Reaction cycle access channel

The structure of beef liver catalase is shown in Fig. 2-16 [2], The heme site IS accessible by a channel 3 nm long. The specificity is largely determined by the ability of a substrate to form H-bonding interactions to some amino acids [2]. Unlike peroxidases, the fifth ligand of catalase is tyrosine. The phenyl ring is tilted 42° inwards to the heme plane with an iron-heme distance of 0.22 nm. This suggest that the phenolic side chain is deprotonated and has a localized charge. The reaction cycle with Fe(III) and Fe(IV) is shown in Eq. 2-6. 

In the cell, compartmentation of enzymes into multienzyme complexes or organelles provides a means of regulation, either because the compartment provides unique conditions or because it limits or channels access of the enzymes to substrates. Enzymes or pathways with a common function are often assembled into organelles. For example, enzymes of the TCA cycle are all located within the mitochondrion. The enzymes catalyze sequential reactions, and the product of one reaction is the substrate for the next reaction. The concentration of the pathway intermediates remains much higher within the mitochondrion than in the surrounding cellular cytoplasm. 

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