Structure
(The discussion of structure, catalytic center, and allosteric site that follows is based on the prokaryotic version of ATCase, to be specific, from E. coli.)
Early studies demonstrated that ATCase consists of two different kinds of polypeptide chains, which have different roles. The catalytic subunits catalyze the carbamylation of the amino group of aspartate but do not have regulatory properties, while the regulatory subunits do not have any catalytic activity but contain the regulatory sites for effector binding. The ATCase holoenzyme is made of two catalytic trimers that are in contact and held together by three regulatory dimers, so the native form of the enzyme contains six chains of each type, with a total molecular weight of 310 kDa.
Each of the catalytic domains is composed of two structural domains, the aspartate domain, which contains most of the residues responsible for binding aspartate, and the carbamoyl phosphate domain, which contains most of the residues that bind to carbamoyl phosphate. Each regulatory domain is also composed of two domains, the allosteric domain, which has the binding site for the nucleotide effectors, and the zinc domain, consisting of four cysteine residues clustered in its C-terminal region. These residues coordinate a zinc atom that is not involved in any catalytic property, but has been shown to be essential for the association of regulatory and catalytic subunits.
The three-dimensional arrangement of the catalytic and regulatory subunits involves several ionic and hydrophobic stabilizing contacts between amino acid residues. Each catalytic chain is in contact with three other catalytic chains and two regulatory chains. Each regulatory monomer is in contact with one other regulatory chain and two catalytic chains. In the unliganded enzyme, the two catalytic trimers are also in contact.
Read more about this topic: Aspartate Carbamoyltransferase
Famous quotes containing the word structure:
“... the structure of our public morality crashed to earth. Above its grave a tombstone read, “Be tolerant—even of evil.” Logically the next step would be to say to our commonwealth’s criminals, “I disagree that it’s all right to rob and murder, but naturally I respect your opinion.” Tolerance is only complacence when it makes no distinction between right and wrong.”
—Sarah Patton Boyle, U.S. civil rights activist and author. The Desegregated Heart, part 2, ch. 2 (1962)
“The question is still asked of women: “How do you propose to answer the need for child care?” That is an obvious attempt to structure conflict in the old terms. The questions are rather: “If we as a human community want children, how does the total society propose to provide for them?””
—Jean Baker Miller (20th century)
“Vashtar: So it’s finished. A structure to house one man and the greatest treasure of all time.
Senta: And a structure that will last for all time.
Vashtar: Only history will tell that.
Senta: Sire, will he not be remembered?
Vashtar: Yes, he’ll be remembered. The pyramid’ll keep his memory alive. In that he built better than he knew.”
—William Faulkner (1897–1962)