Apoprotein-heme enzyme

In protective heme enzymes such as catalase and peroxidases, the dominant process is heterolytic, and amino acids such as histidine in the apoprotein are in close proximity in the active site to transfer protons to the RO in sim. For... 

As well as induction of the synthesis of the apoprotein portion of cytochrome P-450, there is also induction of the synthesis of the heme portion. Clearly, it is also necessary to have an increased amount of heme if there is an increase in the amount of the enzyme apoprotein being synthesized. Thus, the rate-limiting step in heme synthesis, the enzyme 5-aminolaevulinate synthetase, is inducible by both phenobarbital and TCDD. This is the result of transcriptional activation of the gene, which codes for the S-aminolaevulinate synthetase. It may be that the decrease in the heme pool, which results from incorporation of heme into the newly synthesized apoprotein, leads to derepression of the gene and hence increased mRNA synthesis. The gene repression could be heme-mediated, or heme may modulate P-450 genes. 

Flavin-dependent le -transfer in enzymes and chemical model systems can he differentiated from 2e -transfer activities, i.e., (de)hydrogenation and oxygen activation, by chemical structure and dynamics. For le -transfer, two types of contacts are discussed, namely outer sphere for interflavin and flavin-heme and inner sphere for flavinr-fenedoxin contacts. Flavin is the indispensable mediator between 2e - and le -transfer in all biological redox chains, and there is a minimal requirement of three cooperating redox-active sites for this activity. The switch between 2e - and le -transfer is caused by apoprotein-dependent prototropy between flavin positions N(l)/0(2a) and N(5) or by N(5)-metal contact. 

Both myoglobin and hemoglobin contain a prosthetic group (a nonpolypeptide part of a protein), namely heme. Prosthetic groups remain bound to the protein permanently, by covalent bonds, noncovalent bonds, or both. The protein without the prosthetic group is called an apoprotein. When an enzyme is involved, the apoprotein plus prosthetic group may be called the holoenzyme. 

Many enzymes are initially synthesized as inactive forms, which require exogenous cofactors for their activity. Only after the inactive apoprotein combines with the cofactor it becomes the active holoenzyme. Cofactors may be dissociable or tightly bound with the latter often referred to as a prosthetic group. A cofactor may be a metal (e.g., iron or copper), an organic compound (e.g., pyridoxal phosphate or flavin), or an organometallic compound (e.g., heme or cobalamin). Cofactors are necessary to assist amino acid residues at the active sites... 

The heme group in the cytochrome 65 molecule is easily removed by an acid-acetone treatment at low temperature, and the resulting apoprotein rapidly recombines with protohematin. The reconstituted cytochrome shows the same spectral and enzymic properties as the original cytochrome 65 (1 6). The apocytochrome 65 readily combines also with other iron porphyrins (1S5). 

Mechanism-based CYP inhibition or irreversible inhibition, involves permanent inactivation of CYP enzymes during catalysis, where reactive intermediate(s) are formed, leading to apoprotein or heme-ion center modification. Typical characteristics of mechanism-based enzyme inhibition include time-dependent loss of enzyme activity, a rate of inactivation generally following saturation kinetics, enzyme activity that cannot be recovered after... 

In the first step of the heme branch, protoporphyrin IX ferrochelatase (FeCh) inserts Fe + into protoporphyrin IX to produce protoheme (Figure 5.37). Heme oxygenase catalyzes the oxidation and the ring opening of protoheme to give biliverdin EXa. This reaction is followed by the conversion to (3 )-phytochromobilin controlled by phytochromobilin synthase. Isomerization of 3 -phytochromobilin into the (3Z) isomer takes place before the chromophore is bound to the phytochrome apoprotein. Whether such an isomerization step is catalyzed by an enzyme or whether it proceeds spontaneously remained unknown [137]. 

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