Mutations are a random change in the nucleotide sequence in a genome. Genes code for proteins, such as enzymes, and are found in the genome. When a change in the nucleotide sequence within a gene happens, the amino acid sequence (primary structure) of the protein that the gene codes for is possibly changed. This is because each three nucleotides within the sequence of the gene make up one of 64 codons, which codes for one of the 20 amino acids. If the primary structure of the protein is changed, it's possible that the secondary (alpha-helices and beta-sheets) and subsequently tertiary structures (protein folding, i.e. hydrogen bonding, van der Waal's, disulfide bridges) are changed. This means that the activity of the enzyme will either be reduced or outright lost, due to the change in conformation of the specific active site.