Electron delocalisation extends throughout the cyclic tetrapyrrole ring of porphyrins and gives rise to an intense transition at 400 nm called the Soret band.

TLS Online TPP Program

#Id: 3647

#Unit 3. Fundamental Processes

The amino acids for which the Class I synthetases are specific tend to be larger and more hydrophobic than those for Class II synthetases.

TLS Online TPP Program

#Id: 3648

#Unit 3. Fundamental Processes

In ValRS, the side chain of Asp 279 protrudes into the editing active site, where it hydrogen-bonds with the hydroxyl group of threonine. The isosteric (having the same shape) valine lacks this hydroxyl group and is thereby excluded from the editing pocket.

TLS Online TPP Program

#Id: 3649

#Unit 3. Fundamental Processes

Valine cannot coordinate the Zn2+ in this way and hence does not undergo adenylation by ThrRS. A separate editing site deal with misacylated Ser–tRNAThr.

TLS Online TPP Program

#Id: 3650

#Unit 3. Fundamental Processes

TyrRS distinguishes between tyrosine and phenylalanine through hydrogen bonding with tyrosine’s OH group. Since no other amino acid resembles tyrosine, the enzyme can do without an editing function.

TLS Online TPP Program

#Id: 3651

#Unit 3. Fundamental Processes

GlnRS is absent in gram-positive bacteria, archaebacteria, cyanobacteria, mitochondria, and chloroplasts.

TLS Online TPP Program

#Id: 3652

#Unit 3. Fundamental Processes

In many bacteria glutamate is linked to tRNAGln by the same GluRS that synthesizes Glu–tRNAGlu. The resulting Glu–tRNAGln is then converted to Gln–tRNAGln by Glu–tRNAGln amidotransferase in an ATP-requiring reaction in which glutamine is the amide donor.