3.8 : 복합 단백질

Proteins form complexes with other small and large molecules, including nucleic acids, carbohydrates, and lipids. These are called conjugated proteins, meaning their complexes consist of both amino acids and nonprotein components.

Nucleoproteins are comprised of both proteins and nucleic acids. 

Deoxyribonucleoproteins or DNPs contain DNA and are commonly found in the nucleus. In eukaryotic cells, DNA is tightly wrapped around histone proteins, forming nucleosomes, a crucial part of chromosome structure.

Ribonucleoproteins or RNPs contain RNA and can be located in the nucleus, as well as the cytoplasm. The spliceosome is a large complex of five small nuclear RNPs, or snRNPs containing both RNA and protein along with accessory cofactors. The spliceosome removes undesired nucleotides from a precursor gene transcript. 

Glycoproteins and proteoglycans are protein complexes that contain carbohydrates of varying sizes.

Antibodies are glycoproteins with short carbohydrate chains, critical for normal immune function. 

In contrast, proteoglycans are formed from long repeating sugar units.  Often these are modified sugars that contain other functional groups instead of a hydroxyl group. Proteoglycans are a major component of the extracellular matrix in long-bone cartilage, ensuring frictionless motion of the joints.

Lipids also combine with proteins to perform a variety of functions. 

Lipoproteins are water-soluble complexes, having a hydrophobic lipid core and hydrophilic protein surface. Insoluble cholesterol cannot travel freely through blood, so it binds to lipoprotein core, which aids in its transport. 

In contrast, proteolipids are water-insoluble complexes with hydrophobic amino acid residues attached to lipid molecules. Proteolipids are abundant in brain tissues where they can form an insulating coating that maintains normal motor functions.

Additionally, proteins form complexes with many other types of molecules.

Red blood cells derive their color from heme, a nonprotein component of hemoglobin. Heme has a central iron atom that binds and transports oxygen through the bloodstream.