The early detection and diagnosis of cancer lies central to successful treatment 
and improved patient outcome. Current techniques are limited by the nature of 
the biological receptor and the assays available. This paper reports the use of 
novel biological probes, peptide aptamers, in detecting cyclin-dependent protein 
kinases (CDKs) whose activity is important in proliferating and cancerous cells. 
We describe, specifically, the optimization of an orientated peptide aptamer 
surface and its utilization in establishing a highly specific, low-nanomolar 
sensitive, detection protocol for the active form of CDK2. In comparing target 
binding affinity of two different aptamers (pep6 and pep9), both constructed 
through the insertion of peptide sequences into the surface of a scaffold 
protein, one was observed to be consistently more effective. Significantly, the 
pep9 aptamers were able to detect subtle changes in the conformation of CDK2 
associated with activation of its catalytic activity that may be caused by the 
phosphorylation of a single amino acid (threonine 160). A typical response 
toward the inactive form of CDK2 was in the range of 0.5-2% of the binding of 
the active form of CDK2 in the concentration range from 2 to 20 nM. Although 
antibodies are occasionally able to recognize conformations in their targets, 
this is the first time that a nonantibody protein probe has been used to detect 
an active protein isoform. Because peptide aptamers are usually raised against 
full-length proteins, this raises the possibility that peptide aptamers will be 
able to extend the repertoire of probes that recognize protein conformations, 
post-translational modifications (PTMs), or conformations stabilized by PTMs.