Macromolecules are basically large molecules, generally exceeding around 5000 daltons in weight. Figure 1 provides a comparison of the size of macromolecule compared to individual constituents and larger molecules such as cells themselves.
Figure 1. The dimensions of some biomolecules, assemblies and cells (Stryer 1988)
Key: = angstroms (1=0.1nm), m = metres, assemblies (of macromolecules)
Macromolecules form the basis of nearly all biological structures and systems and are formed from simple molecules, linked together with covalent and non-covalent bonds, which confer many of their properties. There are 3 main types of macromolecules in biological systems - proteins, nucleic acids and polysaccharides.
Proteins are polypeptide chains comprised of a varied combination of 20 standard amino acids joined together with covalent peptide bonds. The secondary structure is formed by regular folding and hydrogen bonds in specific regions of the polypeptide chain, to form an -helix or -sheet formation. Further covalent and non-covalent bonds can also form, leading to the tertiary and quaternary, complex folded 3-dimensional structure of biological proteins.
Proteins are arguably the basis of all life systems, as they perform nearly all of the jobs in biological systems (Elliott, and Elliott 2001). Space precludes an extensive discussion so there will be a focus on 3 protein types -enzymes, membrane proteins and oxygen binding proteins.
Within biological systems all processes need to be are catalysed - that is aided and sped up; by enzymes. Enzymes are large molecular weight (10,000+) proteins (Elliott, and Elliott 2001) that remain unchanged during the reaction, thus ensuring their availability for future reactions.
Each enzyme may catalyse more than one reaction but will always have a highly specific active site to which the substrates for the reaction bind via weak noncovalent bonds. The reaction is then usually governed by feedback inhibition, whereby the end product inhibits the further catalysis of the substrate (Stryer 1988).
A specific example of a biological enzyme is adenylate cyclase which catalyses the conversion of adenosine triphosphate (the universal energy donor) to adenosine-3,5-cyclic monophosphate (Elliott, and Elliott 2001).
All biological cells have a cell membrane, a phospholipid bilayer through which anything needing to enter or exit the cell must pass. A pure phospholipid bilayer is impermeable to large polar molecules which ensures that the membrane acts to protect the integrity of the cell contents. However, as there is a need for molecules to cross the membrane a series of large proteins span it to allow molecular passage. There are 5 types of membrane proteins depending on whether they are integral to the membrane or only partially associated; and whether they have an active element inside or outside the cell, as shown in figure 2 below.
Figure 2. The different types of membrane proteins, which are either integral to the membrane or associated to the membrane (adapted from Stryer 1988, figure 12-18).
Membrane proteins allow both the passive and active movement of molecules across the membrane. They also act as receptors - recognition molecules, which respond to a molecule on the outside of the membrane, and cause an intracellular response. One specific example is the -adrenoceptor, which is a large globular protein that spans the membrane and comprises 7 transmembrane helices, with a docking site for the agonist laying within the transmembrane element (Rang, Dale & Ritter 1999).
Figure 3. A molecular model of the -adrenoceptor, showing transmembrane segments and the internal binding site (blue) (Rang, Dale & Ritter 1999)
There are 2 main oxygen binding proteins - myoglobin and haemoglobin; transporting oxygen in the muscles and blood respectively.
Myoglobin is a relatively small protein, comprising 153 amino acids in a single polypeptide chain (Hames, and Hooper 2000). By contrast haemoglobin comprises 4 polypeptide chains (Stryer 1988) and differs at 83% of amino acid residues to myoglobin (Hames, and Hooper 2000). However both molecules have a similar three-dimensional structure, which underlies their oxygen carrying capabilities.
The extensively folded globular nature of both proteins ensures that the hydrophobic amino acid residues are within the centre of the molecule, whilst the hydrophilic, polar residues, are on the outside of the molecule. This allows the non polypeptide ‘prosthetic’ (Stryer 1988) haem (iron) group to bind oxygen molecules but still allows the protein to be carried freely in the blood.
Nucleic acids are very specific molecules found within the nuclei of cells that contain phosphate groups and sugars and bases (1 of 4 amino acids) (Elliott, and Elliott 2001). There are 2 principal nucleic acids - deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
Deoxyrobonucleic acid (DNA)
DNA is a long threadlike macromolecule made up of many deoxyribonucleotides, which are linked via bonds between phosphate groups as well as in a more complex way between molecules. There are 4 bases used in the formation of DNA - adenine, guanine, thymine and cytosine.
DNA is characterised by its double helical structure, as there are 2 separate strands with the bases in the centre, joining across strands, and the sugar-phosphate on the outside (Hale, Margham & and Saunders 1995). Figure 4 below shows the double helix of DNA.
Figure 4. The double helical structure of DNA, showing bases and links between strands (Wikipedia 2005)
DNA forms the basis of the hereditary information of all cells, providing details about all parts of the body via genetic information. A gene is a unit of heredity (Elliott, and Elliott 2001) which carries the information about how a specific structural sequence should be formed, using the type and sequence of bases within the DNA structure to provide this information.
RNA is similar to DNA, except that the 4th base amino acid is uracil instead of thymine (Elliott, and Elliott 2001) and the sugar is ribose, not deoxyribose (Hames, and Hooper 2000).
The majority of RNA molecules are single stranded, but there are regions of double strands, and these often comprise a single strand doubling back on itself (Hames, and Hooper 2000).
RNA is generally concerned with the translation of the genetic information that has been stored in DNA (Allaby 1988) and has a number of different forms, which relate to the function at a given time.
Messenger RNA (mRNA) is a relatively unstable and short lived single strand RNA which acts to copy the information from one unwound strand of DNA during protein synthesis. mRNA contains a codon of 3 nucleotides that codes for an amino acid, or the end of the genetic information (Allaby 1988)
Transfer RNA (tRNA) molecules are small molecules that act as an adaptor (Stryer 1988). tRNA that has a clover leaf end structure which attaches to amino acids and has an anticodon which binds to the codon of the matched mRNA. This acts to ensure that the correct amino acids are joined to the growing protein chain (Hames, and Hooper 2000).
Ribosomal RNA (rRNA) forms 2/3 of the mass of the ribosome, the molecular machines that coordinate the components in protein synthesis (Stryer 1988). There are several forms of rRNA, each being a different sized subunit in the main ribosome. rRNAs are also able to act as enzymes under certain circumstances.
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Hames, B.D. & and Hooper, N.M. 2000, Biochemistry, 2nd edn, BIOS, Oxford.
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Rang, H.P., Dale, M.M. & Ritter, J.M. 1999, "How drugs act : molecular aspects" in Pharmacology, eds. H.P. Rang, M.M. Dale & J.M. Ritter, Fourth edn, Churchill Livingstone, Edinburgh, pp. 19-46.
Stryer, L. 1988, Biochemistry, 3rd edn, W. H. Freeman and Co., New York.
Wikipedia 2005, 30th November 2005-last update, DNA, Available: http://en.wikipedia.org/wiki/DNA, accessed 2005, 30th November.
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