Catalytic mechanism of α-class carbonic anhydrases: CO2 hydration and proton transfer.

The carbonic anhydrases (CAs; EC 4.2.1.1) are a family of metalloenzymes that catalyze the reversible hydration of carbon dioxide (CO2) and dehydration of bicarbonate (HCO3 (-)) in a two-step ping-pong mechanism: [Formula: see text] CAs are ubiquitous enzymes and are categorized into five distinct classes (α, ß, γ, δ and ζ). The α-class is found primarily in vertebrates (and the only class of CA in mammals), ß is observed in higher plants and some prokaryotes, γ is present only in archaebacteria whereas the δ and ζ classes have only been observed in diatoms.The focus of this chapter is on α-CAs as the structure-function relationship is best understood for this class, in particular for humans. The reader is referred to other reviews for an overview of the structure and catalytic mechanism of the other CA classes. The overall catalytic site structure and geometry of α-CAs are described in the first section of this chapter followed by the kinetic studies, binding of CO2, and the proton shuttle network.

What do we know about males and emergency contraception? A synthesis of the literature.

CONTEXT: Unintended pregnancy rates are high in the United States. It is important to know whether improving males' access to emergency contraceptive pills may help prevent unintended pregnancy, especially in cases of -condom failure. METHODS: A search of the PubMed, PsycINFO and Cumulative Index of Nursing and Allied Health Literature databases was conducted to identify studies published from January 1980 to April 2011 concerning males and emergency contraception. Forty-three studies met the specified criteria and examined relevant knowledge, attitudes, beliefs, intentions or behaviors, from the perspectives of males, clinicians or pharmacists. RESULTS: The proportion of males who were familiar with emergency contraception ranged from 38% among teenagers to 65-100% among adults. Small proportions reported that they and their partner had used or discussed using emergency contraceptive pills (13-30%) or that they themselves had ever purchased them (11%). Most providers (77-85%) reported general knowledge about emergency contraceptives, but the proportions who knew the time frame within which the pills can be prescribed were smaller (28-63%). Most males approved of emergency contraceptive use following contraceptive failure (74-82%) or unprotected sex (59-65%), or in cases of rape (85-91%), but both pharmacists and college students reported concerns that females might feel pressured to use the method. No study examined clinicians' attitudes and behaviors (e.g., counseling or advance provision) regarding males and emergency contraception. CONCLUSIONS: Studies are needed to determine whether male involvement in emergency contraception can reduce rates of unintended pregnancy and to assess health professionals' ability to counsel males about the method.