Activation function 2 helix

In the first half of the ping-pong bi-bi reaction, the active site cysteine is acylated by either an acyl-ACP (KAS I and KAS II) or an acetyl-CoA (KAS III), or in the case of the M. tuberculosis KAS III (FabH) a long-chain acyl-CoA. Attack of the active site cysteine thiolate on the donor acyl thioester is aided by the dipole of the active site helix and an oxyanion hole composed of two backbone NH groups. The additional catalytic residues (His, His or His, Asn) are thought to primarily function in the decarboxylation of malonyl-ACP and stabilization of the acetyl-ACP carbanion that is formed. The carbanion subsequently attacks the acyl-enzyme thioester leading to the formation of the (3-ketoacyl-ACP product. 

The prior description of mRNA regulation (see Figure 1.3) has many steps. Thus, it is not surprising to find that other processes can also influence the rate and extent to which the information in a gene can be manifested as an active functional protein. Such translational-level controls can entail competition for ribosomes by the numerous mRNAs from different genes. Alternately, the base interactions that occur in a duplex DNA molecule that lead to the a-helix can also result in structural organization in mRNA. For example, the bases within an mRNA strand can self-complement, thereby leading to the formation of hairpin loops. Such secondary structures that result from a primary structure (the base sequence) can influence how fast and successfully the ribosomal-mediated translation process occurs. 

Although biologically active helical y-peptides have not been reported so far, the striking structural similarities (polarity and helicity) between the a-helix of L-a-peptides and the (P)-2.6i4-hehx of y-peptides suggest that the 2.614-helical backbone might prove useful as a template for elaborating functional mimetics of a-helical surfaces and intervening in protein-protein interactions. 

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