Translocase of the mitochondrial inner membrane

Rassow J, Dekker PJ, van Wilpe S, Meijer M, Soli J (1999) The preprotein translocase of the mitochondrial inner membrane function and evolution. J Mol Biol 286 105-120... 

Meier S, Neupert W, Herrmann JM (2005) Conserved N-terminal negative charges in the Timl7 subunit of the TIM23 translocase play a critical role in the import of preproteins into mitochondria. J Biol Chem 280 7777-7785 Meinecke M et al. (2006) Tim50 maintains the permeability barrier of the mitochondrial inner membrane. Science 312 1523-1526... 

A specific transport protein, the carnitine-acylcarnitine translocase, moves the fatty acylcarnitine into the mitochondrial matrix while returning carnitine from the matrix to the cytoplasm. Once inside the mitochondria, another enzyme, carnitine palmitoyltransferase II (CPT II), located on the matrix side of the mitochondrial inner membrane, catalyzes the reconversion of fatty acylcarnitine to fatty acyl-CoA. Intramitochondrial fatty acyl-CoA then undergoes (3-oxidation to generate acetyl-CoA.Acetyl-CoA can enter the Kreb s cycle for complete oxidation or, in the liver, be used for the synthesis of acetoacetate and P-hydroxybutyrate (ketone bodies). 

The fact that the mitochondrial inner membrane is virtually impermeable to long-chain fatty acyl-CoA, while the fatty acid oxidative machinery is located inside the mitochondrial matrix, a space enclosed by the inner membrane, might create a serious problem for cellular energy production. The problem is solved by the development of a transmembrane carnitine-dependent transport system for the long-chain acyl residue of acyl-CoA. Catalyzed by carnitine acyltransferase I (CAT-I), which is attached to the inner surface of the mitochondrial outer membrane, fatty acyl-CoA is converted to fatty acyl-carnitine by replacing the CoA residue with carnitine (Figure 3). Fatty acyl-carnitine is transported across the mitochondrial inner membrane in exchange for a molecule of free carnitine by carnitine-acylcarnitine translocase. After arrival in the mitochondrial matrix, fatty acyl-carnitine is converted back to acyl-CoA by carnitine acyltransferase II (CAT-II), an enzyme located on the inner surface of the mitochondrial inner membrane. 

Carnitine is linked to acyl groups transported into the mitochondria for oxidation (Figure 18.15). Acyl-CoAs in the cytoplasm are converted to acyl-camitine derviatives by action of carnitine acyltransferase I on the outer portion of the mitochondrial inner membrane. A translocase carries the acyl-carnitine into the mitochondria. Once inside the mitochondrial matrix, carnitine is replaced on the acyl group by CoASH. The acyl-CoA then is free to go through oxidation or elongation. 

Before long-chain fatty acids can enter the mitochondria and get access to the P-oxidation pathway, they must first be activated to acyl-CoA in a reaction that requires ATP and coenzyme-A. The acyl-CoA still cannot cross the mitochondrial inner membrane and must react with carnitine to form the corresponding carnitine ester. This reaction is catalyzed by the enzyme carnitine palmitoyltransferase (CPT). The acylcamitine itself is also unable to diffuse into the mitochondrial matrix so that the transport is achieved by a specific protein, the carnitine acylcamitine translocase. Following transport across the mitochondrial inner membrane, acylcamitines are converted back to the corresponding acyl-CoA and carnitine. This reaction is catalyzed by another carnitine palmitoyltransferase which is a different enzyme than that involved in the formation of the acylcamitine outside the mitochondria. Hence, there are two CPTs, one associated with the inner aspect of the mitochondrial inner membrane, CPT-lP and one that lies... 

Rehling P, Model K, Brandner K, Kovermann P, Sickmann A, Meyer HE, Kuhlbrandt W, Wagner R, Truscott KN, Pfanner N (2003) Protein insertion into the mitochondrial inner membrane by a twin-pore translocase. Science 299 1747-1751 Richards TA, Cavalier-Smith T (2005) Myosin domain evolution and the primary divergence of eukaryotes. Nature 436 1113-1118... 

Acyl CoA is formed in the cytosol, and the enzymes of the (3-oxidation pathway are in the matrix of the mitochondrion. The mitochondrial inner membrane is impermeable to CoA and its acyl derivatives. However, a translocase protein can shuttle carnitine and its acyl derivatives across the inner mitochondrial membrane. The acyl group is transferred to carnitine on the cytosol side of the inner membrane and back to CoA on the matrix side. Thus, carnitine acts as a transmembrane carrier of acyl groups. 

The exchange of ADP and ATP across the mitochondrial inner membrane, mediated by the adenine nucleotide translocase, is thought to be rate-limiting for oxidative phosphorylation (Heldt and Pfaff, 1969). This translocase functions asymmetrically, i.e., it favors a rapid entry of ADP dependent on the availablity of intramitochondrial ATP and results in an asymmetric distribution of ATP and ADP on either side of the membrane (Heldt et al., 1972, Slater et al., 1973). Thus the... 

Carnitine (p-hydroxy-y-trimethylammonium butyrate), (CHjljN"—CH2—CH(OH)—CH2—COO , is widely distributed and is particularly abundant in muscle. Long-chain acyl-CoA (or FFA) will not penetrate the inner membrane of mitochondria. However, carnitine palmitoyltransferase-I, present in the outer mitochondrial membrane, converts long-chain acyl-CoA to acylcarnitine, which is able to penetrate the inner membrane and gain access to the P-oxidation system of enzymes (Figure 22-1). Carnitine-acylcar-nitine translocase acts as an inner membrane exchange transporter. Acylcarnitine is transported in, coupled with the transport out of one molecule of carnitine. The acylcarnitine then reacts with CoA, cat-... 

FIGURE 31-7 Mitochondrial carriers. Ions and small molecules enter the intermembrane space, since the outer mitochondrial membrane is not a significant permeability barrier. However, the inner mitochondrial membrane is impermeable to ions except those for which there are specific carriers. Most of the carriers are reversible, as indicated by two-headed arrows. Compounds transported in one direction are indicated in red. The ATP/ADP translocase and the aspartate-glutamate carrier are both electrophoretic their transport is driven in the direction of the mitochondrial membrane potential, as indicated by red arrows. Glutamine is carried into the matrix by an electroneutral carrier. The unimpaired functioning of mitochondrial carriers is essential for normal metabolism. (Adapted with permission from reference [70].)... 

Fig-i Mitochondrial protein import machinery as defined in S. cerevisiae. TOM translo-case of the outer mitochondrial membrane SAM sorting and assembly machinery TIM translocase of the inner mitochondrial membrane MIA mitochondrial IMS import and assembly machine PAM presequence translocase associated motor IMP inner membrane protease MPP mitochondrial processing peptidase. The numbers on the individual Tom, Sam, Tim or Pam components represent their approximate molecular masses in kDa. See text for mechanistic details. Adopted from Dolezal et al. 2006... 

The answer is b. (Murray, pp 505-626. Scriver, pp 4029-4240. Sack, pp 121-138. Wilson, pp 287-320.) A deficiency in carnitine, carnitine acyl-transferase 1, carnitine acyltransferase 11, or acylcarnitine translocase can lead to an inability to oxidize long-chain fatty acids. This occurs because all of these components are needed to translocate activated long-chain (>10 carbons long) fatty acyl CoA across mitochondrial inner membrane into the matrix where P oxidation takes place. Once long-chain fatty acids are coupled to the sulfur atom of CoA on the outer mitochondrial membrane, they can be transferred to carnitine by the enzyme carnitine acyltransferase I, which is located on the cytosolic side of the inner mitochondrial membrane. Acyl carnitine is transferred across the inner membrane to the matrix surface by translocase. At this point the acyl group is reattached to a CoA sulfhydryl by the carnitine acyltransferase 11 located on the matrix face of the inner mitochondrial membrane. 

Fig. 20.20. Model for the import of nuclear-encoded proteins into the mitochondrial matrix. The matrix preprotein with its positively charged N-terminal presequence is shown in blue. Abbreviations OM, outer mitochondrial membrane IMS, intramembrane space IM, inner mitochondrial membrane TOM, translocases of the outer mitochondrial membrane TIM, translocases of the inner mitochondrial membrane mthspTO, mitochondrial heat shock protein 70.

Carnitine palmitoyltransferase I (CPTI also called carnitine acyltransferase I, CATI), the enzyme that transfers long-chain fatty acyl groups from CoA to carnitine, is located on the outer mitochondrial membrane (Fig. 23.5). Fatty acylcamitine crosses the inner mitochondrial membrane with the aid of a translocase. The fatty acyl group is transferred back to CoA by a second enzyme, carnitine palmitoyl-transferase II (CPTII or CATII). The carnitine released in this reaction returns to the cytosolic side of the mitochondrial membrane by the same translocase that brings fatty acylcamitine to the matrix side. Long-chain fatty acyl CoA, now located within the mitochondrial matrix, is a substrate for (3-oxidation. 

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