protected function getEnzymes(\SimpleXMLElement $xml)
    {
        $enzymes = array();

        foreach ($xml->Enzyme as $xmlEnzyme) {
            $enzyme = $this->getEnzyme($xmlEnzyme);

            if (isset($enzyme['id'])) {
                $enzymes[$enzyme['id']] = $enzyme;
                continue;
            }

            $enzymes[] = $enzyme;
        }

        return $enzymes;
    }