M-subunit

The L and M subunits show about 25% sequence identity and are therefore homologous and evolutionarily related proteins. The H subunit, on the other hand, has a completely different sequence. The fourth subunit of the reaction center is a cytochrome that has 336 amino acids with a sequence that is not similar to any other known cytochrome sequence. 

The L and the M subunits are firmly anchored in the membrane, each by five hydrophobic transmembrane a helices (yellow and red, respectively, in Figure 12.14). The structures of the L and M subunits are quite similar as expected from their sequence similarity they differ only in some of the loop regions. These loops, which connect the membrane-spanning helices, form rather flat hydrophilic regions on either side of the membrane to provide interaction areas with the H subunit (green in Figure 12.14) on the cytoplasmic side and with the cytochrome (blue in Figure 12.14) on the periplasmic side. The H subunit, in addition, has one transmembrane a helix at the car-boxy terminus of its polypeptide chain. The carboxy end of this chain is therefore on the same side of the membrane as the cytochrome. In total, eleven transmembrane a helices attach the L, M, and H subunits to the membrane. 

Alpha helices D and E from the L and M subunits (Figure 12.14) form the core of the membrane-spanning part of the complex. These four helices are tightly packed against each other in a way quite similar to the four-helix bundle motif in water-soluble proteins. Each of these four helices provides a histidine side chain as ligand to the Ee atom, which is located between the helices close to the cytoplasm. The role of the Ee atom is probably to... 

The structurally similar L and M subunits are related by a pseudo-twofold symmetry axis through the core, between the helices of the four-helix bundle motif. The photosynthetic pigments are bound to these subunits, most of them to the transmembrane helices, and they are also related by the same twofold symmetry axis (Figure 12.15). The pigments are arranged so that they form two possible pathways for electron transfer across the membrane, one on each side of the symmetry axis. 

Figure 12.15 Schematic arrangement of the photosynthetic pigments in the reaction center of Rhodopseudomonas viridis. The twofold symmetry axis that relates the L and the M subunits is aligned vertically in the plane of the paper. Electron transfer proceeds preferentially along the branch to the right. The periplasmic side of the membrane is near the top, and the cytoplasmic side is near the bottom of the structure. (From B. Furugren, courtesy of the Royal Swedish Academy of Science.)...

Comparison of the amino acid sequences of the L and M subunits of the reaction centers from three different bacterial species shows that about 50% of all residues in those two subunits are conserved in all three species. In the transmembrane helices, sequence conservation varies. Residues that are buried and have contacts either with pigments or with other transmembrane helices are about 60% conserved. In contrast, residues that are fully exposed to the membrane lipids are only 16% conserved. Clearly, fewer restrictions... 

LD is well known to be the combined activity of 5 isoenzymes (35) which in turn are thought to result from the combination in varying proportions of two basic subunits, heart (H) and muscle (M) subunits, to form a tetramer (36). Additional isoenzymes in skin and testicle have been described, and it may be that the LD isoenzyme spectrum ultimately will turn out to be complex. 

K15. Kanno, T., Sudo, K., Takeuchi, I., Kanda, S., Honda, N Nishimura, Y., and Oyama, K., Hereditary deficiency of lactate dehydrogenase M-subunit. Clin. Chim. Acta 108,267-276 (1980). 

K16. Kanno, T., and Maekawa, M Lactate dehydrogenase M-subunit deficiencies Clinical features, metabolic background, and genetic heterogeneities. Muscle Nerve (Suppl. 3), S54-S60 (1995). 

The clinical picture includes cramps and recurrent myoglobinuria following intense exercise. Aside from episodes of myoglobinuria, none of the patients was weak. Forearm ischemic exercise caused a 1.5-2.0-fold increase in venous lactate concentration, an abnormally low but not absent response. Muscle biopsy showed normal or only moderately increased glycogen concentration. Because other accessible tissues, such as erythrocytes, leukocytes and cultured fibroblasts, express a different isoenzyme, the diagnosis of PGM-M subunit deficiency must be established by biochemical studies of muscle. Four different... 

The enzyme responsible for this topping-up ATP in active muscle is CK. CK is found in high concentration in muscle cells, both free within the sarcoplasm and also associated with membranes of mitochondria and the sarcoplasmic reticulum. Structurally, creatine kinase is a dimeric enzyme of B and/or M subunits, each of about 40 kDa. Three quaternary structure isoenzyme forms arise CK-MM, CK-BB and CK-MB. The predominant form in all muscles is CK-MM, but cardiac muscle also contains a significant amount of CK-MB and this isoenzyme can be used as a specific marker of myocardial damage (see Case Notes at the end of this chapter). 

Sheinin, A, Shavit, S., Benveniste, M. Subunit specificity and mechanism of action of NMDA partial agonist D-Cycloserine, Neuropharmacology 2001, 41, 151-158. 

Quaternary structure of isoenzymes Many isoenzymes contain different subunits in various combinations. For example, creatine kinase occurs as three isoenzymes. Each isoenzyme is a dimer composed of two polypeptides (called B and M subunits) associated in one of three combinations CK1 = BB, CK2 = MB, and CK3 - MM. Each CK isoenzyme shows a characteristic electrophoretic mobility (see Figure 5.21). 

Figure 23-31 (A) Stereoscopic ribbon drawing of the photosynthetic reaction center proteins of Rhodopseudomonas viridis. Bound chromophores are drawn as wire models. The H subunit is at the bottom the L and M subunits are in the center. The upper globule is the cytochrome c. The view is toward the flat side of the L, M module with the L subunit toward the observer. (B) Stereo view of only the bound chromophores. The four heme groups Hel-He4, the bacteriochlorophylls (Bchl) and bacteriopheophytins (BPh), the quinones QA and QB/ and iron (Fe) are shown. The four hemes of the cytochrome are not shown in...

An example of an enzyme which has different isoenzyme forms is lactate dehydrogenase (LDH) which catalyzes the reversible conversion of pyruvate into lactate in the presence of the coenzyme NADH (see above). LDH is a tetramer of two different types of subunits, called H and M, which have small differences in amino acid sequence. The two subunits can combine randomly with each other, forming five isoenzymes that have the compositions H4, H3M, H2M2, HM3 and M4. The five isoenzymes can be resolved electrophoretically (see Topic B8). M subunits predominate in skeletal muscle and liver, whereas H subunits predominate in the heart. H4 and H3M isoenzymes are found predominantly in the heart and red blood cells H2M2 is found predominantly in the brain and kidney while HM3 and M4 are found predominantly in the liver and skeletal muscle. Thus, the isoenzyme pattern is characteristic of a particular tissue, a factor which is of immense diagnostic importance in medicine. Myocardial infarction, infectious hepatitis and muscle diseases involve cell death of the affected tissue, with release of the cell contents into the blood. As LDH is a soluble, cytosolic protein it is readily released in these conditions. Under normal circumstances there is little LDH in the blood. Therefore the pattern of LDH isoenzymes in the blood is indicative of the tissue that released the isoenzymes and so can be used to diagnose a condition, such as a myocardial infarction, and to monitor the progress of treatment. 

LDH is a tetrameric enzyme. In the heart enzyme the H subunits predominate, whereas in skeletal muscle the M subunits predominate. 

Heart muscle in aerobic. It is likely to be converting lactate to pyruvate, rather than the other way around. Skeletal muscle has M subunits, heart muscle H type. 

Figure 2.1 An IDP composed of very small <0.2 p.m subunits. The IDPs show the fluffy structure of particles that is expected for dust agglomerates in protoplanetary accretion disks. The IDPs are suspected to be composed partly from particles of interstellar origin. A small fraction of the subunits can be identified as presolar dust grains. (Courtesy of NASA, http //stardust.jpl.nasa.gov/science/sci2.html.)...

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