History:

• In the mid-twentieth century, scientists were still unsure as to whether DNA or protein was the genetic material of the cell

• It was known that some viruses consisted solely of DNA and a protein coat and could transfer their genetic material into hosts

• In 1952, Alfred Hershey and Martha Chase conducted a series of experiments to prove that DNA was the genetic material

• Viruses (T2 bacteriophage) were grown in one of two isotopic mediums in order to radioactively label a specific viral component

  • Viruses grown in radioactive sulfur (³⁵S) had radiolabelled proteins (sulfur is present in proteins but not DNA)

  • Viruses grown in radioactive phosphorus (³²P) had radiolabeled DNA (phosphorus is present in DNA but not proteins)

• The viruses were then allowed to infect a bacterium (E. coli) and then the virus and bacteria were separated via centrifugation

• The larger bacteria formed a solid pellet while the smaller viruses remained in the supernatant

• The bacterial pellet was found to be radioactive when infected by the ³²P–viruses (DNA) but not the ³⁵S–viruses (protein)

• This demonstrated that DNA, not protein, was the genetic material because DNA was transferred to the bacteria

Summary of the Hershey-Chase Experiment

Rosalind Franklin and Maurice Wilkins used a method of X-ray diffraction to investigate the structure of DNA

• DNA was purified and then fibres were stretched in a thin glass tube (to make most of the strands parallel)

• The DNA was targeted by a X-ray beam, which was diffracted when it contacted an atom

• The scattering pattern of the X-ray was recorded on a film and used to elucidate details of molecular structure

Summary of the Process of X-Ray Crystallography

From the scattering pattern produced by a DNA molecule, certain inferences could be made about its structure

• Composition:  DNA is a double stranded molecule

• Orientation:  Nitrogenous bases are closely packed together on the inside and phosphates form an outer backbone

• Shape:  The DNA molecule twists at regular intervals (every 34 Angstrom) to form a helix (two strands = double helix)

Photo 51 – Evidence for the Structure of DNA via X-Ray Diffraction

Franklin’s data was shared by Wilkins with James Watson (without Franklin’s permission) who, with the help of Francis Crick, used the information to create a molecular model of the basic structure of DNA

In 1962, Watson, Crick and Wilkins (but not Franklin) were awarded the Nobel prize for their contributions to DNA structure identification

Franklin’s x-ray diffraction experiments demonstrated that the DNA helix is both tightly packed and regular in structure

• Phosphates (and sugars) form an outer backbone and nitrogenous bases are packaged within the interior

Chargaff had also demonstrated that DNA is composed of an equal number of purines (A + G) and pyrimidines (C + T) 

• This indicates that these nitrogenous bases are paired (purine + pyrimidine) within the double helix

• In order for this pairing between purines and pyrimidines to occur, the two strands must run in antiparallel directions

When Watson & Crick were developing their DNA model, they discovered that an A–T bond was the same length as a G–C bond

• Adenine and thymine paired via two hydrogen bonds, whereas guanine and cytosine paired via three hydrogen bonds

• If the bases were always paired this way, then this would describe the regular structure of the DNA helix (shown by Franklin)

Replication

Consequently, DNA structure suggests two mechanisms for DNA replication:

• Replication occurs via complementary base pairing (adenine pairs with thymine, guanine pairs with cytosine)

• Replication is bi-directional (proceeds in opposite directions on the two strands) due to the antiparallel nature of the strands

Complementary Base Pairing as a Mechanism for DNA Replication

DNA replication is a semi-conservative process that is carried out by a complex system of enzymes:
Helicase

• Helicase unwinds and separates the double-stranded DNA by breaking the hydrogen bonds between base pairs

• This occurs at specific regions (origins of replication), creating a replication fork of two strands running in antiparallel directions

DNA Gyrase

• DNA gyrase reduces the torsional strain created by the unwinding of DNA by helicase

• It does this by relaxing positive supercoils (via negative supercoiling) that would otherwise form during the unwinding of DNA

Single Stranded Binding (SSB) Proteins

• SSB proteins bind to the DNA strands after they have been separated and prevent the strands from re-annealing

• These proteins also help to prevent the single stranded DNA from being digested by nucleases

• SSB proteins will be dislodged from the strand when a new complementary strand is synthesised by DNA polymerase III

DNA Primase

• DNA primase generates a short RNA primer (~10–15 nucleotides) on each of the template strands

• The RNA primer provides an initiation point for DNA polymerase III, which can extend a nucleotide chain but not start one

DNA Polymerase III

• Free nucleotides align opposite their complementary base partners (A = T ; G = C)

• DNA pol III attaches to the 3’-end of the primer and covalently joins the free nucleotides together in a 5’ → 3’ direction

• As DNA strands are antiparallel, DNA pol III moves in opposite directions on the two strands

  • On the leading strand, DNA pol III is moving towards the replication fork and can synthesise continuously

  • On the lagging strand, DNA pol III is moving away from the replication fork and synthesises in pieces (Okazaki fragments)

DNA Polymerase I

• As the lagging strand is synthesised in a series of short fragments, it has multiple RNA primers along its length

• DNA pol I removes the RNA primers from the lagging strand and replaces them with DNA nucleotides

DNA Ligase

• DNA ligase joins the Okazaki fragments together to form a continuous strand

• It does this by covalently joining the sugar-phosphate backbones together with a phosphodiester bond

Summary of DNA Replication

DNA polymerase cannot initiate replication, it can only add new nucleotides to an existing strand

For DNA replication to occur, an RNA primer must first be synthesized to provide an attachment point for DNA polymerase

DNA polymerase adds nucleotides to the 3’ end of a primer, extending the new chain in a 5’ → 3’ direction

• Free nucleotides exist as deoxynucleoside triphosphates (dNTPs) – they have 3 phosphate groups

• DNA polymerase cleaves the two additional phosphates and uses the energy released to form a phosphodiester bond with the 3’ end of a nucleotide chain

Direction of DNA Synthesis

Leading versus Lagging Strands

Because double-stranded DNA is antiparallel, DNA polymerase must move in opposite directions on the two strands

• On the leading strand, DNA polymerase is moving towards the replication fork and so can copy continuously

• On the lagging strand, DNA polymerase is moving away from the replication fork, meaning copying is discontinuous

  • As DNA polymerase is moving away from helicase, it must constantly return to copy newly separated stretches of DNA

  • This means the lagging strand is copied as a series of short fragments (Okazaki fragments), each preceded by a primer

  • The primers are replaced with DNA bases and the fragments joined together by a combination of DNA pol I and DNA ligase
    
    

DNA sequencing refers to the process by which the base order of a nucleotide sequence is elucidated

The most widely used method for DNA sequencing involves the use of chain-terminating dideoxynucleotides

Dideoxynucleotides (ddNTPs) lack the 3’-hydroxyl group necessary for forming a phosphodiester bond

Consequently, ddNTPs prevent further elongation of a nucleotide chain and effectively terminate replication

The resulting length of a DNA sequence will reflect the specific nucleotide position at which the ddNTP was incorporated

For example, if a ddGTP terminates a sequence after 8 nucleotides, then the 8th nucleotide in the sequence is a cytosine

The Sanger method:

• Four PCR mixes are set up, each containing stocks of normal nucleotides plus one dideoxynucleotide (ddA, ddT, ddC or ddG)

• As a typical PCR will generate over 1 billion DNA molecules, each PCR mix should generate all the possible terminating fragments for that particular base

• When the fragments are separated using gel electrophoresis, the base sequence can be determined by ordering fragments according to length

• If a distinct radioactive or fluorescently labelled primer is included in each mix, the fragments can be detected by automated sequencing machines

• If the Sanger method is conducted on the coding strand (non-template strand), the resulting sequence elucidated will be identical to the template strand

DNA sequencing

DNA sequencing refers to the process by which the base order of a nucleotide sequence is elucidated

• The most widely used method for DNA sequencing involves the use of chain-terminating dideoxynucleotides

Dideoxynucleotides

• Dideoxynucleotides (ddNTPs) lack the 3’-hydroxyl group necessary for forming a phosphodiester bond

• Consequently, ddNTPs prevent further elongation of a nucleotide chain and effectively terminate replication

• The resulting length of a DNA sequence will reflect the specific nucleotide position at which the ddNTP was incorporated

  • For example, if a ddGTP terminates a sequence after 8 nucleotides, then the 8th nucleotide in the sequence is a cytosine

Determining Nucleotide Positions Using Dideoxynucleotides

Sequencing

Dideoxynucleotides can be used to determine DNA sequence using the Sanger method

• Four PCR mixes are set up, each containing stocks of normal nucleotides plus one dideoxynucleotide (ddA, ddT, ddC or ddG)

• As a typical PCR will generate over 1 billion DNA molecules, each PCR mix should generate all the possible terminating fragments for that particular base

• When the fragments are separated using gel electrophoresis, the base sequence can be determined by ordering fragments according to length

• If a distinct radioactive or fluorescently labelled primer is included in each mix, the fragments can be detected by automated sequencing machines

• If the Sanger method is conducted on the coding strand (non-template strand), the resulting sequence elucidated will be identical to the template strand

DNA Sequencing via the Sanger Method

The vast majority of the human genome is comprised of non-coding DNA (genes only account for ~ 1.5% of the total sequence)

• Historically referred to as ‘junk DNA’, these non-coding regions are now recognised to serve other important functions

• Examples include satellite DNA, telomeres, introns, ncRNA genes and gene regulatory sequences 

Types of Non-Coding DNA

DNA profiling is a technique by which individuals can be identified and compared via their respective DNA profiles

• Within the non-coding regions of an individual’s genome there exists satellite DNA – long stretches of DNA made up of repeating elements called short tandem repeats (STRs) 

• Tandem repeats can be excised using restriction enzymes and then separated with gel electrophoresis for comparison

• As individuals will likely have different numbers of repeats at a given satellite DNA locus, they will generate unique DNA profiles

  • Longer repeats will generate larger fragments, while shorter repeats will generate smaller fragments

Comparative STR Lengths at Two Specific Loci

In eukaryotic organisms, the DNA is packaged with histone proteins to create a compacted structure called a nucleosome

• Nucleosomes help to supercoil the DNA, resulting in a greatly compacted structure that allows for more efficient storage

• Supercoiling helps to protect the DNA from damage and also allows chromosomes to be mobile during mitosis and meiosis

Organisation of Eukaryotic DNA

• The DNA is complexed with eight histone proteins (an octamer) to form a complex called a nucleosome

• Nucleosomes are linked by an additional histone protein (H1 histone) to form a string of chromatosomes

• These then coil to form a solenoid structure (~6 chromatosomes per turn) which is condensed to form a 30 nm fibre

• These fibres then form loops, which are compressed and folded around a protein scaffold to form chromatin

• Chromatin will then supercoil during cell division to form chromosomes that are visible (when stained) under microscope

A nucleosome consists of a molecule of DNA wrapped around a core of eight histone proteins (an octamer)

• The negatively charged DNA associates with positively charged amino acids on the surface of the histone proteins

• The histone proteins have N-terminal tails which extrude outwards from the nucleosome

• During chromosomal condensation, tails from adjacent histone octamers link up and draw the nucleosomes closer together

To view the structure of a nucleosome via an interactive pop-up, click on the name of the structure below:

A gene is a sequence of DNA which is transcribed into RNA and contains three main parts:

Promoter

• The non-coding sequence responsible for the initiation of transcription

• The core promoter is typically located immediately upstream of the gene’s coding sequence

• The promoter functions as a binding site for RNA polymerase (the enzyme responsible for transcription)

• The binding of RNA polymerase to the promoter is mediated and controlled by an array of transcription factors in eukaryotes

• These transcription factors bind to either proximal control elements (near the promoter) or distal control elements (at a distance)

Coding Sequence

• After RNA polymerase has bound to the promoter, it causes the DNA strands to unwind and separate

• The region of DNA that is transcribed by RNA polymerase is called the coding sequence

Terminator

• RNA polymerase will continue to transcribe the DNA until it reaches a terminator sequence

• The mechanism for transcriptional termination differs between prokaryotes and eukaryotes

Sections of a Gene

Antisense vs Sense

A gene (DNA) consists of two polynucleotide strands, but only one is transcribed into RNA

• The antisense strand is the strand that is transcribed into RNA

  • Its sequence is complementary to the RNA sequence and will be the "DNA version” of the tRNA anticodon sequence

  • The antisense strand is also referred to as the template strand

• The sense strand is the strand that is not transcribed into RNA

  • Its sequence will be the “DNA version” of the RNA sequence (i.e. identical except for T instead of U)

  • The sense strand is also referred to as the coding strand (because it is a DNA copy of the RNA sequence)

Either of the 2 polynucleotide strands may contain a gene, and hence the determination of sense and antisense is gene specific

Antisense vs Sense Strands

Transcription is the process by which a DNA sequence (gene) is copied into a complementary RNA sequence by RNA polymerase

• Free nucleotides exist in the cell as nucleoside triphosphates (NTPs), which line up opposite their complementary base partner

• RNA polymerase covalently binds the NTPs together in a reaction that involves the release of the two additional phosphates

• The 5’-phosphate is linked to the 3’-end of the growing mRNA strand, hence transcription occurs in a 5’ → 3’ direction

Overview of Transcription

The process of transcription can be divided into three main steps: initiation, elongation and termination

• In initiation, RNA polymerase binds to the promoter and causes the unwinding and separating of the DNA strands

• Elongation occurs as the RNA polymerase moves along the coding sequence, synthesising RNA in a 5’ → 3’ direction

• When RNA polymerase reaches the terminator, both the enzyme and nascent RNA strand detach and the DNA rewinds

Many RNA polymerase enzymes can transcribe a DNA sequence sequentially, producing a large number of transcripts 

• In eukaryotes, post-transcriptional modification of the RNA sequence is necessary to form mature mRNA

Summary of Transcription

In eukaryotes, there are three post-transcriptional events that must occur in order to form mature messenger RNA:

Capping

• Capping involves the addition of a methyl group to the 5’-end of the transcribed RNA

• The methylated cap provides protection against degradation by exonucleases

• It also allows the transcript to be recognised by the cell’s translational machinery (e.g. nuclear export proteins and ribosome)

Polyadenylation

• Polyadenylation describes the addition of a long chain of adenine nucleotides (a poly-A tail) to the 3’-end of the transcript

• The poly-A tail improves the stability of the RNA transcript and facilitates its export from the nucleus

Splicing

• Within eukaryotic genes are non-coding sequences called introns, which must be removed prior to forming mature mRNA

• The coding regions are called exons and these are fused together when introns are removed to form a continuous sequence

• Introns are intruding sequences whereas exons are expressing sequences

• The process by which introns are removed is called splicing

Post-Transcriptional Modifications

Splicing can also result in the removal of exons – a process known as alternative splicing

The selective removal of specific exons will result in the formation of different polypeptides from a single gene sequence

• For example, a particular protein may be membrane-bound or cytosolic depending on the presence of an anchoring motif

Alternative Splicing

Transcriptional activity is regulated by two groups of proteins that mediate binding of RNA polymerase to the promoter

Transcription factors form a complex with RNA polymerase at the promoter

• RNA polymerase cannot initiate transcription without these factors and hence their levels regulate gene expression

Regulatory proteins bind to DNA sequences outside of the promoter and interact with the transcription factors

• Activator proteins bind to enhancer sites and increase the rate of transcription (by mediating complex formation)

• Repressor proteins bind to silencer sequences and decrease the rate of transcription (by preventing complex formation)

The presence of certain transcription factors or regulatory proteins may be tissue-specific

• Additionally, chemical signals (e.g. hormones) can moderate protein levels and hence mediate a change in gene expression

Differential Gene Expression

Control Elements

The DNA sequences that regulatory proteins bind to are called control elements

• Some control elements are located close to the promoter (proximal elements) while others are more distant (distal elements)

• Regulatory proteins typically bind to distal control elements, whereas transcription factors usually bind to proximal elements

• Most genes have multiple control elements and hence gene expression is a tightly controlled and coordinated process

Enhancers and Silencers

Changes in the external or internal environment can result in changes to gene expression patterns

• Chemical signals within the cell can trigger changes in levels of regulatory proteins or transcription factors in response to stimuli

• This allows gene expression to change in response to alterations in intracellular and extracellular conditions

There are a number of examples of organisms changing their gene expression patterns in response to environmental changes:

• Hydrangeas change colour depending on the pH of the soil (acidic soil = blue flower ; alkaline soil = pink flower)

• The Himalayan rabbit produces a different fur pigment depending on the temperature (>35ºC = white fur ; <30ºC = black fur)

• Humans produce different amounts of melanin (skin pigment) depending on light exposure

• Certain species of fish, reptile and amphibian can even change gender in response to social cues (e.g. mate availability)

The Role of Soil pH in the Determination of Hydrangea Flower Colour

Eukaryotic DNA is wrapped around histone proteins to form compact nucleosomes

• These histone proteins have protruding tails that determine how tightly the DNA is packaged

Modification of Histone Tails

Typically the histone tails have a positive charge and hence associate tightly with the negatively charged DNA

• Adding an acetyl group to the tail (acetylation) neutralises the charge, making DNA less tightly coiled and increasing transcription

• Adding a methyl group to the tail (methylation) maintains the positive charge, making DNA more coiled and reducing transcription

Acetylation of Nucleosomes

Types of Chromatin

When DNA is supercoiled and not accessible for transcription, it exists as condensed heterochromatin

When the DNA is loosely packed and therefore accessible to the transcription machinery, it exists as euchromatin

• Different cell types will have varying segments of DNA packaged as heterochromatin and euchromatin

• Some segments of DNA may be permanently supercoiled, while other segments may change over the life cycle of the cell

Nucleosome Packaging in Euchromatin and Heterochromatin

Direct methylation of DNA (as opposed to the histone tails) can also affect gene expression patterns

• Increased methylation of DNA decreases gene expression (by preventing the binding of transcription factors)

• Consequently, genes that are not transcribed tend to exhibit more DNA methylation than genes that are actively transcribed

Factors Contributing to DNA Methylation Patterns

Epigenetics

Epigenetics is the study of changes in phenotype as a result of variations in gene expression levels 

• Epigenetic analysis shows that DNA methylation patterns may change over the course of a lifetime

• It is influenced by heritability but is not genetically pre-determined (identical twins may have different DNA methylation patterns)

• Different cell types in the same organism may have markedly different DNA methylation patterns

• Environmental factors (e.g. diet, pathogen exposure, etc.) may influence the level of DNA methylation within cells

Comparative DNA Methylation Patterns in Twins of Different Ages

Comparative Methylation Patterns in Twins of Differing Health Status (Healthy vs Diseased)

Ribosomes

• Ribosomes are made of protein (for stability) and ribosomal RNA (for catalytic activity)

• They consist of a large and small subunit:

  • The small subunit contains an mRNA binding site

  • The large subunit contains three tRNA binding sites – an aminoacyl (A) site, a peptidyl (P) site and an exit (E) site

• Ribosomes can be found either freely floating in the cytosol or bound to the rough ER (in eukaryotes)

• Ribosomes differ in size in prokaryotes and eukaryotes (prokaryotes = 70S ; eukaryotes = 80S)

Structure of a Ribosome

Transfer RNA (tRNA)

tRNA molecules fold into a cloverleaf structure with four key regions:

• The acceptor stem (3’-CCA) carries an amino acid

• The anticodon associates with the mRNA codon (via complementary base pairing)

• The T arm associates with the ribosome (via the E, P and A binding sites)

• The D arm associates with the tRNA activating enzyme (responsible for adding the amino acid to the acceptor stem)

Structure of tRNA

Ribosome and tRNA Molecules

Each tRNA molecule binds with a specific amino acid in the cytoplasm in a reaction catalysed by a tRNA-activating enzyme

• Each amino acid is recognised by a specific enzyme (the enzyme may recognise multiple tRNA molecules due to degeneracy)

The binding of an amino acid to the tRNA acceptor stem occurs as a result of a two-step process:

• The enzyme binds ATP to the amino acid to form an amino acid–AMP complex linked by a high energy bond (PP released)

• The amino acid is then coupled to tRNA and the AMP is released – the tRNA molecule is now “charged” and ready for use

The function of the ATP (phosphorylation) is to create a high energy bond that is transferred to the tRNA molecule

• This stored energy will provide the majority of the energy required for peptide bond formation during translation

tRNA Activation

Initiation

The first stage of translation involves the assembly of the three components that carry out the process (mRNA, tRNA, ribosome)

• The small ribosomal subunit binds to the 5’-end of the mRNA and moves along it until it reaches the start codon (AUG)

• Next, the appropriate tRNA molecule bind to the codon via its anticodon (according to complementary base pairing)

• Finally, the large ribosomal subunit aligns itself to the tRNA molecule at the P site and forms a complex with the small subunit
  
  

Elongation

• A second tRNA molecule pairs with the next codon in the ribosomal A site

• The amino acid in the P site is covalently attached via a peptide bond (condensation reaction) to the amino acid in the A site

• The tRNA in the P site is now deacylated (no amino acid), while the tRNA in the A site carries the peptide chain

Translocation

• The ribosome moves along the mRNA strand by one codon position (in a 5’ → 3’ direction)

• The deacylated tRNA moves into the E site and is released, while the tRNA carrying the peptide chain moves to the P site

• Another tRNA molecules attaches to the next codon in the now unoccupied A site and the process is repeated

Termination

The final stage of translation involves the disassembly of the components and the release of a polypeptide chain

• Elongation and translocation continue in a repeating cycle until the ribosome reaches a stop codon 

• These codons do not recruit a tRNA molecule, but instead recruit a release factor that signals for translation to stop

• The polypeptide is released and the ribosome disassembles back into its two independent subunits

In eukaryotes, the ribosomes are separated from the genetic material (DNA and RNA) by the nucleus

• After transcription, the mRNA must be transported from the nucleus (via nuclear pores) prior to translation by the ribosome

• This transport requires modification to the RNA construct (e.g. 5’-methyl capping and 3’-polyadenylation)

Prokaryotes lack compartmentalised structures (like the nucleus) and so transcription and translation need not be separated

• Ribosomes may begin translating the mRNA molecule while it is still being transcribed from the DNA template

• This is possible because both transcription and translation occur in a 5’ → 3’ direction 

A polysome (or a polyribosome) is a group of two or more ribosomes translating an mRNA sequence simultaneously

• The polysomes will appear as beads on a string (each 'bead' represents a ribosome ; the ‘string’ is the mRNA strand)

• In prokaryotes, the polysomes may form while the mRNA is still being transcribed from the DNA template

• Ribosomes located at the 3’-end of the polysome cluster will have longer polypeptide chains that those at the 5’-end

Polysomes

All proteins produced by eukaryotic cells are initially synthesised by ribosomes found freely circulating within the cytosol

• If the protein is targeted for intracellular use within the cytosol, the ribosome remains free and unattached

• If the protein is targeted for secretion, membrane fixation or use in lysosomes, the ribosome becomes bound to the ER

Protein destination is determined by the presence or absence of an initial signal sequence on a nascent polypeptide chain

• The presence of this signal sequence results in the recruitment of a signal recognition particle (SRP), which halts translation

• The SRP-ribosome complex then docks at a receptor located on the ER membrane (forming rough ER)

• Translation is re-initiated and the polypeptide chain continues to grow via a transport channel into the lumen of the ER

  • The synthesised protein will then be transported via a vesicle to the Golgi complex (for secretion) or the lysosome

  • Proteins targeted for membrane fixation (e.g. integral proteins) get embedded into the ER membrane

• The signal sequence is cleaved and the SRP recycled once the polypeptide is completely synthesised within the ER

The Role of the Signal Recognition Particle in Protein Destination

Primary (1º) Structure

• The first level of structural organisation in a protein is the order / sequence of amino acids which comprise the polypeptide chain

• The primary structure is formed by covalent peptide bonds between the amine and carboxyl groups of adjacent amino acids

• Primary structure controls all subsequent levels of protein organisation because it determines the nature of the interactions between R groups of different amino acids

Secondary (2º) Structure

• The secondary structure is the way a polypeptide folds in a repeating arrangement to form α-helices and β-pleated sheets

• This folding is a result of hydrogen bonding between the amine and carboxyl groups of non-adjacent amino acids

• Sequences that do not form either an alpha helix or beta-pleated sheet will exist as a random coil

• Secondary structure provides the polypeptide chain with a level of mechanical stability (due to the presence of hydrogen bonds)

• In pictures, alpha helices are represented as spirals (purple ; left) and beta-pleated sheets as arrows (blue ; right)

Tertiary (3º) Structure

• The tertiary structure is the way the polypeptide chain coils and turns to form a complex molecular shape (i.e. the 3D shape)

• It is caused by interactions between R groups; including H-bonds, disulfide bridges, ionic bonds and hydrophobic interactions

• Relative amino acid positions are important (e.g. non-polar amino acids usually avoid exposure to aqueous solutions)

• Tertiary structure may be important for the function of the protein (e.g. specificity of active site in enzymes)

Quaternary (4º) Structure

• Multiple polypeptides or prosthetic groups may interact to form a single, larger, biologically active protein (quaternary structure)

• A prosthetic group is an inorganic compound involved in protein structure or function (e.g. the heme group in haemoglobin)

• A protein containing a prosthetic group is called a conjugated protein

• Quaternary structures may be held together by a variety of bonds (similar to tertiary structure)

Summary of the Four Levels of Protein Structure