Final answer:
Most HSP-90 substrates function as enzymes which act as catalysts in biochemical processes. Ubiquitination may also be necessary to regulate these enzymes' activity or stability.
Step-by-step explanation:
The function of most HSP-90 substrates is that they are primarily enzymes, which are catalysts in biochemical reactions. These enzymes may be involved in various functions such as breakdown, rearrangement, or synthesis reactions within the cell. To aid in these processes, ubiquitination may also be needed, which is the process where proteins are labeled for degradation or for other regulatory functions. This is because enzymes, while they increase reaction rates as organic catalysts, can undergo alterations such as ubiquitination that affect their function and stability within the cellular environment.