Isoenzyme polypeptide chains

The data in Table III giving the amino acid compositions of the isoenzymes of glucoamylase indicate a similarity, if not an identity, in the polypeptide chains in the two forms. Information is not available on the amino acid sequences in the glucoamylase isoenzymes. However, the two isoenzymes contain alanine at the N-terminus, and exhibit similar immunological properties.7,41 All of these observations point to the presence of identical polypeptide chains in the two iso-... 

MPO is a covalently linked dimeric glycoprotein of Mr 140 kDa comprised of 745 amino acids. MPO consists of three different isoenzyme forms termed a, b, and c. The natme of the chemical differences between these isoforms is not fiilly understood, and only isoform c has been crystallized. Catalytically active recombinant human MPO has been expressed in Chinese hamster ovary cells. However, incomplete posttranslational processing of the recombinant enzyme yields a monomeric form of the enzyme consisting of a single polypeptide chain of Mr 84 kDa with altered carbohydrate content. ... 

A single polypeptide chain can in theory exist in an infinite number of different conformations. However, one specific conformation generally appears to be the most stable for any given sequence of amino acids, and this conformation is assumed by the chain as it is synthesized within the cell. Thus, the primary structure of the polypeptide chain also determines its three-dimensional secondary and tertiary structures. It is conceivable that in some cases there may be several alternative conformations ("conforraers ) of a single chain that are of nearly equal stabilities and therefore these alternative forms may coexist. This possibility was first suggested to account for the heterogeneity noted in preparations of the cytoplasmic and mitochondrial isoenzymes of malate dehydrogenase and has also been proposed as an explanation of the multiple electrophoretic zones of erythrocyte acid phosphatase. However, no multiple enzyme forms have been shown unequivocally to be due to conformational isomerism. 

The properties and biological function of LPO have recently been reviewed. This secretory peroxidase is isolated from cow s milk. Crystalline LPO from bovine milk was obtained in 1943 and shown to be green colored like MPO. Its isolation and purification was simplified by precipitation of casein by rennet. The enzyme with a single polypeptide chain has been known to exist as several isoenzyme forms. The Soret maxima of LPO of 412 nm with molar absorptivity of 112 mM cm is typical of most peroxidases. It recently has been demonstrated that the heme in LPO is attached to the protein by ester bonds between the heme 1- and 5-methyl groups, and Glu375 and Asp275, respectively. ... 

To explain the CPK isoenzyme pattern in hereditary muscular dystrophy Schapira et al. (1968) and Cao et al. (1971) suggested that the organism simply fails to convert the fetal to the adult CPK pattern. Hooton and Watts (1966), however, claimed that hereditarily dystrophic mice contained an MM-CPK composed of two nonidentical subunits M and M. Fingerprints of the M polypeptide chain appeared to differ from those of the M subunit by a single peptide. Since the MM dimer showed only half the activity of the normal enzyme it was assumed that the M subunit was inactive. This would also explain why no M M enzyme was detected. Studies of CPK in dystrophic chickens and dystrophic humans could not confirm the existence of a distinct isoenzyme in dystrophic tissues (Roy et al, 1970). Should it exist, moreover, the question would remain whether this alteration is a specific feature of muscular dystrophy or whether it is the mere consequence of increased proteolysis in damaged muscle. 

Certain enzymes exist in several different forms termed isoenzymes which are distinguishable by electrophoresis (mobility in an electric current) and by their immimochemistry. The different forms of the enzyme may occur in different tissues or alter in their relative proportions within the same tissue with time. The case which has been most studied is that of the five isoenzymes of lactate dehydrogenase which occur in mammalian tissues. Each isoenzyme has the same molecular weight (135,000) and is built up of foiu- polypeptide chains of two kinds (a and j8). The isoenzymes differ in their a, p make up thus 4a, 4j8, la + 3/S, 2a + 2/S, 3a - - 1/3. At least 33 different enzymes have been reported to exist as isoenzymes in plant tissues. 

Heavy and light chain polypeptides of the prophenoloxidase-activating isoenzymes (PPAE-1 and PPAE-2) of the silkworm Bombyx mori [467]... 

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