From prokaryotes to eukaryotes

From prokaryotes to eukaryotes

The appearance of eukaryotic cells ~2 billion years ago has been linked to the introduction of oxygen in the atmosphere and therefore aerobic metabolism. The increased presence of oxygen produces a more efficient energy source in the form of aerobic metabolism, producing 16–18 times more adenosine triphosphate (ATP) per hexose sugar than anaerobic metabolism. Since aerobic metabolism generates more energy, approximately 1000 more reactions can occur than under anaerobic metabolism.

This allowed the generation of new metabolites, for example, steroids, alkaloids and isoflavonoids

Steroids and polyunsaturated fatty acids are important elements of membranes; thus, they must have been involved in promoting organelle formation and cell compartmentalisation.

As some of the metabolites produced from respiration are involved in processes that target nuclear receptors, it has been hypothesized that higher ambient oxygen promoted these nuclear signalling systems within cells. Nuclear factors have conserved volumes and are highly hydrophobic, since they must pass through cell membranes. Experiments comparing the volume and hydrophobicity of both aerobic and anaerobic metabolites to those known for nuclear ligands indicate that aerobic metabolites are more hydrophobic and more closely match the required volumes of appropriate molecules for nuclear factors compared to the anaerobic molecules.

Since these appear important in superior eukaryotes, it has been hypothesized that such events influenced by increased oxygen levels have influenced biological evolution.

approximately 68% of transmembrane proteins in humans are high-oxygen content, whereas in most bacteria, such as E. coli, only 36% are oxygen rich. The size of the extracellular domains of transmembrane proteins increased as ambient oxygen concentration increased.

The size of the extracellular domains of transmembrane proteins increased as ambient oxygen concentration increased.

Larger and higher oxygen-containing proteins are more energy demanding than those lacking oxygen in their side chains. However, the primary hypothesis suggests that the presence of large and oxygen-rich amino acids as side chains would have resulted in weak protein structures during anoxic eras

Eukaryotes assign more communication roles to proteins than prokaryotes, allowing for more complex and variable signalling pathways. In order to achieve this, various changes in protein structure and function had to occur during evolution. The secondary structures of transmembrane proteins are more hydrophobic, and oxygen as well as nitrogen is important during the formation of hydrophilic structures. Acquisti et al. studied the oxygen content and the topology of transmembrane proteins of different organisms. The ratio of receptors to channels was higher, with a greater amount of oxygen-rich proteins, in more highly developed and ‘recent’ organisms. For example, approximately 68% of transmembrane proteins in humans are high-oxygen content, whereas in most bacteria, such as E. coli, only 36% are oxygen rich. The size of the extracellular domains of transmembrane proteins increased as ambient oxygen concentration increased. Larger and higher oxygen-containing proteins are more energy demanding than those lacking oxygen in their side chains. However, the primary hypothesis suggests that the presence of large and oxygen-rich amino acids as side chains would have resulted in weak protein structures during anoxic eras. Eukaryotes have an abundance of oxygen in the plasma membrane, as oxygen is utilized in the mitochondria. This compartmentalisation may have evolved as a mechanism to protect the transmembrane proteins, which are rich in oxygen. Through the development of complex compartmentalisation of cells, multiple processes including signalling and oxygen levels could also be controlled in different parts of the cell. In addition, there has been the emergence of multiple cell types within the same ‘greater’ organism, with over 200 different cell types in the adult human body. Multicellular organisms may have required both the accumulation of oxygen-rich amino acids in their transmembrane proteins and the allocation of respiration to specific intracellular compartments, that is, mitochondria.