protected function buildRequestParams($function, array $params)
  {
    $associateTag = array();

    if(false === empty($this->requestConfig['associateTag']))
    {
      $associateTag = array('AssociateTag' => $this->requestConfig['associateTag']);
    }

    return array_merge(
      $associateTag,
      array(
        'AWSAccessKeyId' => $this->requestConfig['accessKey'],
        'Request' => array_merge(
          array('Operation' => $function),
          $params,
          $this->responseConfig['optionalParameters'],
          array('ResponseGroup' => $this->prepareResponseGroup())
    )));
  }