (Alberts et al., 2002), condensing a lot of information in a small area.

Retrieved from http://www.bbc.co.uk/education/guides/z36mmp3/revision

Messenger ribonucleic acid (mRNA) is a chemical "blueprint" for protein production, carrying the coding information from a DNA template to the ribosomes, where the transcription into proteins occurs. In ribosomes, transfer RNA (tRNA) binds on one end to specific codons in the mRNA and on the other end to the AA specified by that codon.

mRNA is synthesized on an anti-sense DNA strand in a process known as DNA transcription. The genetic information is encoded in four nucleotides (A, U, C, G) arranged into codons of three bases each on a single strand mRNA. Each codon encodes for a certain AA, except the stop codons that terminate protein synthesis.

Retrieved from http://www.councilforresponsiblegenetics.org/geneticprivacy/DNA_sci_4.html

References

Alberts, B., Johnson, A. and Lewis, J. (2002), Molecular Biology of the Cell. 4th edition.

Bennett, J.S. (2006), Vasoocclusion in Sickle Cell Anemia: Are Platelets Really Involved?, Arterioscler Thromb Vasc Biol., 26, pp. 1415-1416, doi:10.1161/01.ATV.0000227595.97898.3f

Berg, J.M., Tymoczko, J.L. and Stryer, L. (2002), Glycogen Metabolism, Biochemistry. 5th edition. New York, Available from: http://www.ncbi.nlm.nih.gov/books/NBK21190

Clegg, B., Keratin, Audio Blog Post retrieved from http://www.rsc.org/chemistryworld/podcast/CIIEcompounds/transcripts/keratin.asp

Cummings, J. H. & Englyst, H. N. (1995) Gastrointestinal effects of food carbohydrates. Am. J. Clin. Nutr. 61(suppl): 938S–945S

Eibein, A.D., Pan, Y.T, Pastuszak, I. and Carroll, D. (2003), New insights on trehalose: a multifunctional molecule, Glycobiology, 13 (4), 17R-27R. doi: 10.1093/glycob/cwg047

Festucci-Buselli, R.A., Otoni, W.C. and Joshi, C.P. (2007), Structure, organization, and functions of cellulose synthase complexes in higher plants, Braz. J. Plant Physiol., 19(1), Londrina.

Foltmann, B. (1981), Protein sequencing: Past and present. Biochemical Education, 9, pp. 2–7. doi: 10.1016/0307-4412(81)90049-2

Marengo-Rowe, A. J. (2006), Structure-function relations of human hemoglobins. Proceedings (Baylor University. Medical Center), 19(3), pp. 239–245.

Raphael R.I. (2005) Pathophysiology and treatment of sickle cell disease. Clin Adv Hematol Oncol., 3(6), pp.492–505. [PubMed]

Ritter, S. (2012), Water Bear Inspires Refrigeration-free Storage, Conservation, University of Washington.

http://www.texascollaborative.org/hildasustaita/bonds.gif

https://study.com/academy/lesson/cellulose-in-plants-function-structure-quiz.html

http://chemwiki.ucdavis.edu/Core/Biological_Chemistry/Carbohydrates/Polysaccharides/Cellulose

http://www.mhhe.com/biosci/genbio/raven6b/graphics/raven06b/other/raven06b_03.pdf

http://chemwiki.ucdavis.edu/Wikitexts/Sacramento_City_College/SCC%3A_Nutri_300_(Coppola)/04%3A_Carbohydrates/4.4%3A_The_Functions_of_Carbohydrates_in_the_Body

http://www.bbc.co.uk/education/guides/z36mmp3/revision

http://www.councilforresponsiblegenetics.org/geneticprivacy/DNA_sci_4.html