Aspartate carbamoyltransferase

Gouaux, J.E., Lipscomb, W.N. Crystal structures of phosphonoacetamide ligated T and phosphono-acetamide and malonate ligated R states of aspartate carbamoyltransferase at 2.8 A resolution and neutral pH. Biochemistry 29 389-402, 1990. 

EC2 Transferases methyltransferase, aspartate carbamoyltransferase, etc.) EC2.3 Acvltransferases (amino-acid N-acetyltransferase, ATP citrate synthase, etc.) luciferasa ... 

Doubly wound parallel fi sheet Aspartate carbamoyltransferase, (Monaco et ah, 1978)... 

Catalytic domain 1 doubly wound parallel fi sheet Catalytic domain 2 classic doubly wound fi sheet (Fig. 76) Aspartate transaminase see Aspartate aminotransferase Aspartate transcarbamylase (Monaco et ah, 1978), see Aspartate carbamoyltransferase... 

Figure 3.13 (a) Feedback control of a hypothetical pathway. (b) Feedback control of threonine deaminase in the isoleucine synthetic pathway and of aspartate carbamoyltransferase in the cytidine triphosphate synthetic pathway in the bacterium E. coli. 

A different, simpler , pathway is involved in the synthesis of pyrimidine nucleotides. A pyrimidine base (orotate), is synthesised first. Then the ribose is added from 5-phosphoribosyl 1-pyrophosphate. The two precursors for the formation of orotate are carbamoylphosphate and aspartate, which form carbamoyl aspartate, catalysed by aspartate carbamoyltransferase. 

The regulation of bacterial aspartate carbamoyltransferase by ATP and CTP has been particularly well studied, and is discussed on p. 116. In animals, in contrast to prokaryotes, it is not ACTase but carbamoyl-phosphate synthase that is the key enzyme in pyrimidine synthesis. It is activated by ATP and PRPP and inhibited by UTP. 

Like aspartate carbamoyltransferase (see p. 116), Hb can exist in two different states (conformations), known as the T form and... 

ASPARTYLCLUCOSAMINIDASE ASPARTATE AMINOTRANSFERASE ASPARTATE AMMONIA-LYASE ASPARTATE CARBAMOYLTRANSFERASE ASPARTATE a-DECARBOXYLASE ASPARTATE /3-DECARBOXYLASE ASPARTATE KINASE d-ASPARTATE OXIDASE ASPARTATE RACEMASE... 

ASPARTATE CARBAMOYLTRANSFERASE ATMOSPHERE ATOM PERCENT EXCESS TRACER/TRACEE RATIO COMPARTMENTAL ANALYSIS ISOTOPE EXCHANGE KINETICS ATOMIC MASS UNIT ATOMIC ORBITAL ATOMIZATION ATP... 

ASPARTATE CARBAMOYLTRANSFERASE NEGATIVE COOPERATIVITY POSTTRANSLATIONAL MODIEICATION OE PROTEINS... 

Proteins are often organized into large complexes, sometimes for the purpose of regulating metabolism. An example is aspartate carbamoyltransferase... 

Figure 7-20 (A) Subunit assembly of two C3 catalytic trimers (green) and three R2 regulatory dimers around the periphery in aspartate carbamoyltransferase. After Krause et a/.109 Courtesy of William N. Lipscomb. The aspartate-and carbamoylphosphate-binding domains of the catalytic subunits are labeled Asp and CP, respectivley, while the zinc and allosteric domains of the regulatory subunits are labeled Alio and Zn, respectively. (B) Ribbon drawing of a single pair of regulatory (left) and catalytic (right) subunits with the structural domains labeled. MolScript drawing from Thomas et al.no...

Aspartate aminotransferase 57s, 135s, 753 absorption spectra 749 active site structure 744 atomic structure 750 catalytic intermediates, models 752 NMR spectra 149 quinonoid intermediate 750 Ramachandran plot 61 sequence 57 transamination 742 Aspartate ammonia-lyase 685 Aspartate carbamoyltransferase 348s active sites 348 regulation 540... 

Mixed subunits Aspartate carbamoyltransferase, pertussis toxin Allosteric enzymes different subunits have separate functions... 

Stevens, R. C., J. E. Gouaux, and W. N. Lipscomb, Structural consequences of effector binding to the T state of aspartate carbamoyltransferase Crystal structures of the unligated and ATP- and CTP-complexed enzymes at 2.6 A resolution. Biochem. 29 7691, 1990. 

Aspartate carbamoyltransferase is an allosteric enzyme in which the active sites and the allosteric effector binding sites are on different subunits. Explain how it might be possible for an allosteric enzyme to have both kinds of sites on the same subunit. 

How do you explain the observation that pyrimidine biosynthesis in bacteria is regulated at the level of aspartate carbamoyltransferase, whereas most of the regulation in humans is at the level of carbamoyl phosphate synthase ... 

Aspartate transcarbamoylase (aspartate carbamoyltransferase ATCase), a key enzyme in pyrimidine biosynthesis (see Topic FI), provides a good example of allosteric regulation. ATCase catalyzes the formation of N-carbamoylaspar-tate from aspartate and carbamoyl phosphate, and is the committed step in pyrimidine biosynthesis (Fig. 2). The binding of the two substrates aspartate and carbamoyl phosphate is cooperative, as shown by the sigmoidal curve of V0 against substrate concentration (Fig. 3). 

Aspartate carbamoyltransferase catalyzes the formation of carbamoyl aspartate from carbamoyl phosphate and aspartate in the first committed step of pyrimidine biosynthesis (Chap. 15). The enzyme from the bacterium E. coli (Mr = 310,000) consists of 12 subunits, six regulatory and six catalytic. CTP is a negative effector i.e., it inhibits the enzyme, and does so through binding to the regulatory subunits. ATP is a positive effector that acts through the regulatory subunits, while succinate inhibits the reaction by direct competition with aspartate at the active site (see Chap. 9 for more on effectors). 

Carbonic anhydrase has one Zn2+ ion per molecule of enzyme, and the metal ion resides in the active site. Aspartate carbamoyltransferase has six Zn2+ ions per dodecamer these are required fo the stabilization of the complex, since without Zn2+, the hexamer dissociates. 

Alternatively, if Cs>B, then C would activate the production of B and this again would tend to equalize the concentrations of B and C. This activation by C is usually the result of C competing for the same binding site as B on E[, and thus reducing the inhibition by B. The first enzyme of pyrimidine synthesis, aspartate carbamoyltransferase, in E. coli, is subject to this type of control (Example 8.5 Chap. 15) in this case B is CTP, C is ATP, and D is the nucleic acids. 

Fig. 1. The structure of ZnS4 cores of (a) aspartate carbamoyltransferase (4) and (b) alcohol dehydrogenase [Zn(II)] (5).

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