There are two types of coenzymes – cosubstrates and prosthetic groups. Cosubstrates are coenzymes that bind tightly but transiently to an enzyme. Cosubstrates may be released from the enzyme at some point, and then bind again at a later time. Prosthetic groups are enzyme partner molecules that bind tightly or even covalently to an enzyme. Unlike cosubstrates, the bond between prosthetic groups and enzymes is permanent. Prosthetic groups act as structural elements and charge carriers. They also help enzymes bind to other molecules. Despite these differences, both cosubstrates and prosthetic groups have the same function. They both aid in the reaction of enzymes and proteins.