1. Proteins containing non-heme iron.

Ferritin (about 20% iron) is concentrated mainly in splean, liver and bone marrow. It plays the role of iron storage in the body.

Serum transferrin (about 0.13% iron) transports iron ions in reticulocytes, which made the biosynthesis of hemoglobin.

2. Metalloenzymes are proteins that have enzymatic activity and containing metal cations. In metalloenzymes protein bond with the metal more stable. Enzymes that are activated by metal ions are less strongly associated with metals.

ATPase contains Na, K, Ca, Mg, alcohol dehydrogenase - Zn, cytochrome oxidase - Cu, proteinase - Mg, K.

6. Nucleoproteins (NP) are stable complexes of nucleic acids and proteins.

Nucleic acids

Nucleic acids – RNA and DNA – are polymers of nucleotides.

DNA is found in the cell nucleus and mitochondria.

RNA is found in all parts of the cell. We distinguish messenger RNA (mRNA). It synthesized by DNA and determines the order of amino acids in a protein molecule. Ribosomal RNA (r-RNA) is part of the ribosome. Transport RNA (t-RNA) carries amino acids to the place of protein synthesis.

NA provide storage and transmission of genetic information by programming the synthesis of cellular proteins.

Bases of NA – DNA: purine - A, G, pyrimidine – C, T; RNA: purine – A, G, pyrimidine – C, U. One of the important properties of free nitrogenous bases is that they can exist in two tautomeric forms. Also NA containe carbohydrates (ribose and deoxyribose) and phosphoric acid residues.

Nucleotides consist of three components: a pyrimidine or purine base, a pentose and phosphoric acid (fig. 7). Nucleotides are nucleoside phosphates.

Fig. 7. Nucleotide structure

To study the chemical composition of NA using sequencing - splitting up into fragments by enzymes or chemical reagents. Products are analyzed by electrophoresis, chromatography, etc.

DNA isolated from different tissues of the same species, has the same composition of the nitrogenous bases. Analysis of DNA composition and the quantity of bases was established for the first time by Erwin Chargaff, Austrian-born American biochemist. The quantitative relations were named Chargaff's rules.

1. The quantity of purine bases (in moles) is equal to the quantity of pyrimidine bases: A + G = C + T.

2. The quantity of adenine and cytosine is equal to the quantity of guanine and thymine: A + C = G + T.

3. The quantity of adenine is always approximately equal to that of thymine, and the quantity of guanine is always approximately equal to that of cytosine.

A = T, G = C.

4. The A/G ratio varies widely from species to species. The coefficient of specificity is (G + C) / (A + T) (0,54 - 0,94 in animals, 0,45-2,57 in microorganisms).

The structure of nucleic acids

The primary structure of nucleic acids is the sequence of mononucleotides in the polynucleotide chain of DNA and RNA. Monomers in nucleic acid molecules are connected by an ester bond formed by the phosphate residue of one mononucleotide and 3'-hydroxyl group of the pentose residue of another mononucleotide (3', 5'-phosphodiester bond) (fig. 8). To study the chemical composition of NA sequencing is used. It is NA splitting into fragments by enzymes or chemical reagents, and analysis of the products by electrophoresis, chromatography, etc.

Fig. 8. The primary structure of DNA

Secondary structure. DNA is composed of two strands, forming a double helix in which the two polynucleotide chains twisted around the same axis. The bases are inside, and carbohydrate components are outside (fig. 9). Bases are stacked in pairs: purine from the one strand and pyrimidine - from another. The interaction of pairs of A-T and G-C called complementarity, and the corresponding bases - complementary. The stability of A - T pairs is provided by two hydrogen bonds, and pairs of G - C - by three hydrogen bonds. DNA strands are complementary to each other.

The forces of hydrophobic interactions (stacking interactions) occure between the bases, assembled in a stack along the DNA molecule. They make a great contribution to the stabilization of the double helix. The intensity of stacking:

Both chains in the DNA molecule have opposite polarity: internucleotide bond in one chain has a direction of 5'  3', the other - 3'5'.

Fig. 9. Secondary structure of DNA
Fig. 10. Complementary of strands in DNA.

The configuration of the double helix of DNA varies from the quantity of water and ionic strength of solution. Existence more than 10 forms of DNA was established by X-ray analysis.

RNA molecules are constructed from a single polynucleotide chain. In this chain there are complementary areas, which form a double helix. At the same time the pair A - U and G - C are connected by hydrogen bonds. Helical regions of RNA (hairpin) contain 20-30 base pairs and alternate with non-helical parts.

R. Holley suggested two-dimensional cloverleaf model for t-RNA. There is a helix polynucleotide chain on itself in strictly fixed locations. Features of the t-RNA structure are directly related to the processes of translation, so they are considered in more detail in the section of protein biosynthesis.

The secondary structure of the messenger and ribosomal RNA is not studied as well. Secondary structure of r-RNA and m-RNA is characterized by helix formation on itself.

Some nucleotide sequences in the secondary structure of DNA and RNA are called palindromic (inverted repeations). These repeations are the basis for the formation of hairpins or triple helices.

The tertiary structure of nucleic acids. DNA double helix in some areas may be further coiling to form a superhelix. Some viruses detected single-stranded DNA of linear and circular forms. The tertiary structure of the m-RNA and t-RNA is packed more compact by folding of various parts of the molecule.

Nucleic acids are parts of nucleoproteins (NP). They perform functions of storage and transmission of genetic information.

There are 2 types of NP, which differ from each other in composition, size, and physical and chemical properties - deoxyribonucleoproteins (DNP, contain DNA) and ribonucleoproteins (RNP, contain RNA). DNP predominantly located in the cells nuclei and PNR - in the cytoplasm. Typical representatives of the NP are ribosome (ribosomal RNA complexes with proteins). DNP-chromatin is the complex of DNA with histone and non-histone proteins.

Histones are strongly alkaline proteins of low molecular weight, soluble in dilute acids. They contain large amounts of lysine and arginine. 5 classes of histones vary in size, amino acid composition and amount of charge (always positive). They take part in the structural organization of chromatin by neutralizing negatively charged phosphate groups of DNA by positive charges of amino acid residues.

The nature of non-histone proteins is not found out enough.

Many chromatin proteins are characterized by specific structural motifs that provide their binding to DNA: “leucine zipper”, “α-helix - turn - α-helix”, "zinc finger".