The main reason that proteins outperform other macromolecules in terms of catalytic ability is because see section 3.4 (page

Among the four main biological macro-molecules, the proteins are capable of outperforming the others as far as their catalytic ability is concerned. This ability of the proteins to outshine in terms of catalytic activity is because of the presence of they contain a variety of the alkyl (R) groups. These alkyl groups attached to the chains of proteins enhances the catalytic activity.

Hence, the given blank can be filled with 'variety of alkyl groups'.

Answer;

-Proteins can contain a variety of R groups

Explanation;

-All amino acids have the alpha carbon bonded to a hydrogen atom, carboxyl group, and amino group. The R group varies among amino acids and determines the differences between these protein monomers.

Hydrogen bonds in secondary structure involve the polar amino and carboxyl groups of amino acids, but do not involve R-groups.As the peptide chain continues to grow, interactions between amino acid side chains (R-groups) form the tertiary structure of a polypeptide.