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Intracellular Proteins

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Intracellular Proteins

Intracellular Proteins Background

Intracellular proteins are synthesized by cytoplasmic free ribosomes, which do not undergo endoplasmic reticulum, processing of Golgi and exocytosis of cell membranes, and produce only a class of proteins that are affected in cells. Intracellular proteins are synthesized on cytoplasmic free ribosomes and do not need to be transported outside the cell membrane to function in cells. For example, respiratory enzyme DNA polymerase, various transaminase, DNA helicase, and RNA polymerase. The cells have free ribosomes and ribosomes attached to the endoplasmic reticulum. Free ribosomes are produced by intracellular proteins, which do not pass through the endoplasmic reticulum and the Golgi apparatus. In the ribosome attached to the endoplasmic reticulum, the ribosome only converts the amino acid into a peptide chain, and further processing is carried out by the endoplasmic reticulum and the Golgi apparatus, and finally secreted by the cell membrane. In fact, there are many types of intracellular proteins, and different ways of protein formation may be different.

Degradation pathway of intracellular protein

There are three main pathways for degradation of proteins in eukaryotic cells, the lysosomal pathway, the ubiquitination pathway, and the caspase pathway.

1. Lysosome pathway: Protein is degraded by the corresponding enzyme in the acidic environment of the same enzyme, and then transported to the cell fluid through the carrier protein of the lysosomal membrane to supplement the cytosol metabolism library. Intracellular proteins: Some proteins in the cytosol contain a KFERQ signal at the N-terminus, which can be recognized by HSC70. HSC70 helps these proteins to enter the lysosome and be degraded by proteolytic enzymes. Extracellular protein: enters cells through endocytosis or pinocytosis and degrades in lysosomes.

2. Ubiquitin-proteolytic enzyme pathway: an important pathway for specific protein degradation, involved in various metabolic activities of the body, mainly degrading Cyclin, spindle-related proteins, cell surface receptors such as epidermal growth factor receptor, Transcription factors such as NF-KB, tumor suppressor factors such as P53, oncogene products, etc.; intracellular denaturing proteins and abnormal proteins are also degraded by this pathway under stress conditions. This pathway is dependent on ATP and consists of two steps, namely polyubiquitination of the target protein. The polyubiquitinated protein is hydrolyzed by the 26S proteolytic enzyme complex.

3. Caspase pathway: a protein degradation pathway for apoptosis.

The meaning of caspase means that the active site of this type of protease is the highly conserved cysteine ​​(cysteine) and the asparagine (aspase) which specifically cleaves the substrate, referred to as caspase. According to its specific function, it is divided into caspase (caspase1, 2, 4, 5, 8, 9, 10) and effect caspase (caspase 3, 6, 7, 11). Caspase is present in normal cells as a zymogen and is activated upon initiation of apoptosis. One pathway is mediated by the death domain of the death signaling molecule and the receptor, which causes caspase-8 to catalyze itself as a protease with hydrolase activity, hydrolyzing downstream caspase-3,6,7, etc., caspase-3,6 , 7 acts on the substrate to degrade, leading to apoptosis; the other pathway is mediated by cytochrome C located on the mitochondria, activates caspase-9, activates caspase-9 and activates caspase-3. The proteins that are degraded in apoptosis are: DNA damage repair enzyme, U1 small nuclear ribonucleoprotein component, lamin, actin and lining protein. The degradation of these enzymes and proteins leads to small apoptosis of cells. The body is eventually engulfed and digested by phagocytic cells.

Examples of Intracellular Proteins

  1. Hemoglobin

    Hemoglobin is a protein responsible for carrying oxygen in higher organisms. It is located in red blood cells and is an intracellular protein. In addition to carrying oxygen, hemoglobin can also be combined with carbon dioxide, carbon monoxide, and cyanide ions, and the combination is exactly the same as oxygen. The only difference is the firmness of the combination. Once carbon monoxide and cyanide ions are combined with hemoglobin, it is difficult to leave. This is the principle of gas poisoning and cyanide poisoning. In this case, other substances with stronger binding ability to these substances can be used for detoxification. For example, carbon monoxide poisoning can be used for veins. The method of injecting methylene blue is used for treatment.

    Structure of human haemoglobin. Figure 1. Structure of human haemoglobin.
  2. Respiratory enzymes

    Respiratory enzymes include both aerobic and anaerobic respiration enzymes, all located in cells and belonging to intracellular proteins. The first phase of aerobic respiration occurs in the cytoplasmic matrix and requires the catalysis of related enzymes in the cytoplasmic matrix; the second and third phases of aerobic respiration occur in the mitochondria and require enzymes on the mitochondrial inner membrane to catalyze; anaerobic respiration All occur in the cytoplasmic matrix and must be catalyzed by enzymes in the cytoplasmic matrix.

  3. DNA polymerase, DNA helicase, and RNA polymerase

    DNA polymerase, DNA helicase, and RNA polymerase are all located in the nucleus, both of which belong to intracellular proteins. DNA is unfolded by DNA helicase. When DNA is copied, both strands act as templates, and under the action of DNA polymerase, deoxynucleotides are linked into a chain, and the newly synthesized daughter strand and the original parent strand are reassembled into new DNA molecules; when DNA is transcribed One strand is a template, and under the action of RNA polymerase, ribonucleotides are linked into a chain, and new RNA molecules are generated.

    3D structure of the DNA-binding helix-turn-helix motifs in human DNA polymerase beta. Figure 2. 3D structure of the DNA-binding helix-turn-helix motifs in human DNA polymerase beta.
  4. Transaminase

    Transaminase is a kind of enzyme that catalyzes the amino transfer between amino acid and keto acid. It is common in animal tissues such as myocardium, brain, liver and kidney, and belongs to intracellular proteins. Transaminase is involved in the decomposition and synthesis of amino acids. The keto acid or aldehyde acid produced by transamination of amino acids can be oxidatively decomposed to supply energy or converted into sugars or fatty acids. Conversely, keto acid or aldehyde acid can also form non-essential amino acids through the action of transaminase, and transmutation between certain amino acids is also involved in transaminase.

Aspartate transaminase from E. coli with Pyridoxal 5' Phosphate cofactor. Figure 3. Aspartate transaminase from E. coli with Pyridoxal 5' Phosphate cofactor.


  1. Karmen A.; et al. Transaminase activity in human blood. The Journal of Clinical Investigation. 1995, 34 (1): 126–31.

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