Cofactors and Coenzymes Some enzymes do not need any additional components to show full activity. However, others require non-protein molecules called cofactors to be bound for activity. Cofactors can be either inorganic (e.g., metal ions and iron-sulfur clusters) or organic compounds, (e.g., flavin and heme).

Cofactors Active enzyme / Holoenzyme : – Polypeptide portion of enzyme (apoenzyme) – Nonproteineous group (cofactor) Cofactors are bound to the enzyme for it to maintain the correct configuration of the active site – Metal ions – Organic compounds – Organometallic compounds

Organic cofactors can be either:  prosthetic groups, which are tightly bound to an enzyme, or coenzymes, which are released from the enzyme's active site during the reaction.  carbonic anhydrase, with a zinc cofactor bound as part of its active site. These tightly- bound molecules are usually found in the active site and are involved in catalysis.

For example, flavin and heme cofactors are often involved in redox reactions. Most cofactors are not covalently attached to an enzyme, but are very tightly bound. However, organic prosthetic groups can be covalently bound.

Coenzymes Coenzymes are small organic molecules that transport chemical groups in an enzymatic reaction. OR Any of a number of freely diffusing organic compounds that function as cofactors with enzymes in promoting a variety of metabolic reactions. Some of these chemicals such as riboflavin, thiamine and folic acid are vitamins, (acquired).

 The chemical groups carried include the hydride ion (H - ) carried by NAD or NADP +, the acetyl group carried by coenzyme A, … etc.  Since coenzymes are chemically changed as a consequence of enzyme action, it is useful to consider coenzymes to be a special class of substrates, or second substrates, which are common to many different enzymes.  About 700 enzymes are known to use the coenzyme NADH.

 Coenzymes are usually regenerated and their concentrations maintained at a steady level inside the cell: for example, NADPH is regenerated through the pentose phosphate pathway and S-adenosylmethionine by methionine adenosyltransferase  Coenzymes participate in enzyme-mediated catalysis in stoichiometric (mole-for-mole) amounts and are modified during the reaction.

Coenzymes A coenzyme is required by some enzymes – An organic molecule bound to the enzyme by weak interactions / Hydrogen bonds – Most coenzymes carry electrons or small groups – Many have modified vitamins in their structure

Nicotinamide Adenine Dinucleotide in Oxidized and Reduced Forms 19.7 Cofactors and Coenzymes

NAD + to NADH Mechanism The nicotinamide part of NAD + accepts a hydride ion (H plus two electrons) from the alcohol to be oxidized The alcohol loses a proton ( H + ) to the solvent Oxidized formReduced form +H Cofactors and Coenzymes

Two Other Adenine Dinucleotide Coenzymes 19.7 Cofactors and Coenzymes