title: Why multiple small subunits (Y2 and Y4) for yeast ribonucleotide reductase? Toward understanding the role of Y4.
abstract: Ribonucleotide reductases (RNRs) catalyze the conversion of nucleotides to deoxynucleotides. Class I RNRs are composed of two homodimeric subunits: R1 and R2. R1 is directly involved in the reduction, and R2 contains the diferric-tyrosyl radical (Y*) cofactor essential for the initiation of reduction. Saccharomyces cerevisiae has two RNRs; Y1 and Y3 correspond to R1, whereas Y2 and Y4 correspond to R2. Y4 is essential for diferric-Y* formation in Y2 from apoY2, Fe(2+), and O(2). The actual function of Y4 is controversial. Y2 and Y4 have been further characterized in an effort to understand their respective roles in nucleotide reduction. (His)(6)-Y2, Y4, and (His)(6)-Y4 are homodimers, isolated largely in apo form. Their CD spectra reveal that they are predominantly helical. The concentrations of Y2 and Y4 in vivo are 0.5-2.3 microM, as determined by Western analysis. Incubation of Y2 and Y4 under physiological conditions generates apo Y2Y4 heterodimer, which can form a diferric-Y small middle dot when incubated with Fe(2+) and O(2). Holo Y2Y4 heterodimer contains 0.6-0.8 Y* and has a specific activity of 0.8-1.3 micromol.min.mg. Titration of Y2 with Y4 in the presence of Fe(2+) and O(2) gives maximal activity with one equivalent of Y4 per Y2. Models for the function of Y4 based on these data and the accompanying structure will be discussed.
authors.affiliation: Department of Chemistry, Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: J
authors.initials: J
authors.lastname: Ge
authors.forename: D L
authors.initials: DL
authors.lastname: Perlstein
authors.forename: H H
authors.initials: HH
authors.lastname: Nguyen
authors.forename: G
authors.initials: G
authors.lastname: Bar
authors.forename: R G
authors.initials: RG
authors.lastname: Griffin
authors.forename: J
authors.initials: J
authors.lastname: Stubbe
mesh_terms: D001051:Apoenzymes / Q000737:chemistry / Q000378:metabolism; D001483:Base Sequence; D015153:Blotting, Western; D002942:Circular Dichroism; D017931:DNA Primers / Q000235:genetics; D019281:Dimerization; D007501:Iron / Q000378:metabolism; D017433:Protein Structure, Secondary; D021122:Protein Subunits; D012264:Ribonucleotide Reductases / Q000737:chemistry* / Q000235:genetics; D012441:Saccharomyces cerevisiae / Q000201:enzymology / Q000235:genetics
chemical_list: D001051:Apoenzymes; D017931:DNA Primers; D021122:Protein Subunits; D007501:Iron; D012264:Ribonucleotide Reductases