thumb|Chemical structure of a [[polypeptide macromolecule]]

A macromolecule is a "molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass." Polymers are physical examples of macromolecules. Common macromolecules are biopolymers (RNA and DNA, proteins, and carbohydrates), polyolefins (polyethylene) and polyamides (nylon).

Synthetic macromolecules

thumb|[[Polyethyleneterephthalate (PET), used to make beverage containers]]

Many macromolecules are synthetic polymers (plastics, synthetic fibers, and synthetic rubber). Polyethylene is produced on a particularly large scale such that ethylenes are the primary product in the chemical industry.

Examples or different types of synthetic macromolecules include:

  • Thermoplastic polymers
  • Thermoset polymers
  • Dendrimers
  • Vitrimers / Covalent adaptable networks (CANs)
  • Covalent organic frameworks (COFs)

When considering organometallic materials within the scope, this may also include metal organic frameworks (MOFs). Additionally, when also considering other non-covalent bonding, such as hydrogen bonds or pi-stacking many different types of supramolecular networks are also included.

Macromolecules in nature

  • Proteins are polymers of amino acids joined by peptide bonds.
  • DNA and RNA are polymers of nucleotides joined by phosphodiester bonds. These nucleotides consist of a phosphate group, a sugar (ribose in the case of RNA, deoxyribose in the case of DNA), and a nucleotide base (either adenine, guanine, thymine, uracil, or cytosine, where thymine occurs only in DNA and uracil only in RNA).
  • Polysaccharides (such as starch, cellulose, and chitin) are polymers of monosaccharides joined by glycosidic bonds.
  • Some lipids (organic nonpolar molecules) are macromolecules, with a variety of different structures.

Linear biopolymers

All living organisms are dependent on three essential biopolymers for their biological functions: DNA, RNA and proteins. Each of these molecules is required for life since each plays a distinct, indispensable role in the cell. The simple summary is that DNA makes RNA, and then RNA makes proteins.

DNA, RNA, and proteins all consist of a repeating structure of related building blocks (nucleotides in the case of DNA and RNA, amino acids in the case of proteins). In general, they are all unbranched polymers, and so can be represented in the form of a string. Indeed, they can be viewed as a string of beads, with each bead representing a single nucleotide or amino acid monomer linked together through covalent chemical bonds into a very long chain.

In most cases, the monomers within the chain have a strong propensity to interact with other amino acids or nucleotides. In DNA and RNA, this can take the form of Watson–Crick base pairs (G–C and A–T or A–U), although many more complicated interactions can and do occur.

Structural features

{| class="wikitable floatright"

!

! DNA

! RNA

! Proteins

|-

| Encodes genetic information

| Yes

| Yes

| No

|-

| Catalyzes biological reactions

| No

| Yes

| Yes

|-

| Building blocks (type)

| Nucleotides

| Nucleotides

| Amino acids

|-

| Building blocks (number)

| 4

| href="dendrimer" | 4

| 20

|-

| Strandedness

| Double

| Single

| Single

|-

| Structure

| Double helix

| Complex

| Complex

|-

| Stability to degradation

| High

| Variable

| Variable

|-

| Repair systems

| Yes

| No

| No

|}

Because of the double-stranded nature of DNA, essentially all of the nucleotides take the form of Watson–Crick base pairs between nucleotides on the two complementary strands of the double helix.

In contrast, both RNA and proteins are normally single-stranded. Therefore, they are not constrained by the regular geometry of the DNA double helix, and so fold into complex three-dimensional shapes dependent on their sequence. These different shapes are responsible for many of the common properties of RNA and proteins, including the formation of specific binding pockets, and the ability to catalyse biochemical reactions.

DNA is optimised for encoding information

DNA is an information storage macromolecule that encodes the complete set of instructions (the genome) that are required to assemble, maintain, and reproduce every living organism.

DNA and RNA are both capable of encoding genetic information, because there are biochemical mechanisms which read the information coded within a DNA or RNA sequence and use it to generate a specified protein. On the other hand, the sequence information of a protein molecule is not used by cells to functionally encode genetic information.

The single-stranded nature of protein molecules, together with their composition of 20 or more different amino acid building blocks, allows them to fold in to a vast number of different three-dimensional shapes, while providing binding pockets through which they can specifically interact with all manner of molecules. In addition, the chemical diversity of the different amino acids, together with different chemical environments afforded by local 3D structure, enables many proteins to act as enzymes, catalyzing a wide range of specific biochemical transformations within cells. In addition, proteins have evolved the ability to bind a wide range of cofactors and coenzymes, smaller molecules that can endow the protein with specific activities beyond those associated with the polypeptide chain alone.

RNA is multifunctional

RNA is multifunctional, its primary function is to encode proteins, according to the instructions within a cell's DNA.]]

See also

  • List of biophysically important macromolecular crystal structures
  • Small molecule
  • Soft matter

References

  • Synopsis of Chapter 5, Campbell & Reece, 2002
  • Lecture notes on the structure and function of macromolecules
  • Several (free) introductory macromolecule related internet-based courses
  • Giant Molecules! by Ulysses Magee, ISSA Review Winter 2002–2003, . Cached HTML version of a missing PDF file. Retrieved March 10, 2010. The article is based on the book, Inventing Polymer Science: Staudinger, Carothers, and the Emergence of Macromolecular Chemistry by Yasu Furukawa.