What Are Histones - Their Types, Structure And Function - Microbial Tones (2023)

What are Histones?

• It is a group of relatively small proteins that are strongly alkaline and have a strong positive charge, because they have a high amino acid content, for example.lysine to arginine,and are present in the nuclei of eukaryotic cells. These are basic amino acids and give histones a positive charge. DNA is negatively charged and DNA binding to histones is stabilized by ionic bonds. These opposite charges help DNA bind to histone proteins known as nucleosomes.
• Histones were discovered inin 1884alreadyAlbrecht Kossel.It is very important to maintain the pH of histones as it is lowerpH 4, lose their specific secondary and tertiary structure, undergo non-specific aggregation and partially develop.
• Histones play an important role in maintaining structurechromosomes. Inside the nucleus is DNA, the length of which is about a meter or more if we extend it completely until the diameter of the nucleus does not exceed 5-10 nm. Folding this long DNA into a nucleus a million times smaller than DNA is something very strange.
• Chromosomal DNA is coiled hierarchically. Chromosome structure is highly ordered in a eukaryotic cell and each chromosome undergoes some level of condensation or complexity. DNA is complexed with various proteins, and to achieve this level of complexity, DNA changes into the form of chromatin. DNA is one2 nmthick double helix DNA is wrapped around histone protein complexes to fit into the cell nucleus and give the chromosomes a more compact shape.

histone species

There are five main types of histones.

1. H1
2. H2A
3. H2B
4. H3
5. H4

These species fall into two main classes.

1. Opcore histonesZonde H2A, H2B, H3 and H4.
2. Opbind histonesthey are H1 and H5(highest ratio of lysine to arginine). Linker histones participate in the highly ordered chromatin structure. To form a highly ordered DNA structure, the histone binding protein H1 locks the DNA in place by binding to nucleosomes at the DNA start and end sites. Histone H5 are individual proteins and play an important role in packaging a specific region of DNA. The mass of histones in chromatin is approximately equal to the mass of DNA in most cells.

The chromatin contains an equal amount of H2A, H2B, H3 and H4 molecules, but about half that number of H1 molecules. In different types of eukaryotic cells, this ratio remains constant.

Functions of histones

1. Their main function is to bind to DNA and help DNA to maintain its shape and control gene activity.
2. An active gene is less histone bound compared to an inactive gene which is highly histone bound. In particular, all types of nuclei contain histone proteins. They have basic properties and play an important role in the regulation and function of chromosomal DNA.
3. These proteins also play an important role in the regulation of DNA replication and transcription; in simple terms, they regulate cellular events.

histone structure

• The structural pattern of core histones is known asHistone-vouwdomein(The 3D structure of proteins is determined by their amino acid sequence, and folding proteins into their correct local structure is very important to perform their function). They consist of three alpha helices linked together by two loops.
• Histones are composed of high levels of basic amino acids, lysine and arginine, giving them a positive charge, and that negative charge helps them bind to negatively charged DNA.
• Oktamer histona- It is formed by a complex of eight proteins present in the center of the central particle of the nucleosome and these proteins play an important role in DNA packaging. The histone octamer consists of two copies each of the histone proteins H2A, H2B, H3 and H4.
• the nucleosomeIt consists of eight histone molecules or proteins (two H2A-H2B dimers and H3-H4 tetramers) that form a tertiary structure and these histone proteins are enveloped by a series147base pairs of double-stranded DNA. DNA and proteins are linked by salt bonds (unstable bonds of ionic nature). They are called salt bonds because of specific metal ions, for example Ca++ and Mg++. Nucleosomes are attached to other nucleosomes using a DNA linker that is short from 8 to 147 base pairs. The diameter of the nucleosome is 10 nm. In the nucleosome, histone proteins have tails known as N-terminal tails. These tails are made up of specific amino acids and with the help of these terminal tails undergo various translational modifications, preferably acetylation, phosphorylation and methylation. Using these tails, various proteins can bind to chromatin and, as a result, influence chromatin condensation and transcriptional activity. Nucleosomes are then packaged in the form of chromatin fibers and chromosomes.
• Solenoid– When a chain of nucleosomes (consisting of six nucleosomes) is coiled into a 30 nm coil, it is called a solenoid. In simple terms, we can say that it is a condensed chromatin fiber with a diameter of 30 nm and plays a role in DNA packaging. Chromatin fibers look like balls on a string, with the balls representing proteins and the strands of DNA. When the DNA is wrapped around a protein, it is known as a nucleosome. Chromatin fibers present in chromatin are 10 nm in diameter, but in intact cells, the chromatin forms a thick fiber 30 nm in diameter called a solenoid.

Effect of histone modification on chromatin packaging.

• Eight histone proteins have tails known as N-terminal tails that can be chemically modified. These modifications can take place by adding or removing acetyl and methyl groups. Loose nucleosome packing is favored by histone acylation, and tighter packing is favored by histone methylation.
• In different cells, different tails have different modification patterns. The general pattern of modification causes a change in the activity of the associated DNA known as the histone code (a series of histone modifications that indicate whether the chromatin is active or not). These histone codes are read by the proteins involved in DNA translation and gene expression. expression.
• Histonacetylering en deacetylering– Tails are formed by specific amino acids, the addition of an acetyl group (acetylation) to the chain of amino acids is carried out by the enzyme Histone acetyltransferase(SOMBRERO)This causes decondensation of chromatin, reduces the positive charge and weakens histone-DNA interactions. It also makes DNA more accessibleRNA-polymeraseII and thereby facilitates the rewriting process. Similarly, if we remove the acetyl group, transcription is suppressed.
• Histon-methylering- Methylation of the amino acid lysine is carried out usingHiston metil transferaza (HMT).For example, methylation of the fourth lysine.(H3K4)on histone H3 indicates that the genes are active, while lysine methylation at positions 9 and 27 indicates that the gene is inactive. Methylation cannot neutralize the charge, but it can suppress regulatory proteins that perform the function of binding methylated histones.

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