(1) A bank organized under this act shall have capital in an amount the commissioner considers adequate to conduct its business.

  (2) A bank shall not be authorized to commence business until it has surplus at least equal to 20% of its capital.

  (3) After organization, each bank shall maintain an adequate capital structure appropriate to conduct its business and the protection of its depositors. The capital adequacy of a bank shall be analyzed and appraised in relation to the character of its management, the liquidity of assets, history of earnings and of the retention of earnings, the potential volatility of the deposit structure, and its risk management, with due regard to the bank’s capacity to furnish the broadest service to the public.

  (4) At all times a bank shall maintain surplus in an amount equal to at least the amount of its capital, except as provided in subsection (2) and except as provided in section 3806, and shall not reduce surplus without the approval of the commissioner.