Proteins can modulate the properties of ligands to which they bind. For instance heme in solution would not make a good oxygen carrier because it binds too tightly to CO , but when myoglobin binds the heme protein residues are present that change the relative affinity of myoglobin for oxygen and the molecule against which oxygen competes. However the side chains of these amino acids do not bring enough chemical diversity to catalyze all of the kinds of reactions that enzymes catalyze. In the graph to the right the free energies of A and B are shown. The first is for the reaction without enzyme present and the second is the same reaction in the presence of an enzyme specific for that reaction.
Enzymes are remarkable.
Nucleotides have many functions in the cell. The protein DNA ligase joins Okazaki fragments 51. The protein Primase synthesizes short RNA oligonucleotides required for DNA polymerase to begin incorporating deoxynucleotides 52. The protein Helicase separates the DNA duplex at replication forks 53. The protein required to prevent separated DNA strands from rehybridizing is SSB 54.