Valyl-tRNA synthetase can sterically exclude isoleucine from its catalytic pocket because isoleucine is larger than valine. In contrast, valine should slip easily into the catalytic pocket of the isoleucyl-tRNA synthetase. Although both amino acids will fit into the isoleucyl-tRNA synthetase amino acid–binding site, interactions with the extra 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: 7298

#Unit 6. System Physiology – Plant

PPDK-RP make PPDK active removing phosphates of catalytic his/threo residues using ADP as phosphoryl donor, rather than ATP, and the phosphorolytic dephosphorylation of the (PPDK)−Thr−O−P with formation of pyrophosphate, rather than the hydrolysis employed by most protein

phosphatases.

TLS Online TPP Program

#Id: 7299

#Unit 6. System Physiology – Plant

TLS Online TPP Program

#Id: 7300

#Unit 6. System Physiology – Plant

Crassulacean acid metabolism (CAM). Temporal separation of CO2 uptake from photosynthetic reactions: CO2 uptake and fixation take place at night, and decarboxylation and refixation of the internally released CO2 occur during the day. The adaptive advantage of CAM is the reduction of water loss by transpiration, achieved by the stomatal opening during the night.

TLS Online TPP Program

#Id: 7301

#Unit 6. System Physiology – Plant

Typically, a CAM plant loses 50 to 100 g of water for every gram of CO2 gained, compared with values of 250 to 300 g and 400 to 500 g for C4 and C3 plants Thus, CAM plants have a competitive advantage in dry environments.

TLS Online TPP Program

#Id: 7302

#Unit 6. System Physiology – Plant

In CAM plants, formation of the C4 acids is both temporally and spatially separated. At night, CO2 is captured by PEP carboxylase in the cytosol, and the malate that forms from the oxaloacetate product is stored in the vacuole

TLS Online TPP Program

#Id: 7303

#Unit 6. System Physiology – Plant

PEPCase‐kinase transcripts accumulate during the night, controlled by the circadian oscillator and metabolites. Nocturnal phosphorylation of PEPCase controls primary carboxylation.