The major facilitator superfamily represents the largest group of secondary membrane transporters in the cell. Here we report the 3.3 angstrom resolution structure of a member of this superfamily, GlpT, which transports glycerol-3-phosphate into the cytoplasm and inorganic phosphate into the periplasm. The amino- and carboxyl-terminal halves of the protein exhibit a pseudo two-fold symmetry. Closed off to the periplasm, a centrally located substrate-translocation pore contains two arginines at its closed end, which comprise the substrate-binding site. Upon substrate binding, the protein adopts a more compact conformation. We propose that GlpT operates by a single-binding site, alternating-access mechanism through a rocker-switch type of movement.

With the capability of inducing elevated expression of ACE2, the cellular receptor for SARS-CoV-2, angiotensin II receptor blockers or angiotensin-converting enzyme inhibitors (ARBs/ACEIs) treatment may have a controversial role in both facilitating virus infection and reducing pathogenic inflammation. We aimed to evaluate the effects of ARBs/ACEIs on COVID-19 in a retrospective, single-center study. 126 COVID-19 patients with preexisting hypertension at Hubei Provincial Hospital of Traditional Chinese Medicine (HPHTCM) in Wuhan from January 5 to February 22, 2020 were retrospectively allocated to ARBs/ACEIs group (n=43) and non-ARBs/ACEIs group (n=83) according to their antihypertensive medication. 125 age-and sex-matched COVID-19 patients without hypertension were randomly selected as non-hypertension controls. In addition, the medication history of 1942 hypertension patients that were admitted to HPHTCM from November 1 to December 31, 2019 before COVID-19 outbreak were also reviewed for external comparison. Epidemiological, demographic, clinical and laboratory data were collected, analyzed and compared between these groups. The frequency of ARBs/ACEIs usage in hypertension patients with or without COVID-19 were comparable. Among COVID-19 patients with hypertension, those received either ARBs/ACEIs or non-ARBs/ACEIs had comparable blood pressure. However, ARBs/ACEIs group had significantly lower concentrations of hs-CRP (p=0.049) and procalcitonin (PCT, p=0.008). Furthermore, a lower proportion of critical patients (9.3% vs 22.9%; p=0.061), and a lower death rate (4.7% vs 13.3%; p=0.216) were observed in ARBs/ACEIs group than non-ARBs/ACEIs group, although these differences failed to reach statistical significance. Our findings thus support the use of ARBs/ACEIs in COVID-19 patients with preexisting hypertension.

The crystal structure of rat liver S-adenosyl-L-homocysteine hydrolase (AdoHcyase, EC 3.3.1.1) which catalyzes the reversible hydrolysis of S-adenosylhomocysteine (AdoHcy) has been determined at 2.8 A resolution. AdoHcyase from rat liver is a tetrameric enzyme with 431 amino acid residues in each identical subunit. The subunit is composed of the catalytic domain, the NAD+-binding domain, and the small C-terminal domain. Both catalytic and NAD+-binding domains are folded into an ellipsoid with a typical alpha/beta twisted open sheet structure. The C-terminal section is far from the main body of the subunit and extends into the opposite subunit. An NAD+ molecule binds to the consensus NAD+-binding cleft of the NAD+-binding domain. The peptide folding pattern of the catalytic domain is quite similar to the patterns observed in many methyltransferases. Although the crystal structure does not contain AdoHcy or its analogue, there is a well-formed AdoHcy-binding crevice in the catalytic domain. Without introducing any major structural changes, an AdoHcy molecule can be placed in the catalytic domain. In the structure described here, the catalytic and NAD+-binding domains are quite far apart from each other. Thus, the enzyme appears to have an "open" conformation in the absence of substrate. It is likely that binding of AdoHcy induces a large conformational change so as to place the ribose moiety of AdoHcy in close proximity to the nicotinamide moiety of NAD+. A catalytic mechanism of AdoHcyase has been proposed on the basis of this crystal structure. Glu155 acts as a proton acceptor from the O3'-H when the proton of C3'-H is abstracted by NAD+. His54 or Asp130 acts as a general acid-base catalyst, while Cys194 modulates the oxidation state of the bound NAD+. The polypeptide folding pattern of the catalytic domain suggests that AdoHcy molecules can travel freely to and from AdoHcyase and methyltransferases to properly regulate methyltransferase activities. We believe that the crystal structure described here can provide insight into the molecular architecture of this important regulatory enzyme.

Methyltransfer reactions are some of the most important reactions in biological systems. Glycine N-methyltransferase (GNMT) catalyzes the S-adenosyl-l-methionine- (SAM-) dependent methylation of glycine to form sarcosine. Unlike most SAM-dependent methyltransferases, GNMT has a relatively high value and is weakly inhibited by the product S-adenosyl-l-homocysteine (SAH). The major role of GNMT is believed to be the regulation of the cellular SAM/SAH ratio, which is thought to play a key role in SAM-dependent methyltransfer reactions. Crystal structures of GNMT complexed with SAM and acetate (a potent competitive inhibitor of Gly) and the R175K mutated enzyme complexed with SAM were determined at 2.8 and 3.0 A resolutions, respectively. With these crystal structures and the previously determined structures of substrate-free enzyme, a catalytic mechanism has been proposed. Structural changes occur in the transitions from the substrate-free to the binary complex and from the binary to the ternary complex. In the ternary complex stage, an alpha-helix in the N-terminus undergoes a major conformational change. As a result, the bound SAM is firmly connected to protein and a "Gly pocket" is created near the bound SAM. The second substrate Gly binds to Arg175 and is brought into the Gly pocket. Five hydrogen bonds connect the Gly in the proximity of the bound SAM and orient the lone pair orbital on the amino nitrogen (N) of Gly toward the donor methyl group (C(E)) of SAM. Thermal motion of the enzyme leads to a collision of the N and C(E) so that a S(N)2 methyltransfer reaction occurs. The proposed mechanism is supported by mutagenesis studies.